1983 Porsche 944 DME Relay Location

[onthesolidrock]

We just bought a 1983 Porsche 944 and we are having issues getting spark to the distributor. We have followed several threads on this forum but we still cannot get the engine to fire.

We have installed a new coil and we have power to the coil 12 Volts and when the ignition is on, the negative side has has 12 volts too, until we start cranking, then the voltage drops off to 0.

We do have fuel pressure and the injectors are firing, but we still cannot get power out of the coil to the distributor.

We have spent the last couple of hours trying to locate the DME relay to check that, but for the life of me we cannot find it! We are beginning to wonder if it even has one!!! Can someone pont us in the right direction to locate this relay?

Secondly, we see a wire connection that appears to be broken, but we don't know what it is or if it has anything to do with our problme. A photo is attached; can someone let us know what this is?

Thanks so much for your help

[nightheart]

The pictured is the 02 sensor connection. The DME relay is under the dash, drivers side along with the 2 separate fuse panels and a collection of other relays.

[nightheart]

Pelican Parts: 944 Early Fuse and Relay Layout

Its called the fuel pump relay on early cars

[onthesolidrock]

Thanks for the quick reply, I think since the fuel pump is working and the injectors are firing, that the relay is good.

I wonder if the oxygen senor wire was broke at the connection I referenced, if that would have anything to do with the car not firing

[nightheart]

i doubt it. The O2 sensors were an American option in 83. The Euro cars didnt have them and the car defaulted into an "open loop" mode running richer than the American cars did. Also the O2 sensor (if its the one wire version) doesn't kick in till the exhaust and sensor are warmed up. The car runs in an open loop mode till then.

Check out Clark's Garage Home Page for some trouble shooting ideas. I can't think of anything off the top of my head. The DME unit itself could be bad? Someone else will probably chime in with a few other ideas

[pfarah]

Check to see if you get the infamous tach bounce while turning over the starter. You may need to replace the speed/reference sensors, particularly if they're original units.

[sausagehacker]

Replace the ignition switch, it's ~$5 from our host and they go bad all the time. Losing the coil power when cranking makes it a possibility. Worst case, you replace it now, and headaches are saved later on because it WILL go bad. I'm on my third since I owned my car (bought in 2013).

[onthesolidrock]

We do not see the tach bouncing at all while turning the engine over. I think it would be a good idea to replace the two sensors; I can find the crank position sensor here on Pelican, but where do I find the speed sensor?

Thanks for the help

[Rasta Monsta]

One in the same they are, padawan.

[JonA]

If you do not have tach bounce I believe the injectors will not fire and the fuel pump will not run! Replacing the speed and reference sensors is a challenge and my advise would be not to go there unless you are certain one is bad. Check the connectors for the speed and reference sensors at the firewall at the top of the engine first. Take out the DME relay and put in the 3 wire jumper and test.
Read some of the posts on this site, read all the information at Clark's Garage. Many of us have been down this road and there is a bunch of info out there.

Hope this helps,
Jon

[JohnnyZ]

Had similar problem with my 83. Was getting no spark. Temporary fix by running a switched jumper from battery to positive side of coil. Car started immediately. Remember to switch off jumper after starting or the key will not shut down the engine.

[jpk]

The Bosch Motronic system bases everything on three key inputs: airflow, engine speed, and engine position. Anything else is a secondary input. Without all three of those signals, the engine will not run correctly.

The DME gets power through the DME relay. That same relay will also send power to the fuel pump if the DME tells it to (it's really 2 relays in one case).

The engine speed sensor tells the DME how fast the engine is spinning by reading the teeth on the flywheel. The engine reference sensor tells the DME when the engine gets to TDC by reading a steel set screw on the flywheel. Both of those sensors are on the bracket at the back of the engine on the top side of the flywheel. The later 3.0l cars have a single sensor that does both jobs.

Airflow is measured by the "barn boor" AFM between the airbox and the throttle body.

A few key points to keep in mind when doing backyard diagnosis:

-Power to the coil means nothing. That happens entirely outside of the DME. The DME takes the coil to ground to make a spark (like a points and condensor)

-Tach bounce during cranking means only that the engine speed sensor is sending a singal. Nothing else. (Well, I guess it also means you're getting power to the DME...)

-Speed and reference sensors are passive and send a very low level voltage pulse to the DME. The signal is very dependent on the condition of the wire and connectors.

- A regular volt meter can't measure the transient signals from things like sensor pulses or coil grounding pulses. At best, you can check continuity and circuit resistance with the ohm meter. The only way to truely measure the low level, transient pulses is with an oscilloscope.

The Bosch Motronic system is very different than a lot of systems used in the 80's and 90's. Bosch published a very good reference (I believe it was called the Bosch Fuel Injection handbook; I have a copy somewhere...) There used to be a website published by FR Wilk which had a really good explaination of the DME (with 944 specifics) but I haven't seen that in a while.

[dgcantrell]

I have that Bosch EFI book and it is excellent. I recommend it because it covered not on the Motronic stuff, it covers most of the other Bosch flavors of EFI.