DME Troubleshooting

[salayc]

My car is running fine with the DME relay bypassed but not once the DME is installed. I bought a solid state DME relay, but no luck. Can anyone tell me the pinout for the DME relay connector?
I currently get 12v constant (unswitched) at 30 and a brief 12v on 86 when I turn the ignition to ON.

[FrankM_]

30 is 12V power
86 is 12V for coil1
85 is GND
87 is output 12v when coil1 is energized
87 internally also provides power for coil2
85b is switched to ground by the ecu (in my case, a 3.2) when the ecu wants the fuel pump on
87b is the 12v output for the fuel pump

Those are the six terminals on the dme relay

If you only get a short 12v on 86 and the nothing, your problem seems to be upstream ?
See here for a good diagram (page 6 for the dme relay): http://911c1.de/porsche/Schaltplaene/plaene1-10_carrera_32.pdf

Do you have an alarm installed maybe ?


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[salayc]

Thanks.

What's upstream from 86? I jumped 86 to a switched source and fuel pump goes but no start.

[FrankM_]

What year 911 ?

As said in my post, if you have an alarm, that is upstream of 86 for example and of course your key contact switch.


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[ischmitz]

As others said: power for T86 comes from the ignition switch and might have an alarm relay in its path. That's how Porsche's factory alarm was implemented and how many aftermarket alarms are done as well. With the alarm active it interrupts power from ignition switch to T86.

If you supply +12V to T86 with the DME relay installed the fuel pump should have not started to run. Power to T86 just turns on the first stage of the DME relay and thereby powers the DME. It is equivalent with turning the ignition key to RUN. You can actually listen to the DME relay click when you turn the ignition to RUN.

Once you turn the ignition key to START for cranking it signals the DME to provide GND to T85b. The DME will do so when it either sees the key in START or when it detects flywheel motion above maybe 100 RPM or both. If that DME output is defect that would explain your issue. In that case you can make the engine run by removing the DME relay jumpering T30 (Perm +12V) to T87 and T87b. This is the classic "jumper instead of a DME relay".

Ingo

[salayc]

Originally Posted by FrankM_

What year 911 ?

As said in my post, if you have an alarm, that is upstream of 86 for example and of course your key contact switch.


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It's an '87. It had factory alarm removed by previous owner, and an aftermarket stereo which I removed.

Originally Posted by ischmitz

As others said: power for T86 comes from the ignition switch and might have an alarm relay in its path. That's how Porsche's factory alarm was implemented and how many aftermarket alarms are done as well. With the alarm active it interrupts power from ignition switch to T86.

If you supply +12V to T86 with the DME relay installed the fuel pump should have not started to run. Power to T86 just turns on the first stage of the DME relay and thereby powers the DME. It is equivalent with turning the ignition key to RUN. You can actually listen to the DME relay click when you turn the ignition to RUN.

Once you turn the ignition key to START for cranking it signals the DME to provide GND to T85b. The DME will do so when it either sees the key in START or when it detects flywheel motion above maybe 100 RPM or both. If that DME output is defect that would explain your issue. In that case you can make the engine run by removing the DME relay jumpering T30 (Perm +12V) to T87 and T87b. This is the classic "jumper instead of a DME relay".

Ingo

Ingo, the pump runs with power to 86 because I installed a new solid state relay with a prime function (I know, not very clear in my post, I should have said the pump primes.) It wouldn't work with the mechanical relay either, same symptoms.

What I'm not understanding is why pin 86 only gets momentary power. I guess that means there's a relay in the circuit that's interrupting, but I have no idea where. By jumping switched power directly to 86, I assumed it would start.

I have made a jumper and the jumper works, the car starts and runs with the jumper installed.

[FrankM_]

An 87 so the link to the schematics of the 88 in my previous post will help you.
If I trace it back using the diagram, check for the alarm unit. In the wiring diagram there is also a note on how non-alarm cars are using 2 bridges to make the connections work (maybe the bridges are not seated properly?).
Another thing to check is your terminal 15 ‘output’ from your key contact - it should provide the stable 12v.


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[salayc]

Alarm bypassed using a method found on this forum (https://forums.pelicanparts.com/porsche-911-technical-forum/1039422-clarify-alarm-module-bypass-sc.html) but still no start. I'm tracing as much as I can and removing / repairing modified wiring as I go.
So on to the next step: where is the DME grounded? I see on the eoectrical diagram that pins 5 and 10-16-17-19 lead directly to ground. Is that one of the ground connections near the fuel filter? Are there multiple ground locations?

