GRS from Kzoo

McKinlay, I'm glad to see, even after all these years, that the all the posting saved somebody a lot of headaches (as well as backaches crawling under the instrument panel). I actually still think about the silly way Porsche engineered the cruise control circuit almost every time I'm out driving my car.

I had pretty well forgotten about that! I think there's a lesson for all of you out there that you'll want to think twice about doing any business with the firm named in my post of August 10, 2007. Their claim that my unit checked out bad is very suspicious, especially since they now seem to have one listed for sale at $289 + $15 shipping. Looks like they have changed their name but the link in that earlier post will take you to their site. They did not charge me for their "analysis" but by telling me the eprom was fried they got me to let them keep it to avoid paying $100 for their effort. Very suspicious!

I can finally report success and unequivocally state that a dead light bulb in the instrument panel will render the cruise control inoperative on a 1998 Boxster! After 5 years, many hours and hundreds of dollars it is all working again. I am thrilled but feel like a fool since I have actually had a spare light bulb in my ash tray the entire time. Thanks once again to everybody here.

Jon, One more thing... Do you happen to have the part number for the bulb? I see three different bulb numbers for the cluster! GRS

I'm anxious to get to this as soon as I have the use of two hands again. I'm presuming you are referring to the instrument cluster removal at http://boxsters.addr.com/hacks/cluster.htm and that the mesh thing is the grill to the left of the speedometer. Can you describe the baggie method? I've had the cluster off before to replace lights in the IP but never needed to use a baggie for anything. GRS

No I haven't but you've giving me sufficient motivation to replace the bulb as soon as I can use my hand again. (I had surgery on my right hand to repair my thumb after an injury and this will take a couple months to heal). Thanks 00BoxsterS!

Well, I received a used cruise control module from a recycler in Wisconsin, installed it, and the cruise on my car still doesn't work. Same symptoms as before, no green light, no functions (and I put about 800 miles on the car this weekend for a thorough test). Before removing it, I again checked out the pins on the harness connector and everything is still as it was before (per my post on July 14), even to the point where Pin C has less than 12 volts (although this time it was 11.63 and the brake switch had been replaced with a new one). The module was guaranteed so I will pack it up and get a refund, but this does have me wondering what I may have missed. Power, ground and all switch functions (both for the stalk and pedals) all check out OK. I have a few other used modules lined up to try but this has me quite baffled and thinking I must have missed something. Any other suggestions out there? Thanks, GRS

Well, after testing the part, Auto and Truck Electronics tells me that the unit is not repairable as the eprom is shot and they cannot replace that (probably because they do not have the software to put in it). Looks like I need to find a new unit. Leads me to wonder what would have caused the eprom to go. The only thing I can figure is an electrical glitch in the power supply to it. I did need to replace the ignition switch as I had erratic operability of things like the HVAC fan. Since I was in the habit of leaving the cruise stalk "on" switch in the on position, do you think the ignition switch issue could have led to the failure of the cruise control ECU? Once it's fixed I'll be sure to keep it off when not in use. Thanks to all on this board for your help. I've learned a lot and at least I feel certain that I've got to the root cause of the problem. GRS

Just an update... After resoldering a few questionable solder joints and still not getting any voltage out of pin "K", I've shipped off the ECU to Auto & Truck Electronics in West Palm Beach, Florida for rebuild (www.autoecu.com). They rebuild all sorts of automotive controllers and gave me a price of $229 to do the job, which includes a 1 year warranty. Best I was able to find for used ECUs is $250-300. I'll let you know how it works out. GRS

OK, I finally have more to report! I replaced the brake switch a couple of weeks ago, and it made no difference (but then again I don't think any of us expected it to). BTW, the four terminal switch appears to have replaced the three terminal version which was originally in my car and is what I installed. Today I was finally able to locate a control unit from a dismantler ($200), which motivated me to get the old one out and look at it. Not so easy to get out, but if it helps somebody in the future, here's the steps I took: 1) Remove the duct that runs transversely from the HVAC unit to the left side of the footwell. 2) Remove the visible screw in the remaining duct to the HVAC and pull it aside to observe the controller. 3) There are three 10 mm bolts holding the control unit to key-slotted holes in the brake pedal bracket. Loosen the top two 1/2 turn or less and leave them in place. (These are the harder two to access). Remove the third bolt entirely. I found that I could not remove the unit with this bolt in place as it made it too difficult to get sufficient clearance to pull out the controller. 4) Pull the entire controller (still attached to the throttle pedal) down. 5) The electronic control unit is held to the controller by four phillips head screws at its corners. Of course, the part number on my unit (996-617-021-04) is not the same as the one the dismantler has (996-618-051-02). First question: Does anyone out there know if the dismantler's part will work on my '98? In looking my unit over closely I did find one transistor which appears to have a weak solder joint at its connection to the top of the board, although the corresponding joint at the bottom of the board looks fine. I'm no EE, but it seems like that may be an issue, especially since there do not appear to be any traces to that lead on the bottom. Hopefully, I can get someone who knows to look at it. Any thoughts? Thanks again for all of the help so far. GRS

