Hey all,

Newbie here (relatively speaking), though I've spent quite a bit of time going over the threads on various issues. Never quite seen this, so I thought I'd ask the experienced crowd...

'87 951, random no start a bit ago. Background information that may or may not be helpful, car has an AFM Link control unit and the no start came after the battery had drained (old battery, sat for a bit) and I attempted (to no avail) to jump it. With no luck there, I pulled the battery, charged it, put it back in, and the car cranks all day but won't fire. I've run through a litany of the no start diagnostics from Clark's, as well as some helpful hints from the boards. Not a DME relay (swapped for a known good spare). Fuel pump runs properly (though I don't have a gauge to check pressure at the rail), can hear the injectors firing and I can smell fuel coming out of the exhaust when cranking, so I'm working on the assumption that fuel is being injected to the chambers - pressure could still be wrong I suppose. Speed/ref sensors appear ok via tach bounce when cranking and by checking resistance across the terminals at the DME harness. Wiggling connections changed nothing.

The issue seems to be no spark, as I'm getting nothing with a spark tester either at the plug wires or direct from the ignition coil output wire. Working on the assumption that the ignition coil is good after I swapped for a new one I had on hand. The interesting bit came when I followed one of the testing procedures on Clark's where you connect a spark plug to the output wire from the ignition coil, ground it via the threads (I put it to the ground behind the right strut tower, just cleaned, so the ground should have been good), disconnect the negative lead from the coil, run a new ground wire to the negative lead on the coil, and tap the free end of that wire to a ground with the ignition on (not cranking) to simulate a firing signal. I used the battery negative terminal, as suggested by Clark's. Instead of getting a spark at the plug, I'm arcing where I tap the ground from the negative lead on the ignition coil. My question is this - does this sound like a bad output wire from the ignition coil? I don't have any surefire way of testing this other than a known good spare, which I don't have, but it would seem to fit the symptoms of both the no start and this odd test result. Also of note, both leads to the ignition coil are getting a proper 12v read.

I'm hoping it turns out to be this simple, because after this, I think I'm into the rabbit hole of the DME and the (gulp) Link system... Thanks in advance!

 

Hi Rooster,

Just a short reply...I think you already know.

You might get a NOID light to test the signal and make sure the AFM is securly connected and the "door" swings freely.

You implied the car ran fine until the battery drained.



GL & welcome,
John

 

Thanks for the reply John, much appreciated. As a new-ish owner, wish I'd seen that post from Ian a little sooner!

Any thoughts on ways it may be possible to check the DME? I pulled it the other day and inspected it visually, no signs of water or burn damage to the board, but clearly the circuits can fry with no visible damage. Don't know if there's a way to check pertinent circuits with a multimeter, etc. I'm reasonably mechanically inclined, but I'm definitely learning electronics on the fly, so I'm not sure I could figure that one out myself.

Also, thanks for the thought on the AFM. The stock airbox and AFM were deleted by the PO when the AFM Link unit went in. Could be something in that unit is fried/reset, and thus sending the wrong voltage...

Guess I need to find someone in the area with a known good DME.

 

This will explain a few tests:

http://www.944time.com/porsche/dme/index.php

GL
John

 

Thanks John, I'll check these out. Again, I appreciate the help!

 

So I swapped out the DME, car starts and runs!

Of course, there's a new problem - the idle surges and then stalls. Sometimes it does this immediately upon starting, but sometimes it will idle fine indefinitely and drive for a little bit. However, if it will drive, it will usually stall immediately after accelerating as it should be dropping back to idle. When it is idling fine, it runs smoothly, no apparent issues. After it stalls, it will restart, but with difficulty.

Sound like an ISV this time? Thanks in advance!

 

Interesting - might have caused a problem on my S2 this way (jumping it) that progressively got worse until I had to replace the DME.

Is it ok to have a trickle charger or to use a regular battery charger on it without disconnecting the positive cable (but not trying to start it)? I keep having battery drain issues....

ThanKs!!