Please Help, Wiring/DME Problem...

Ehryk · 10-08-2009, 08:15 AM

I am at the end of my rope on this one.

I have a 1983 944 Non-turbo, and it will not start. This is a large, convoluted problem that I have tried to solve for weeks now that is probably something simple.

First off, I have no spark out of the coil, 12V at the coil during cranking (so not the faulty ignition switch), blah blah. Replaced the coil, both of them will produce spark artificially and test between 0.8 and 1.0 ohms, slightly out of spec of 0.6 but still good enough.

Now, the first portion of the DME relay does click over with the ignition switch, but the second part controlling the fuel pump does not. I have tested both portions of the relay and it is good. When I manually jump the relay terminal the DME should activate, the pump runs but still no spark. This points me at the DME.

I have purchased on ebay another DME, which was pulled from a working 1984 944 and has a warrantee, and the other ECU which has already been replaced once. Both DMEs will not run the fuel pump, making me think that it's a wiring issue, since both these ECUs do the same thing and both have been purchased after the vehicle was bought.

When I pull out the relay box, the wiring behind the relays are solid. there is a loose red wire and a loose fused red/black wire, but they do nothing with 12v on them and look like they were coming off nonimportant relays like the rear defroster. Everything pertaining to the DME and fuel pump checks out, except for the black/gray wire that needs gnd from the DME to activate the Fuel pump.

There are multiple pins in the DME connector that have both 12v and GND, though I don't know if I'm perhaps missing one that is causing the DME to not be on. I would assume that even without speed/reference mark sensors, the ecu would turn the fuel pump relay on and it isn't. There is a chance, I suppose, that both replacement DMEs are bad.

What else can I check? What am I missing? I am not worried about spark / fuel injector pulsing / speed/ref mark sensor mark etc if the ECU is not "turning on" - if this applies and it's not just comprised of separate circuits. Any ideas what makes the DME turn the fuel pump on and such?

Grandad#3 · 10-08-2009, 08:34 AM

Hi Ehryk,

Have you checked the fuse for the fuel pump? It is on the upper fuse panel 2nd from the left and is 16 amp. Have you checked the reference sensor and its plug? While cranking the engine do you get TACH bounce? The needle only moves about 1/16" so you have watch close.

Cheers,
Larry

Ehryk · 10-08-2009, 10:32 AM

The Fuel Pump Fuse is good. I'm not hard jumping the relay on the high current side, just grounding the terminal that the DME should. The fuel pump would not run if the fuse was blown unless I bypassed the fuse, which I have not.

I haven't seen any tach bounce, at all. I will look harder this evening, but again the ref mark sensor should have NOTHING TO DO with the DME switching on the Fuel Pump as the key is turned to ignition, or when cranking, and the DME isn't doing even this simple function. I would think that an advanced calculation like spark timing would be out of the question if the DME isn't even doing the simple things. I will post again when I check 100% on the tach bounce.

Slam · 10-08-2009, 10:40 AM

If the ignition switch is bad you can crank and get no tach bounce.

Grandad#3 · 11:52 AM

Hi Ehryk, If you don't have TACH bounce the DME will not send a signal to the DME Relay. The reference sensor must see 200 rpm to send the start signal for fuel and spark. Jiggly the reference sensor plug wire to see if that helps. Check out the link below for helpful info.

http://www.clarks-garage.com/shop-manual/repair-procedure-index3.htm

Cheers,
Larry

Gary2397 · 01:20 PM

I do not think your DME is bad since you swapped with different one and
it does not start. If you turn the key the engine turn over, then it is not
DME.

One way to check an ignigtion switch is by turning the key to crank
position to see if dash lights will go off. If it does , switch is bad.
If you have a spare main relay, swap it this could be the problem.

Another one is bad speed/ ref mark sensors if you do not have spark
out of the coil.

One always carry spare main relay for all 944s. Another budget alternative is that
speed/ ref mark sensor(some times call crank angle sensors) from 84-86 BMW 325E
will do just fine except little bit longer wire.

AKCJ · 10-08-2009, 01:29 PM

Quote:

Originally Posted by Grandad#3

Hi Ehryk, If you don't have TACH bounce the DME will not send a signal to the DME Relay. The reference sensor must see 200 rpm to send the start signal for fuel and spark. Jiggly the reference sensor plug wire to see if that helps. Check out the link below for helpful info.

Clark's Garage Shop Manual Index

Cheers,
Larry

This is what I was thinking (except the rpm reading would come from the speed sensor, not the reference sensor). My understanding is that the DME does not energize the fuel pump until it detects enough engine speed.

I think it's more likely a problem with the speed or ref sensor (or wire plugs) instead of the DME.

Grandad#3 · 10-08-2009, 02:37 PM

Thank you, AKCJ, for correcting my mistake on the speed sensor. You are right on your call. In a No Start I always look at both and for the TACH bounce.

Cheers,
Larry

Ehryk · 10-09-2009, 08:02 PM

Very interesting, that it needs engine speed before it energizes the fuel pump, this I was not aware of. I will check the sensors as best I can with my multimeter. Which one is it checking for the 200RPM on, specifically - Speed or Reference Mark sensors?