[FrankM_]

Referring to the electrical diagram link I shared above : both the relay for the ECU ('dme relay') and the ecu itself are grounded on 'MassenPunkt XII' (MP XII) and on the small car diagram on each sheet you can see that it is on the intake runner of cyl 1 ('Im Motorraum' is their generic description)

Also the correct way to bridge the alarm is stated on 'Blatt 5' (page 5) : Alarmanlage M533 option
kl 61 (pin 61) to Kl 61 , so BL (Blue in German!) to blue-red wire
kl 87a with pin 15, so black wire to black-red wire

[ischmitz]

Ground is provided in the engine bay. If that was your issue the car would not start at all.

Have you tried to verify if the DME relay will work and turn on the fuel pump when you turn it on as follow:

- remove the 35-pin connector from the DME
- turn ignition to RUN
- use a wire and connect pin 20 inside the 35-pin DME connector to GND. For example pin 16 or 17.

With that the fuel pump needs to come on. If that works AND the car runs with a jumper instead of DME relay you have more or less isolated your issue to the DME.

Ingo

[salayc]

Originally Posted by ischmitz

Ground is provided in the engine bay. If that was your issue the car would not start at all.

Have you tried to verify if the DME relay will work and turn on the fuel pump when you turn it on as follow:

- remove the 35-pin connector from the DME
- turn ignition to RUN
- use a wire and connect pin 20 inside the 35-pin DME connector to GND. For example pin 16 or 17.

With that the fuel pump needs to come on. If that works AND the car runs with a jumper instead of DME relay you have more or less isolated your issue to the DME.

Ingo

I'm on my second DME relay, the one I am using now is a solid state, so I don't think that's the problem. Just to make sure I ordered a mechanical DME so I can open it and see what it's doing.

Yes, the car runs with a jumper, but the initial problem of pin 86 only getting momentary power means there was likely an alarm problem (alarm not sending signal through 15) Has to be, right?

So now with the alarm bypass there's still no start with power confirmed to 86. I opened up a mechanical relay so I could see what it's doing, and the second contact is not connecting. It moves slightly then disconnects. I can start the car with the mechanical DME if I manually connect the second contact. The ground at 85b makes that circuit activate, so it seems that it has to be a ground, although when I manually ground the pin, it makes no difference.

I'm going to verify (again) that the DME Relay is getting ground. It's possible that I have a bad solid state DME relay I guess.
Also possible that there's something wrong with the DME, but I don't really have a way to test that.

And thank you gentlemen for the advice.

[ischmitz]

You’re describing what sounds like a DME (not DME relay) issue. The DME acts as low-side switch and provides GND to the relay’s second stage via pin 20. In your case that appears broken.

Try my test in the previous post - don’t waste more money on DME relays.

Ingo

[FrankM_]

As Ingo said earlier, there are a few conditions that need to be met before the DME actually switches pin 20 to LOW (near ground) and thereby energises coil2 of the DME relay (pin 20 goes to 85b on the relay).

The CPU can decide to do so if it sees a run condition (see the ADV7 connection that can trigger the darlington pair to switch to ground, or in this case 'near ground' as there is a 5.6 ohm resistor it will be closer to 1..2,5 Volts I guess) or if via pin 4 the darlington transistor gets triggered and pin 4 is powered by the same power that goes to the starter.

But you are saying that pin 86 of the DME relay was not getting power , only briefly - that pin is powered without interference of the ECU itself. If your alarm is correctly bridged now, that means pin 86 from the DME goes back to 87a from the alarm socket, now bridged to pin 15 of the alarm socket that comes directly from the base of the fuse 7 (bridged to fuse 8) in fuse box 1. That same fuse in it's turn should be connected by a thick (2,5mm) Red/Black cable to pin 15 of your key contact which is active in 'Run&start' only.

Check that while starting you are getting 12V on pin 4 of the DME
Still a mystery why your bus 15 is not giving 12V while starting with the relay in the socket, I would check the output on the contact itself to eliminate a wiring issue of bad contacts ?




Key contacts :
15 = run (but also 12V when starting)
50 = start
30 = always hot (Vbat) !

[FrankM_]

For completeness sake

DME relay



30 = always hot, directly wired, unfused to battery, 4mm2 diameter red wire (take care !)
86 = coming from alarm plug 87a (black wire), if no alarm bridged to bus 15 of key contact (RUN), after starting should have permanent 12V-14V
85 = brown wire, straight to ground MPXII, intake runner cyl 1
87 = 12V output of first stage of relay for ECU (pin 18, 35), ICV (pin 4) and all fuel injectors (still unfused !), thick red wire (6 mm2)

85b = ground provided by DME for stage 2 of the relay (black/white wire), see above
87b = 12V output of second stage of the relay running to fuse 3 (red/green wire) and then the fuel pump and also powering the oxygen sensor heater (yellow wire) (unfused)


Pinout of the DME for a 3.2
While key is in start position (bus 50 on 12V) , pin 4 will also receive 12V and connect pin 20 to ground, energising the 2nd coil of the DME relay.