DME Problems ?
 

My 1988 Carrera will not start. it cranks but will not fire. I Have a spark, so I figured that it was a fuel problem. I bypassed the DME ( fuel relay) as mentioned in the forums, and the car started right up.
Ordered a new DME relay. Same symptoms. car will not start. Any thoughts ?

The first thing I would check is your fuses. Make sure they are not only good but the proper values. Many here have found that they were running incorrect fuse values and accidentally toasting components before toasting fuses.

It is possible (although highly unlikely) your replacement DME is a dud, did you get one of the cheap replacement ones or a genuine Porsche one? For the record, you should have at least 2 spares and a spare 3 point wire jumper in your car at ALL times, least you get stranded.


Regards
Dave

dave-- I checked the fuses prior to making the 3-wire jumper. the fuses are good.
I had the same thoughts about a " bad out of the box" relay, and ordered another one. Any other thoughts ? Thanks Brian

The 3-wire jumper basically hot wires the fuel pump. This deals with bad relays, but also masks other problems such as no trigger signals.

The relay switches power to the fuel pump only when the engin is spinning at almost idle speed or above, unless the starter is engaged. This could be your issue as well. I'm not certain of the circuit path that starter signal passes, but I'd also be wondering about a worn ignition switch, or other similar bits.

It could be a DME problem or the alarm or the DME relay. But it's not the sensors. By using a jumper instead of the DME relay you effectively disable the factory alarm.

The DME provides "GND" to turn the 2nd stage of the DME relay on once you crank (yellow wire to starter powered) or there is flywheel motion above 200 RPM. If the portion inside the DME that controls the DME relay is damaged the DME relay's second stage (fuel pump, O2 sensor heater) never gets turned on.

You could try and bench-test the DME relay (need tons of wires) or try and simulate the GND from the DME with the DME relay in the car to see if the relay clicks on. For bench-testing provide +12V to pin 87 and GND to pin 85B and you need to hear a click. Then with power to those pins measure resistance between pin 30 and pin 87B.

To do the test with the relay in the car locate the wire going into the DME relay socket that turns the second stage on. It's a blk/wht wire thin wire. Shove another wire with 1/4 inch of its insulation removed where that wire goes and remove the insulation on the other end of that wire. Then turn the ignition to ON. That will make the DME relay click once (1st stage). If not the relay or your alarm unit is faulty. Then connect the wire with the other end to a bare metal area under the seat. If that makes the relay click and makes the fuel pump come on the DME relay is healthy. If not the DME relay is bad.

If the DME relay checks out healthy your DME needs repair.

Good luck,
Ingo

The jumper masks a bad alarm module but not "missing trigger signals" or a bad ignition switch. If one of the sensors for the DME is bad there won't be any fuel or spark signals ever. And if the starter switch was bad there would either be no cranking or no running with a jumper.

The 2nd stage of the DME relay (fuel pump, O2 heater) is controlled by a logic OR inside the DME. Once T50 (start) is powered (cranking) OR once flywheel motion above ~200 RPM is detected the DME turns the 2nd stage of the DME relay on.

Id say if you can get ahold of a known good relay and try it (or try your relay in another car) you can figure out if its a trigger problem or not. If you post/add your location a local member may be able to lend a hand.

Regards
Dave

Sorry, not so for the 3 wire jumper that I carry, and that has been posted in several previous threads. That jumper simply jumps power to the triggers for both ingnition and fuel. It is intended for emergency use to get you home only. When connected, the fuel pump stats running constantly as long as the key is in the run position. It does not accept signals from the DME to energize.

Arne,

Not sure what you refer to with "trigger signals". Fact is the 3-wire jumper is equivalent to a DME relay with both stages turned on. And yes, that makes the fuel pump run when the ignition is in ON position as it is powered by the 2nd stage. However, it still won't bypass or mask a bad ignition switch or bad flywheel sensors and make the engine run if either has an issue. The OP says his engine runs with the jumper. That leaves the DME relay, the DME and the alarm module.

The sensor signals from the flywheel are one of the two conditions required to make the DME turn on the DME relay but they do all the other stuff in the DME, too. So yes in a pinch this jumper can replace a bad DME relay and can get you home.

Ingo

When I use the 3-wire jumper to bypass the DME , the fuel pumps runs even without a key in the ingnition.

OK, that's fine. But you still need to sort out whether your alarm module or the DME or the DME relay is the root cause of your issue.

