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Alternator terminal B+ access


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Any tricks to accessing the B + terminal on my alternator ? Wanting to test terminal voltage prior to ordering new cable and there is absolutely no room behind the alternator to get to the terminal. Short of removing it, how does one test the voltage output at this terminal ?

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Any tricks to accessing the B + terminal on my alternator ? Wanting to test terminal voltage prior to ordering new cable and there is absolutely no room behind the alternator to get to the terminal. Short of removing it, how does one test the voltage output at this terminal ?

Well, you could hook it up to the Durametric software and read it.

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Is that the voltage that shows as terminal 30 ? Was not sure if that was the same result as taking a physical reading at the alternator itself. If not , what specific section would give me the reading ?

Edited by dphatch
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The DME monitors the output voltage of the alternator, should be in the actual values section.

What exactly are you trying to accomplish? You mentioned changing cables; if you are having issues with the charging system involving the cables, you should be looking at resistance, not voltage, as corrosion inside the cables causes the resistance to rise, restricting current (amperage) flow. This is often difficult to catch by monitoring voltage, but easy to see when looking at resistance. And you cannot read resistance off of any scanning system I am aware of, you need a multimeter and access to the cable, which means pulling the alternator if you cannot get behind it.

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Looking to varify that the variance between the voltage reading at the alternator and the voltage reading at the battery is possible cause of slow cranking when car is hot. From what I have read the normal voltage at the alternator should be 14.7 volts. A drop of .5 volts or more at the battery suggests resistance exists in the charging curcuit between these two points and the most common cause appears to be the alternator/starter/battery harness. I have also read that the issue is not limited to the '05 and '06 cars. Before I replace it I'm looking for confirmation of same. Battery checks out as being sound and, with the exception of hot starts, the car starts from cold or after a sufficient cooling period.

If I'm going about this the wrong way would appreciate any guidance you can offer.

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Hi dphatch,

You need to do a voltage drop test on both the positive side and the negative of the circuit using a DVOM. I found a video that will make it simple to understand however its not on a Porsche :). As the saying goes a picture is worth a thousand words. Hope this helps.

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Looking to varify that the variance between the voltage reading at the alternator and the voltage reading at the battery is possible cause of slow cranking when car is hot. From what I have read the normal voltage at the alternator should be 14.7 volts. A drop of .5 volts or more at the battery suggests resistance exists in the charging curcuit between these two points and the most common cause appears to be the alternator/starter/battery harness. I have also read that the issue is not limited to the '05 and '06 cars. Before I replace it I'm looking for confirmation of same. Battery checks out as being sound and, with the exception of hot starts, the car starts from cold or after a sufficient cooling period.If I'm going about this the wrong way would appreciate any guidance you can offer.

When we get a car with a suspect cable problem, we disconnect the cable and check it for internal resistance using a digital multimeter. These cables should have no resistance; if we find any, the cable gets replaced. This is a fairly common problem.

You can do the same thing by measuring voltage drop to get to the same place, but you need to be cautious because that method cannot pinpoint which cable is the problem if the harness has more than one cable or a connector in it.

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Hi dphatch,

You need to do a voltage drop test on both the positive side and the negative of the circuit using a DVOM. I found a video that will make it simple to understand however its not on a Porsche :). As the saying goes a picture is worth a thousand words. Hope this helps.

That would be a big help as an alternator is an alternator, but I'm not seeing the video attachment. Could you try it again ?

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Looking to varify that the variance between the voltage reading at the alternator and the voltage reading at the battery is possible cause of slow cranking when car is hot. From what I have read the normal voltage at the alternator should be 14.7 volts. A drop of .5 volts or more at the battery suggests resistance exists in the charging curcuit between these two points and the most common cause appears to be the alternator/starter/battery harness. I have also read that the issue is not limited to the '05 and '06 cars. Before I replace it I'm looking for confirmation of same. Battery checks out as being sound and, with the exception of hot starts, the car starts from cold or after a sufficient cooling period.If I'm going about this the wrong way would appreciate any guidance you can offer.

When we get a car with a suspect cable problem, we disconnect the cable and check it for internal resistance using a digital multimeter. These cables should have no resistance; if we find any, the cable gets replaced. This is a fairly common problem.

