Ed Hughes

To have a sensor go out and all of a sudden, a no-start issue does seem to be a bit of a low percentage problem. Typically, the engine runs like crap, but it runs, I believe.

Also, as an FYI, I replaced my sensors when I built my engine and went with a couple of 944 sensors that were literally 1/2 the price. Kirk @ Zim's and I both looked at my old sensor and one of the new 944units for 10 minutes, and agreed there was no physical difference. It came down to whether they may not work, but they did.

Droops83

OK, I hate seeing people waste money on buying parts and installing them in the hope that the problem will be fixed. The test plan in the factory manual is the way to go, but if you can't access that I think the Bentley could at least point you in the right direction.

So it sounds like you have a crank but no start problem. Seems like the DME relay wasn't the problem since you replaced it with no success, but that can be easily checked by jumpering terminals 30 to 87 on the connector. I wasn't able to tell from the thread if you were able to determine if there was power to the fuel pump and if it was working, but you said that you replaced it to no avail.

I also see that you replaced the coil to no avail. No spark from the coil doesn't necessarily mean the coil is bad. You need to check to see if the coil has power at the positive terminal, and then see if it is being switched on and off by the DME. To do so, use a test light on the negative terminal while someone else cranks it. The light should flash on and off. If there is no light, or it stays on (which is usually the case), you know the problem is likely in the ignition driver portion of the DME or the reference mark/engine speed sensors. You already tried a known good DME, so if everything else is good I'd look at the ref/speed sensors next.

The often prescribed resistance checks of sensors when they are unplugged are almost totally worthless (except maybe testing a temp sensor in hot water and at different temps); sensors fail under load, when they are being used. You really need an oscilliscope to check the speed/ref mark sensors and check the voltage waveform to know if they are giving and adequate signal to the DME. You can check the output with a DMM and some numbers will flash if the sensor is working somewhat, but this is inaccurate and will not tell you much.

Sounds like you have the sensors coming in the mail, but double check the power to the coil like I mentioned above in the meantime. Let us know if you have any more questions

cal44

Well, I installed two new sensors........nothing, zip, nada. Still no spark.
The big question now is........The other brain I tested with came from an '86 911 and the car that will not start is an '84 911. I wonder, is there is a difference in the mapping of the circuts?
I am getting alot of feedback over on Pelican tech but I thought I would keep you guys in the loop as well as this helps all of us.

And the hunt continues. I sure will fell foolish if it is something simple.
I have just sent the DME/brain to Otto's Porsche in Venice, Ca.

Ed Hughes

That sucks. FWIW, I've got a set of shop manuals for Carreras-there are 18 pages of troubleshooting on the engine ignition system-you're welcome to borrow them. You probably need to start from scratch and check each point methodically with your new meter. Some tests require oscilloscopes, but my guess is you could go thru most of these with your Fluke.

Lorenfb

"If you can hear the dme relay "click" when you turn on the ignition, then it's ok."

Hardly! Let's not mislead this poster by such naive info.
Just because a relay "clicks" does not indicate that it can provide power:

1. It may have bad contacts.
2. It's contacts may not make good connections, e.g. a circuit board.

One of the easiest initial tests when troubleshooting is to always check for a spark.

"To have a sensor go out and all of a sudden, a no-start issue does seem to be a bit of a low percentage problem."

Ed Hughes

Actually, that's incorrect. The majority of sensor failures occur suddenly with
no warnings, e.g. poor running!

Bottom line: Let's avoid misleading Rennlist members as usually occurs over there on the "dark side".

theiceman

Hey Loren glad to hear your back, haven't seen you around for a while. i think your expertise is needed on quite a few threads here.

Peter Zimmermann

cal: Have you removed the fuel gauge sending unit from the gas tank to see if the tank is empty? I remember a couple of occasions when the sender failed and a car ran out of gas...

theiceman

I needed an idler roller for my audi and got it from the dealer , it had both an Audi and a VW symbol stamped on it. I inquired and the guy says if we look it up in the audi parts list it costs more. on the VW price list it costs less...

go figure..

rusnak

so now that Porsche owns over 50% of VW, we can buy our parts from VW..woohoo!

Cal, I missed the part where you diagnosed the "no spark" condition. How did you test that, and did you see if you are getting power to the fuel pump by jumpering the dme relay socket? If a coil were going bad I believe you'd have a weak spark, and I think it would run worse as the engine warmed up. But you'd still have a spark. Sears has an inexpensive spark tester by Lisle tool. You just touch it to each spark plug wire, no need to disconnect the spark plug wire.

And by all means, we are all trying to help Cal out. This thread should not be a magnet for condescending and arrogant quips unless your last name is Bosch, and your first name is Robert.

theiceman

.. I have a 78 .. I always could

Amber Gramps

Our brake light switch is $24.00 at the P-dealer and $6.99 at Save-A-Bug. Even the dealer told me to go across the street.

theiceman

parts pricing tips allmost deserves a thread of its own

rusnak

I was stranded in downtown Oakland once and bought a distributor rotor for about $8 bucks at Napa Auto. You'd be surprised at what they carry. Sorry Cal! I forgot Sears is on your blacklist. I believe Pep Boys and Napa also carry Lisle tools.

About 9 months ago I had a no-start situation in my 3.2 911. I replaced the dme relay, checked power, etc. Everything seemed ok. Still no start. No spark and no fuel unless I jumpered the fp. I took the dme brain out, tried to look for solder joint cracks, did a fuel pressure test, etc. I mean everything. I was about to order a new motronic brain, when after 3 days of desperation I put my old one back in, and the car fired right up. I have no idea why, since I did not replace anything, just cleaned up ALL of the contacts EVERYWHERE. Probably the act of connecting and disconnecting all of the leads, cleaning contacts to do resistance checks, etc may have cleaned off corrosion that I could not see. Also, the dme sat on my desk for about two days. Maybe it helped to disconnect and re-connect the dme, I don't know. I must have done every test outlined in Bentley at least 2-3 times. I read and re-read the Bosch Engine Managment handbook. After that I bought a new dme brain and replaced all of my fuel injectors, new fuel pump, new intake boot, new intake gaskets, and new mongo Delco battery just 'cause the no-start thing totally sucked.

If nothing else, I feel your pain.

cal44

Thanks Lads. Otto's will send my DME back and all will be fine.........right? I have read the injectors can short out and cause a no spark. This is something. At least I will be better versed on my car. I do read all feedback because I want to learn. Now, if anyone wants to learn to weld on live natural gas or tap out a pressure control fitting under pressure, I'm your man. Fuel Injection.....not so much. Thanks all. Until follow up post.

Ed Hughes

I'm probably stating the obvious here, but all Rennlisters should be warned that there is a Sewer Troll alert in effect here on Rennlist.....you are advised to thoroughly disinfect your computers and monitors as "its" electron trail is known to be highly toxic.