My point was that with the jumper you can simulate a working DME relay but you will also mask issues with the alarm module or the DME (part that controls the relay).

That's because the jumper uses the #30 power which is directly from the battery.
So both the DME ECM and the FP are powered without the ignition switch, the
DME relay, and the alarm ECU, i.e. bypassing the #15 power from the ignition switch
thru the alarm ECU.

With the DME relay installed:

1. Use a paperclip from pins #30 to 87. This will power the DME ECM and bypass the ignition
switch and the alarm ECU only. If the engine runs, you've localized the problem.
2. Next use a grounded testlight to pin 85b of the DME relay. If the engine runs, the DME ECM
is not providing a ground from pin 20 to run the FP. Remove the paperclip in the #1 to confirm
where the problem is.

Ingo,
I tested the relay with 12V to pin 87, and grounded pin 85. No click. I tapped the relay a couple of times and retested. the relay then clicked on. Put it back in the car with no start.
Tried placing a wire at Black/white wire connection. no click when the ignition was turned on. grounded the wire without any change. what do you think ? Brian

Dave,
I connected pins# 30 and 87. The car started right up. So, you are saying that this isolates the problem to the ignition switch or the alarm module ? what is the next step ? thanks in advance.
Brian

I had a DME relay get funky after 200 miles. Replaced and car run fine.

Ordered a new "glovebox" unit and noticed was made in Germany , not Hungary as previous units.

Also realized that there are probably fewer than 2000 of these units sold in a year and we may be working off a 10k Hungarian batch. Not a manufacturing expert but I think it's difficult to stabilize and perfect any automated process including wave soldering in a new "low cost" plant, especially in low volumes.

Hopefully, The Germans will do a better job

Your results are inconsistent: The 2nd stage relay will have to click when you power pin 85B and 87. So either above is a typo where you really were on 85B and 87. In that case I would be concerned that tapping is required. Or if you really powered pin 85 and 87 there would never be a click.

As to your second question: The fact that connecting pin 30 (permanent +12V) to pin 87 (working contact of 1st relay stage) made the car run clears your DME and points fingers squarely at the alarm module.

Do this: Turn the ignition switch to ON and then check if your ICV on top of the engine vibrates? If not your first stage of the DME relay doesn't turn on. If your oil pressure light and all the rest comes on the ignition switch is healthy, too. The way a 1988 is wired is like this:

When you turn the ignition switch to ON the output of that switch (T15) goes to items like the coil, the injectors and other consumers that are only life with T15 powered. The ignition switch output also goes to the alarm module input. Once the alarm is satisfied it routes power to turn on the DME relay's 1st stage. You actually can hear that click when turning the ignition switch. If the first stage is on the ICV needs to vibrate indicating the DME is powered up and has a heart beat.

In your case this most likely doesn't happen. If you then connect pin 30 and pin 87 the ICV will vibrate. With that check all your door switches, the key switch and the trunk switches. Either one of those is bad or your alarm module is bad.

Ingo

Ingo,
You are correct. I made had a typo in the prior message. I connected 12V to pin 87 and a ground to pin 85b. no click. after a couple of taps the relay clicked.

As you suggested , I turned on the ignition switch. no vibration at the ICV. Oil lights, etc, came on.
there is no click at the DME relay when I turn the ignition switch on.

I will check the various alarm related switches tomorrow.
Thanks again, I really appreciate the help. Brian

So with the DME relay installed, you ONLY jumpered 30 to 87 on the back of the relay and did not
ground 85b, right? If so, then ignition switch thru the alarm ECU is not powering the pin 86 of the
relay. If so, then your DME ECU is O.K.

One last test: Use the paperclip to jumper the 30 pin to the 86 pin on the backside of the relay
when it's installed. This will confirm the missing power from the ignition switch thru the alarm ECU
and that you have a good relay.

Note: The alarm ECU is typically damaged if the battery is reversed jumped.

Ingo,
I went back and retested the relay . 12V to pin 87. Ground to 85 B . the relay clicks inconsistently. I then replaced the relay into the car. put a wire at Black/white wire connection which is 85B . turned on the ignition. no click. grounded the wire= nothing.
I'm thinking that maybe the relay is damaged.
I have no " key on power" to pin 86. I jumped pin 30 to 86 without any results.
maybe the alarm system is a problem as well.

Dave,
Yes if I connect Pin 30 to Pin 87 ( no ground to 85b) the car runs.
I connected pin 30 to Pin 86 without any results. Brian