You can do the same thing by measuring voltage drop to get to the same place, but you need to be cautious because that method cannot pinpoint which cable is the problem if the harness has more than one cable or a connector in it.

So comparing the results from the Durametric to the voltage reading at the battery + terminal won't offer confirmation ?

As an update, checked the Durametric voltage at DME and it reads anywhere from 12.96 to 13.12 with AC, Radio and front cooling fans and engine at idle. Voltage at battery + terminal under the same conditions varied between 13.10 and 13.56. Engine was fully warmed up.

Edited by dphatch
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Voltage drop testing is the best way to find the problem because it is dynamic. Your testing the actual components with out disturbing any connections that might be the issue. As a factory trained tech I use voltage drop testing as one of my first diagnostic testing strategy's. Today's cars have up to 75 + computer modules on board and most of our problems come form bad grounds and or resistance connecting circuits.The problem with only using an Ohm meter is your not testing the circuit under load so is might ohm out as good but will fail when you put a load on it. I have attached one more video that shows how to pinpoint with voltage drop testing. Hope this helps.

BingDiagnostic Auto Lab

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Looking to varify that the variance between the voltage reading at the alternator and the voltage reading at the battery is possible cause of slow cranking when car is hot. From what I have read the normal voltage at the alternator should be 14.7 volts. A drop of .5 volts or more at the battery suggests resistance exists in the charging curcuit between these two points and the most common cause appears to be the alternator/starter/battery harness. I have also read that the issue is not limited to the '05 and '06 cars. Before I replace it I'm looking for confirmation of same. Battery checks out as being sound and, with the exception of hot starts, the car starts from cold or after a sufficient cooling period.If I'm going about this the wrong way would appreciate any guidance you can offer.

When we get a car with a suspect cable problem, we disconnect the cable and check it for internal resistance using a digital multimeter. These cables should have no resistance; if we find any, the cable gets replaced. This is a fairly common problem.

You can do the same thing by measuring voltage drop to get to the same place, but you need to be cautious because that method cannot pinpoint which cable is the problem if the harness has more than one cable or a connector in it.

So comparing the results from the Durametric to the voltage reading at the battery + terminal won't offer confirmation ?

As an update, checked the Durametric voltage at DME and it reads anywhere from 12.96 to 13.12 with AC, Radio and front cooling fans and engine at idle. Voltage at battery + terminal under the same conditions varied between 13.10 and 13.56. Engine was fully warmed up.

A system that is in good shape (battery fully charged, cables good) should show a static voltage (engine not running) of around 12.6V at the battery. Engine running on a system in good shape, you should be seeing 13.5-14.5V at the battery. At a minimum, your voltages are a bit low. Check the system again, engine running but everything else turned off; if the Durametric voltage is not in the 13.5-14.5V range either there is an alternator/regulator problem, or one or more of the cables are high in internal resistance.

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Static voltage at battery was 12.59, at idle 13.80

DME voltage, engine compartment fan running but all else turned off, 13.71

Ordered the cable as all appears within the JFP's recommendations . Will retest after install of the cable and post the results as a followup.

Edited by dphatch
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Looking to varify that the variance between the voltage reading at the alternator and the voltage reading at the battery is possible cause of slow cranking when car is hot. From what I have read the normal voltage at the alternator should be 14.7 volts. A drop of .5 volts or more at the battery suggests resistance exists in the charging curcuit between these two points and the most common cause appears to be the alternator/starter/battery harness. I have also read that the issue is not limited to the '05 and '06 cars. Before I replace it I'm looking for confirmation of same. Battery checks out as being sound and, with the exception of hot starts, the car starts from cold or after a sufficient cooling period.If I'm going about this the wrong way would appreciate any guidance you can offer.

