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Car Forum / Porsche / Porshe 944 / February 2004

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Help - Need DME Connections

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Reno - 10 Feb 2004 17:52 GMT
Help - Need DME Connections

I have a 1987, 944 NA with a no start, no fuel problem. I'm sure I blew
something in my computer. I managed to turn a simple problem in to a huge
problem.

Background: Car would not start. No spark, no fuel. I checked out my DME
relay and it is good. Turned out to be "no voltage" at pin #86 of the DME
socket (with ignition switch turned on). I removed the DME relay cover and
jammed the main relay closed with a tooth pick and the car started right up.
By closing the main relay contacts, battery voltage was then sent to DME
wich in turn closed the fule pump relay and energized the fuel pump. I
suspected the ingnition switch went bad (unlikely since every thing else
worked fine) or the wire going to pin #86 of the DME relay socket may have
slipped down thus not making contact with the relay. All I have is a haynes
manual and it shows a green/red wire going from DME relay socket pin #86 to
the compressor. (?) Could this be right?

Now it gets worse. In an attempt to check the connections on the bottom of
the DME relay socket, I attempted to lift the fuse board up to look
underneath. I was in too much of a hurry to disconnect the battery and DME.
(If you find yourself in a hole, stop digging). No luck, too hard to pull
the board up far enough to work with. Now, not only do I not have power at
pin #86, but the tooth pick trick doesn't energize the fuel pump relay
anymore. Jumping from battery voltage (pin #30) to DME/Fuel Rail (pin #87)
and Fuel Pump (pin #87b) powers the fuel pump but not the injectors.
Apparently DME is not sending signal to pin #85b of DME Relay. I still have
spark, but no fuel. Fuel pressure is approx 39 psi upon cranking, but
injectors aren't working. I think I blew something in the DME. I have the
great schematics of the DME form FRWILKS, but don't know all the connections
to the 35 pins of the DME.

Just so we don't get off track, Fuel pressure regulator and damper are good.
My DME Relay and spare relay are good. Speed and Refernce marks sensors are
good. Coil and wires are good (I have spark). I am in the process of
checking/replacing ignition switch in case it feeds pin #86 of DME relay,
but I don't think so.

Was all this too much? Just wanted to document in case anyone else had same
problem. I know 944 no start problems are a royal pain. I've had 3 no start
problem. First was bad fuel pressure regulator that shut down computer.
Problem pin pointed by disconnecting on injector, reduce injector current
draw. Second was bad solder joint in computer. Resoldered board, worked
fine. By the was I have really abused the computer and now I finally broke
it. Third was no ignition power to working DME Relay, still trying to work
it out.

Forget everything above and please let me know if you have answers to the
following 3 questions.

Questions:

1. Does anyone have a list of the connections to the 35 pins of the DME?
What sensor/relay goes where?

2. Where does pin #86 of DME RELAY get its power from? What color wire to
where?

3. Where do pins #87 and #85b from the DME RELAY end up at the DME
connector? Need to identify possible problem in associated circuit of the
DME.

Thanks

Reno
Paul Sure - 10 Feb 2004 23:18 GMT
I tried to send you the schematic from the work shop manual that has the pin
connections but the email bounced

Paul
Reno - 11 Feb 2004 02:05 GMT
Paul

Sorry about email address, don't like spammers.  I'm at biznet19@att.net.

Anyway,  I have schematics for the computer, but they don't show what
external relays and sensors are connected to what on the 35 pins of the
computer.

The second stage of the DME relay (fuel pump relay) is not being energized
by the DME.  I'm trying to figure out were pins #85b and #87 of the fuel
pump relay end up on the DME connector.  Then with the schematics that I
have, I can test computer components.

Thanks
Reno

> I tried to send you the schematic from the work shop manual that has the pin
> connections but the email bounced
>
> Paul
Martin? - 11 Feb 2004 02:45 GMT
The DME computer GROUNDS pin 85 of the DME relay WHEN the engine starts
cranking.
The relay takes power from pin 30, always on.
If your DME doesn't ground pin 85, you can take out the relay, connect pins
87 and 87b and connect them by wire to a fuse switched on by ignition
circuit (e.g. auto tranny, even on manual).
The DME computer can be repaired, contact F R Wilks or Don Istook via
rennlist.org forum.
Regards,
Martin
Reno - 13 Feb 2004 00:54 GMT
I don't believe this is accurate.  Pin 85 is always grounded and does not go
through the DME.  Pin 85 is the ground for the main relay that is activate
when power is applied to Pin 86 by the ignition switch.  The main relay
supplies power to the DME and Fuel Rail.  The DME then grounds pin 85b which
activates the fuel pump relay and powers the fuel pump.

My question is, if you can't get ignition power to pin 86 and it IS NOT the
ignition switch, were would the fault be?

