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Alternator terminal B+ access


Go to solution Solved by dphatch,

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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.

That's fine if he happens to have a $400 digital multimeter; most backyard DIY'ers have the $10 Harbor Freight version that lack this function, so they end up having to pull the fuse or relay..........

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I ran the voltage drop test (cold) on my 997 alternator B+-to-battery-POS and got dynamic readings

ranging from ~150 millivolts to ~80 millivolts. I noticed the millivolts started dropping as the car ran

for a minute or two.

Can I assume there is virtually no drop?

I am going to test again after driving on a hot day.

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I ran the voltage drop test (cold) on my 997 alternator B+-to-battery-POS and got dynamic readingsranging from ~150 millivolts to ~80 millivolts. I noticed the millivolts started dropping as the car ranfor a minute or two.Can I assume there is virtually no drop?I am going to test again after driving on a hot day.

The higher v drop at cold start is normal due to the high current drawn by the secondary injection pump and the current charging the battery after the starter current draw.Not sure what other loads you had at the time. 150mV drop is excellent if you had the a/c and low beam ON (~54A total load).

Just for reference. On my 996 with the new cable, my B+-to-battery-POS v drop is 180mV @ 54A on a hot engine.

Edited by Ahsai
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So I did a couple tests when the engine was hot, at idle, hi beams were on and A/C full blast.

The drop test showed about 250mV so I figure that's a good sign as far as the cable.

However the voltage from the alt to the battery was 13.4-13.5. Is this acceptable in a 997.

I have read other's comment saying the voltage should always be between 13.8 and anywhere

over 14.0. Is the alternator on the weak side or is this considered normal given the load and low rpms?

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So I did a couple tests when the engine was hot, at idle, hi beams were on and A/C full blast.

The drop test showed about 250mV so I figure that's a good sign as far as the cable.

However the voltage from the alt to the battery was 13.4-13.5. Is this acceptable in a 997.

I have read other's comment saying the voltage should always be between 13.8 and anywhere

over 14.0. Is the alternator on the weak side or is this considered normal given the load and low rpms?

Your voltage is a bit low, it should be between 13.5 and 14.5V with the engine running. Could just be the voltage regulator, but I would get the alternator checked out.

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Now feel free to shoot me down but I suggest that if the battery is somewhat depleted with all these starts and load on it, surely the volts will be 'low' and the current high until the alternator has restored it to its fully charged state again? 'low' being 13.5 ish.

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Now feel free to shoot me down but I suggest that if the battery is somewhat depleted with all these starts and load on it, surely the volts will be 'low' and the current high until the alternator has restored it to its fully charged state again? 'low' being 13.5 ish.

You should be starting the diagnostic process by load testing the battery, if it passes a load test, only then should you be looking at load testing the alternator. You always need to first eliminate the battery from being part of the problem.

That said, even a battery with multiple starts on it will not pull the voltage output down on a running alternator; the alternator should be reading 13.5 - 14.5V at idle.

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