Posted results When we get a car with a suspect cable problem, we disconnect the cable and check it for internal resistance using a digital multimeter. These cables should have no resistance; if we find any, the cable gets replaced. This is a fairly common problem. You can do the same thing by measuring voltage drop to get to the same place, but you need to be cautious because that method cannot pinpoint which cable is the problem if the harness has more than one cable or a connector in it.
So comparing the results from the Durametric to the voltage reading at the battery + terminal won't offer confirmation ? As an update, checked the Durametric voltage at DME and it reads anywhere from 12.96 to 13.12 with AC, Radio and front cooling fans and engine at idle. Voltage at battery + terminal under the same conditions varied between 13.10 and 13.56. Engine was fully warmed up.
A system that is in good shape (battery fully charged, cables good) should show a static voltage (engine not running) of around 12.6V at the battery. Engine running on a system in good shape, you should be seeing 13.5-14.5V at the battery. At a minimum, your voltages are a bit low. Check the system again, engine running but everything else turned off; if the Durametric voltage is not in the 13.5-14.5V range either there is an alternator/regulator problem, or one or more of the cables are high in internal resistance.
Posted results, with everything I could turn off turned off, in above post. Many thanks for your help. The voltage readings you listed where most helpful and the first time I have seen them listed together presenting a much clearer undersanding of what I should be looking for in the results. As noted will post retest results once I install he new harness. Elimination of my starting issues further supports the fact that this issue is not limited to the '05-'06 models.
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Looking to varify that the variance between the voltage reading at the alternator and the voltage reading at the battery is possible cause of slow cranking when car is hot. From what I have read the normal voltage at the alternator should be 14.7 volts. A drop of .5 volts or more at the battery suggests resistance exists in the charging curcuit between these two points and the most common cause appears to be the alternator/starter/battery harness. I have also read that the issue is not limited to the '05 and '06 cars. Before I replace it I'm looking for confirmation of same. Battery checks out as being sound and, with the exception of hot starts, the car starts from cold or after a sufficient cooling period.If I'm going about this the wrong way would appreciate any guidance you can offer.

Posted results When we get a car with a suspect cable problem, we disconnect the cable and check it for internal resistance using a digital multimeter. These cables should have no resistance; if we find any, the cable gets replaced. This is a fairly common problem. You can do the same thing by measuring voltage drop to get to the same place, but you need to be cautious because that method cannot pinpoint which cable is the problem if the harness has more than one cable or a connector in it.
So comparing the results from the Durametric to the voltage reading at the battery + terminal won't offer confirmation ? As an update, checked the Durametric voltage at DME and it reads anywhere from 12.96 to 13.12 with AC, Radio and front cooling fans and engine at idle. Voltage at battery + terminal under the same conditions varied between 13.10 and 13.56. Engine was fully warmed up.
A system that is in good shape (battery fully charged, cables good) should show a static voltage (engine not running) of around 12.6V at the battery. Engine running on a system in good shape, you should be seeing 13.5-14.5V at the battery. At a minimum, your voltages are a bit low. Check the system again, engine running but everything else turned off; if the Durametric voltage is not in the 13.5-14.5V range either there is an alternator/regulator problem, or one or more of the cables are high in internal resistance.
Posted results, with everything I could turn off turned off, in above post. Many thanks for your help. The voltage readings you listed where most helpful and the first time I have seen them listed together presenting a much clearer undersanding of what I should belooking for in the results. As noted will post retest results once I install he new harness. Elimination of my starting issues further supports the fact that this issue is not limited to the '05-'06 models.

Hot starting issues associated with internal cable corrosion are unfortunately all too common across several model years; Porsche actually released updated cables with improvements designed to address this exact problem. While the replacement cables are not all that expensive, the confined spaces they run through make replacement a bit of time consuming but otherwise not difficult job.

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Any tricks to accessing the B + terminal on my alternator ? Wanting to test terminal voltage prior to ordering new cable and there is absolutely no room behind the alternator to get to the terminal. Short of removing it, how does one test the voltage output at this terminal ?

My "secret weapon" for the B+ terminal :) A solid core insulated stiff wire hooking from the back of the alternator, with about 1/4" of metal wire exposed.

Just connect it to your multimeter first, then probe the back for a quick reading. You have to hold it by hand otherwise it can't stay connected though but it's good eough for checking v drop of that starter/alternator cable under load. Wear protective glasses and be very careful poking around the rotating pulleys and the always live terminal though.

I got a v drop of 0.4v sometimes between the B+ and the engine +ve jump start point in the engine bay (low beam and a/c ON) so I knew the cable is bad. Removed it and saw lots of corrosion at the starter end. Replaced it with the latest cable and now the v drop is not even 0.1v under the same condition.

post-5282-0-17778200-1374180568_thumb.jp

Edited by Ahsai
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Any tricks to accessing the B + terminal on my alternator ? Wanting to test terminal voltage prior to ordering new cable and there is absolutely no room behind the alternator to get to the terminal. Short of removing it, how does one test the voltage output at this terminal ?