OK, checking out the voltages between pin E and the others, here's what I get: Pin H – Supply – should have 12v+ when the ignition is switched on. YES! Pin D – Set/Acc – should have 12v+ when the ignition is switched on, the cruise switch is depressed on the end of the stalk, and the lever is pushed to set/acc. YES! Pin B – Off - should have 12v+ when the ignition is switched on, the cruise switch is depressed on the end of the stalk, and the lever is pulled to off. I have just the opposite, pin B gives 12v with ignition on but goes to 0 when I pull the lever to off. Pin G - should have 12v+ when the ignition is switched on, the cruise switch is depressed on the end of the stalk, and the lever is lowered to resume. YES! Pin F – Clutch Switch - should have 12v+ when the ignition is switched on and the clutch is depressed. Again, I have the opposite, it is 12v with the ignition on and the clutch up and 0v when the clutch is depressed. Pin A – Speed Signal – I think this is a pulse which will vary with speed, and would be difficult to test. If your other speed-related devices (spoiler etc) are working, I would assume this to be OK. I did not check anything on this. Pin C – Stop Light Switch - should have 12v+ when the brake pedal is pressed. Very interesting, here I only get 11.58v, which is .6 less than the others. I'm pretty certain this is due to the brake switch situation where the normally closed terminals work reliably but the normally open terminals act a bit flaky. Pin J – Stop Light Switch - should have 12v+ when the brake pedal is up. YES! Pin K – Signal out to the Diagnostic Socket & Cluster Light – the control module should give a 12v+ signal out when the ignition is switched on and the cruise switch is depressed on the end of the stalk. It should also have 12v+ for a few seconds when the ignition is switched on with the cruise switch off. The connector would have to be plugged in to test this. I get 0 volts here, but did not try momentary on with ignition. I do not get the light to go on with a normal start bulb check. I also checked terminal 8 of the dignostic plug and do not get power on that pin either. So I think I have confirmed that the stalk switches all work. The reversal of function on pins B and F is interesting. Richard, can you confirm how they work on yours? I checked mine many times and figure if the logic is reversed it either means that the logic changed in the controller with a model year or my controller has problems. Does anybody have a lead on replacement controllers? I still plan to replace the brake switch but did not find one this week due to a little confusion over part numbers. According to the dealers, the switch listed for my '98 is supposed to be PN 996.613.114.01 but it apparently has 4 terminals, not 3 like the one in mine. I looked at my switch today and confirmed it is a 996.613.110.01, which appears on the list as the '97 model part. The connector for mine looks like it would work with the 4 pin version since it has a space without a terminal in the 4th position. Has anybody out there replaced a 3 terminal switch with a 4? As always, I look forward to your analysis. Thanks, GRS

There is absolutely no cruise function insofar as setting or holding speed. I hoped that the non-functional light might have been a helpful clue for diagnosing the problem. If the switch on the end of the stalk were defective, I don't think the light would illuminate since it used to go on when the switch was depressed. From the circuit, it does appear that there could be other switch issues that would not have showed up by measuring continuity directly at the stalk switch as pins D and B both depend on the switch for power. I still haven't figured out where the cruise lamp gets its power from but it may get illuminated through the diagnostic pin K. With Richard's detailed Input/output list below, I should be able to verify whether any other of the switch functions beside on/off are at fault. If it checks out completely, the only thing left will be the controller. Thanks again for the insights.

Thank you, Richard! This looks to be exactly what I need to thoroughly check it out. It may need to wait until next weekend before I can dig back into it, but I'll let you know how it goes. GRS

Maybe this link will view directly on the page: Cruise Wiring Harness Connectivity

OK, I've spent quite a few hours today digging into this (and may be ready to join the circus as a contortionist). No luck thus far getting the cruise to work, but here is what I have found: 1) Clutch switch appears to work fine when checked with a VOM when the connector is off. 2) Although my brake lights work fine (albeit only when the ignition is on) the brake switch appeared to act erratically with a VOM directly on the switch. I'll order a new one on Monday, but figured I would take it apart and see if I could at least determine what was wrong. The contacts were somewhat burned so I disassembled the switch elements, filed them clean and was actually able to get it all back together and seemingly functional with a VOM check. It is a three terminal switch (I think that's a SPST) with a copper terminal between two plated ones. The plated terminals are normally closed and the copper terminal is normally open, at least once I got it all together. I was very hopeful that I resolved the problem (at least for the time) but in a subsequent test drive I still did not get a green cruise control light or any cruise control function. 3) I popped off the cover from the cruise stalk and checked the switch operation directly at its terminals (with the control unit unplugged). The switch works fine! Then, using the diagram and the ohm meter on the control unit terminals, here is what I found for the wiring harness (perhaps this will be a useful reference someday): CruiseWiring (Note: I've did this as an Excel spreadsheet and then made an image of it to upload. Hopefully it will view properly, I'll check to see if it does.) Looking at the diagram and the chart together, much of this seems to make sense. Notice that a lot of the readings to the speed circuit terminal (A) get a resistance reading instead of open or closed. It also shows that when the cruise stalk main switch is closed, this closes the circuit to terminals C (stop light), E (circuit G162 which I'm guessing is the green dash light), F (clutch switch), H (power supply to controller) and J (another stop light switch). It seems reasonable that the main switch would direct power to those functions. As I said, I will get a new brake switch but since the terminal cleaning had no effect whatsoever I'm not very certain that this will take care of it and am still wondering if the control unit is the problem. Does anyone have any ideas about how to go about checking it (short of buying a new one)? The only thing I haven't done is pulled the dash apart to check the cruise indicator bulb but I can't imagine the failure of the bulb would cause the system not to function. Is that a reasonable assumption? The saga continues, comments are welcome (and encouraged). GRS