> The DME computer GROUNDS pin 85 of the DME relay WHEN the engine starts
> cranking.
[quoted text clipped - 6 lines]
> Regards,
> Martin
Martin? - 13 Feb 2004 01:47 GMT
My post is correct. The whole point of the DME relay is to switch off the
fuel pump in when the engine stops for fire safety reasons i.e. in a crash.
Therefore the DME computer (switched on via ignition key) detects from the
engine speed sensor when it is turning then uses a transistor to ground pin
85 which triggers the relay, which then powers the fuel pump and (87b) idle
stabilizer circuit and the O2 sensor.
I suggest you by-pass the relay as I previously described, and is you have
spark the car should start and run normally.
Regards,
Martin
Reno - 13 Feb 2004 01:58 GMT
Ok.  When I jump from Pin 30 to 87b and 87, car starts and runs fine. When I
put the DME relay back in, no start.  I have checked out the relay and my
spare relay, both work fine.  I just can't get voltage to pin 86.  Replace
ignition switch, still no power between pin 86 and ground.  Any ideas?

> My post is correct. The whole point of the DME relay is to switch off the
> fuel pump in when the engine stops for fire safety reasons i.e. in a crash.
[quoted text clipped - 6 lines]
> Regards,
> Martin
DCorley - 13 Feb 2004 06:11 GMT
do you have the wiring diagrams for your car,  they go a long way
towards tracing issues....

> Ok.  When I jump from Pin 30 to 87b and 87, car starts and runs fine. When I
> put the DME relay back in, no start.  I have checked out the relay and my
[quoted text clipped - 20 lines]
>>Regards,
>>Martin
Reno - 13 Feb 2004 17:27 GMT
Up until now I was trying to use that worthless Haynes manual.  One  of the
guys in this group was kind enough to send me a couple of pages out of the
porsche manual.  Just waiting to hook up with a friend who has the program
to open the files.

> do you have the wiring diagrams for your car,  they go a long way
> towards tracing issues....
[quoted text clipped - 23 lines]
> >>Regards,
> >>Martin
DCorley - 14 Feb 2004 01:05 GMT
if they are PDF files, you can download the reader for free from
www.adobe.com   you want Acrobat Reader ...

> Up until now I was trying to use that worthless Haynes manual.  One  of the
> guys in this group was kind enough to send me a couple of pages out of the
[quoted text clipped - 46 lines]
>>>>Regards,
>>>>Martin
Paul Sure - 14 Feb 2004 13:29 GMT
I made them in PS format so you can send them directly to a postscript
capable printer. Or get ghostview (free) and view the pages on the screen.

Paul

85 must somehow be connected with ground and 86 need to get a +12V from
somewhere. Does your car has a anti theft installation?
Reno - 14 Feb 2004 22:58 GMT
FIXED!!!!

You don't need to be an auto mechanic to work on a 944, just a computer tech
and electrician.

I appreciate all of your inputs, gave me ideas for trouble shooting.  Paul,
Thanks for the pages.

I now know pin 86 of the DME relay comes out of the fuse/relay pannel as a
black/yellow wire from E32 (that's E block of wires, row 3, second pin).
The wire then goes back into the fuse pannel at E44 (Yes, E block, forth
row, forth wire).  Then comes out of fuse pannel again at F14 as green wire
and goes to alarm.  In my case I don't have a factory installed alarm, so
the green wire is connected to the black/white wire that powers the alarm
from ignition switch #15 (ignition switch on).

After much troubleshooting, found power in green wire from alarm but not the
black/yellow wire that came out of and back into the fuse pannel.  So took
the fuse pannel apart.  What a job.  Seven laters of contacts and copper
buses.  Cleaned everything up and put it back together.  Now have ignition
power to DME relay.  Car starts right up.

BTW, the computer is pretty tough.  I took it out and put in in a few times
without disconnecting battery.  Shorted some circuits.  Even resoldered the
board twice.  And it still keeps on ticking.

Again, thanks to all.  When you're stuck, every idea helps in
troubleshooting.

Reno

> I made them in PS format so you can send them directly to a postscript
> capable printer. Or get ghostview (free) and view the pages on the screen.
[quoted text clipped - 3 lines]
> 85 must somehow be connected with ground and 86 need to get a +12V from
> somewhere. Does your car has a anti theft installation?
Martin? - 14 Feb 2004 01:40 GMT
Reno,
exactly as I thought, you have the same problem as I have, your DME computer
doesn't ground pin 85 when the engine starts cranking. You have three
choices.
1. Wire pins 87 and 87b to the auto tranny fuse (or any that is switched on
with ignition) and live with it, as I do.
2. Repair the DME computer, it' just question of replacing a transistor,
(once you have found it !).
3. Get replacement DME, possibly an exchange one from Bosh.
Regards,
Martin
 
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