My "secret weapon" for the B+ terminal :) A solid core insulated stiff wire hooking from the back of the alternator, with about 1/4" of metal wire exposed. Just conenct it to your multimeter first, then probe the back for a quick reading. You have to hold it by hand otherwise it can't stay connected though but it's good eough for checking v drop of that starter/alternator cable under load. Wear protective glasses and be very careful pooking around the rotatating pulleys and the always live terminal though. I got a v drop of 0.4v sometimes between the B+ and the engine +ve jump start point in the engine bay (low beam and a/c ON) so I knew the cable is bad. Removed it and saw lots of corrosion at the starter end. Replaced it with the latest cable and now the v drop is not even 0.1v under the same condition.
Clever approach. Is the engine running or are you measuring while energizing the starter ? Where is the +ve jump start point in the engine bay ? Not familiar with that but will google it .
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Any tricks to accessing the B + terminal on my alternator ? Wanting to test terminal voltage prior to ordering new cable and there is absolutely no room behind the alternator to get to the terminal. Short of removing it, how does one test the voltage output at this terminal ?

My "secret weapon" for the B+ terminal :) A solid core insulated stiff wire hooking from the back of the alternator, with about 1/4" of metal wire exposed. Just conenct it to your multimeter first, then probe the back for a quick reading. You have to hold it by hand otherwise it can't stay connected though but it's good eough for checking v drop of that starter/alternator cable under load. Wear protective glasses and be very careful pooking around the rotatating pulleys and the always live terminal though. I got a v drop of 0.4v sometimes between the B+ and the engine +ve jump start point in the engine bay (low beam and a/c ON) so I knew the cable is bad. Removed it and saw lots of corrosion at the starter end. Replaced it with the latest cable and now the v drop is not even 0.1v under the same condition.
Clever approach. Is the engine running or are you measuring while energizing the starter ? Where is the +ve jump start point in the engine bay ? Not familiar with that but will google it .

I measured everything only when engine is running because I don't have a helper to help me crank.

Actually just realized your car does NOT have that jump start terminal in the engine bay. I was thinking about my 996. Sorry about that. Your cable actually goes from B+ to the starter, then to the battery (with a connection point in between at the transmission).

In you case, you will have to measure the v drop between the B+ and the battery +ve post in the frunk then. When you load the car with a/c and low beam, the load is about 50A. If you further add rear defroster and two heated seats, you get about 90A going through that cable. If there's corrosion, v drop will show up (0.4v in my case @50A only) between the B+ and the engine jump terrminal (only 3 feet of cable).

Even when cranking, the current is only about 200A momentarily and then tappered to 100-60A. So 90A load on the alternator is enough to test that cable.

Edited by Ahsai
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As an update, finished installing te new cable and the difference is night and day. Car starts as it should when hot or cold. Hasn't been on the charger for a week and shows no signs of weakness when starting. Re-testing voltage readings. Battery static - 12.81, battery with engine running and full load (AC,RADIO, FAN, both heated seats) 14.7 . Durametric reading with car running and full load 14.6.

Now, one odd test result I received was the Ohms reading I took on the old cable after it was removed. While I can't zero out my DMM, the reading while shorting the DMM cable + to - and the readings from the cable were the same which suggests there is no resistance evident on the cable. All the connections when I removed the old cable where tight and clean with no signs of corrosion. Am stumped on this one as the old cable was deffinately the weak link in the system. Short of both my DMM's being bad (a bit of a stretch) or a weak connection point, how is the reading of no resistance in the cable possible ?

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Now, one odd test result I received was the Ohms reading I took on the old cable after it was removed. While I can't zero out my DMM, the reading while shorting the DMM cable + to - and the readings from the cable were the same which suggests there is no resistance evident on the cable. All the connections when I removed the old cable where tight and clean with no signs of corrosion. Am stumped on this one as the old cable was deffinately the weak link in the system. Short of both my DMM's being bad (a bit of a stretch) or a weak connection point, how is the reading of no resistance in the cable possible ?

Voltage drop testing is the best way to find the problem because it is dynamic. Your testing the actual components with out disturbing any connections that might be the issue. As a factory trained tech I use voltage drop testing as one of my first diagnostic testing strategy's. Today's cars have up to 75 + computer modules on board and most of our problems come form bad grounds and or resistance connecting circuits.The problem with only using an Ohm meter is your not testing the circuit under load so is might ohm out as good but will fail when you put a load on it.

Diagnostic Auto Lab

Edited by binger
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Now, one odd test result I received was the Ohms reading I took on the old cable after it was removed. While I can't zero out my DMM, the reading while shorting the DMM cable + to - and the readings from the cable were the same which suggests there is no resistance evident on the cable. All the connections when I removed the old cable where tight and clean with no signs of corrosion. Am stumped on this one as the old cable was deffinately the weak link in the system. Short of both my DMM's being bad (a bit of a stretch) or a weak connection point, how is the reading of no resistance in the cable possible ?

Voltage drop testing is the best way to find the problem because it is dynamic. Your testing the actual components with out disturbing any connections that might be the issue. As a factory trained tech I use voltage drop testing as one of my first diagnostic testing strategy's. Today's cars have up to 75 + computer modules on board and most of our problems come form bad grounds and or resistance connecting circuits.The problem with only using an Ohm meter is your not testing the circuit under load so is might ohm out as good but will fail when you put a load on it.

Diagnostic Auto Lab

I tested the cable after it was removed so there were no other connections involved. Was going to do the drop test first but the b+ lug on the alternator isn't very accessable and wasn't sure if the " bent coat hanger" approach would give me a valid reading. Your point about the cable not being under load makes sense and eases my mind a bit as there was no doubt that changing the cable corrected the problem. Was just expecting to get further confirmation from the Ohm readings. Thanks for your help.

Edited by dphatch
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As an update, finished installing te new cable and the difference is night and day. Car starts as it should when hot or cold. Hasn't been on the charger for a week and shows no signs of weakness when starting. Re-testing voltage readings. Battery static - 12.81, battery with engine running and full load (AC,RADIO, FAN, both heated seats) 14.7 . Durametric reading with car running and full load 14.6.

Now, one odd test result I received was the Ohms reading I took on the old cable after it was removed. While I can't zero out my DMM, the reading while shorting the DMM cable + to - and the readings from the cable were the same which suggests there is no resistance evident on the cable. All the connections when I removed the old cable where tight and clean with no signs of corrosion. Am stumped on this one as the old cable was deffinately the weak link in the system. Short of both my DMM's being bad (a bit of a stretch) or a weak connection point, how is the reading of no resistance in the cable possible ?

Easy, the resistance rating for that cable when new is a fraction of an ohm per thousand linear feet of cable. So a cable with a problem may only be showing a very small increase in internal resistance over its length of just a couple of feet. To see it, you need to set the meter to its lowest ohm setting (milliohms would be best) at full scale.

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Just to add to Bing's and JFP's comments, take my example of losing 0.5V on that 3 feet cable when the load is 50A. The equivalent resistance is then 0.5V/50A =0.01 Ohm. Even a Fluke 87V has a resolution of only 0.1 Ohm when measring resistance, hence NOT capable of measuring such low resistance.

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I would like to test the cable during cranking but the engine starts quick so I don't get a valid reading.

On the 997 is there an easy was to disable the engine from starting so I can get a few seconds of crank load

on my voltmeter to check for drop.

In olden days I could just remove the rotor.

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I would like to test the cable during cranking but the engine starts quick so I don't get a valid reading.

On the 997 is there an easy was to disable the engine from starting so I can get a few seconds of crank load

on my voltmeter to check for drop.

In olden days I could just remove the rotor.

Sure, either pull the fuel pump fuse or the fuel pump relay, either one will prevent the car from starting.

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I would like to test the cable during cranking but the engine starts quick so I don't get a valid reading.

On the 997 is there an easy was to disable the engine from starting so I can get a few seconds of crank load

on my voltmeter to check for drop.

In olden days I could just remove the rotor.

Sure, either pull the fuel pump fuse or the fuel pump relay, either one will prevent the car from starting.

I agree with JFP, however here is a tip / trick of the trade I use so I dont have to pull the fuse or relay. If you have a good DVOM you can use the Min Max feature. Instead if a step by step writeup please watch the video to explain how this works in detail. Using Min Max is an invaluable tool for all diagnostic testing.

Edited by binger
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