New DME Relay did not make my Fuel Pump Work

seafood10 · 09-23-2008, 03:38 PM

Hi, I have an 84 and went to start it the other day and it turned over but no ignition. I jumped the fuse and the fuel pump did work and the car started, I removed the jumper and drove the car home, which was a couple of miles, and it drove as normal even though the jumper was not in place at the fuse?
Anyway, I replaced the DME relay with a new one and am still experiencing the issue described above. If I jump the fuse to the one next to it the fuel pump will work.
Can anyone lend me some help on this one,I would really appreciate it.
Thanks

Rick V · 09-23-2008, 04:25 PM

Did you check the integrity of the contacts on the fuse panel?

johnsjmc · 09-23-2008, 04:27 PM

I would suspect one of the 2 flywheel position sensors(I think they are both the same so you can buy or borrow only one) I believe the computer needs a signal from the flywheel movement to activate the fuel pump through the DME relay

ischmitz · ischmitz's Garage

What you describe is that the signal from the igition switch (START, TERM 50) to the DME box pin #4 is interrupted. The DME switches the DME relays' second stage (fuel pump & O2 sensor heater) on due to to different events:

- During cranking pin #4 of the DME box receives a signal from Terminal 50 yellow wire (same that controls the starter soleniod)

- During sufficient flywheel movement by the DME computer. During cranking the RPM are not high enough for the DME to activate the fuel pump.

Anything else would have not allowed you to drive the car anywhere.

Chances are that either the signal doesn't make it to pin #4 or internal damage to the DME. A simple test is to disconnect the DME box and locate pin #4 on the 55-pin connector. Measure its voltage when you crank. If it shows +12V your DME is bad. If it doesn't show any voltage during cranking you need to trace back the signal to the ignition switch and see where it is interrupted.

Ingo

ischmitz · ischmitz's Garage

Quote:

Originally Posted by johnsjmc

I would suspect one of the 2 flywheel position sensors(I think they are both the same so you can buy or borrow only one) I believe the computer needs a signal from the flywheel movement to activate the fuel pump through the DME relay

This is true for the 3.6 engines. The 3.2 has an internal OR function and uses Terminal 50 to sense cranking.

seafood10 · 09-23-2008, 04:54 PM

Great, thanks for the response. I do not have much experience with the flywheel sensors but will do some searching online to see where they are and how to change them out.
I did check the fuse older integrity and it tested normal, so that is ruled out but thanks for the reply.
Do you think that since the car ran without the jumper powering the fuel pump that would more than likely point to the sensors?

ischmitz · ischmitz's Garage

Quote:

Originally Posted by seafood10

Do you think that since the car ran without the jumper powering the fuel pump that would more than likely point to the sensors?

no - it would never have so much as coughed. If one or the other sensor is bad there is no spark and no fuel injector signal. Simple as that. Read the schematics if you don't believe it.

Ingo

seafood10 · 09-23-2008, 06:27 PM

OK ischmitz, I believe you and thanks for the clarification, I will rule out the sensors as the issue and follow your suggestion above.

rick-l · 09-23-2008, 09:05 PM

Here is what ischmitz is talking about. D481 and D752 OR the two signals together.

The schematic below is what FR Wilk says is in the DME.

The fuel pump runs when pin 20 of the DME is pulled to ground energizing the DME relay (green arrow current flow).

The blue arrow is the direction current will flow when 12 volts from the yellow wire (ignition start position) is applied to pin 4 (current into the base turns on NPN darlington).

The red arrow is the direction current will flow when the DME detects the engine is running and commands the pump on (also turns on transistor).

Note that either of these conditions will run the fuel pump. The red current path is working from your description of the problem (runs after started). The blue path is not (won't start unless fuel pump independently powered).

I'd stick a meter on pin 4 and crank it.

[/B

DonMo · DonMo's Garage

My bet is on a cracked solder joint in the Motronic or even a failed diode. Mine started doing the same and it took a motronic rebuild to make it go away. I chased sensors and wires and finally found the bad boys. It was a combination of bad solder joints and a failed diode in the brain.

Good luck on your easter egg hunt.

seafood10 · 09-24-2008, 06:02 AM

Thanks again for the replies.
Rick-I, if I understand correctly, you are suggesting checking for 12 V on Pin 4 of the DME socket when cranking? ALso, if I do not find Voltage there, what would you suggest my next step be after that, or if I DO find voltage what do you suggest.
Again, thanks to everyone assisting me with this.

seafood10 · 09-24-2008, 06:12 AM

Nevermind, I tink I got it now, it is pin 4 of the large 50? pin connector to the motornic.
But again, if there is or is not voltage to that pin, what do you suggest I do after determining that?
thanks

ischmitz · ischmitz's Garage

Quote:

Originally Posted by ischmitz

Chances are that either the signal doesn't make it to pin #4 or internal damage to the DME. A simple test is to disconnect the DME box and locate pin #4 on the 55-pin connector. Measure its voltage when you crank. If it shows +12V your DME is bad. If it doesn't show any voltage during cranking you need to trace back the signal to the ignition switch and see where it is interrupted.

Ingo

Yes it is the 55pin connector on the DME harness.

If you see the + 12V during cranking it is a fault inside the DME
If you do not see the +12V it is a fault in the harness or wiring between the connector and the igition switch

rick-l · 09-24-2008, 06:51 AM

Quote:

Originally Posted by seafood10

checking for 12 V on Pin 4 of the DME socket when cranking?

It is clear that you are trying to measure the voltage on pin 4 of the connector attached to the wire harness and not on the DME that is isolated with the connector removed right.

seafood10 · 09-24-2008, 07:01 AM

Loud and clear now, thanks. I will find my meter and give it a try in the next day or 2. I was bummed when the new DME relay didnt do the trick, but it could never be as easy as that.

seafood10 · 09-24-2008, 06:39 PM

OK, I held the new DME relay and turned the key, felt the relay click and then turned the key to start and the relay clicked again as it is supposed to. I don't have a pin diagram but did check voltage at both ends of the connector, working my way in from the outside in to pin number 6 on both ends, so I think I covered number 4 and they had voltage to them.
I removed the Motronic and am about to start my discovery on the 2 circuit boards, anyone have any last minute advice? I have my multimeter and am ready to go.
thanks again in advance for you taking time to help me with my issues.

rick-l · 09-24-2008, 06:45 PM

Quote:

Originally Posted by seafood10

OK, I held the new DME relay and turned the key, felt the relay click and then turned the key to start and the relay clicked again as it is supposed to.

That says the relay is being energized and power is not getting to the fuel pump or the original diagnosis was wrong.

Quote:

Originally Posted by seafood10

I removed the Motronic and am about to start my discovery on the 2 circuit boards,

Why? It appears to be working fine

seafood10 · 09-24-2008, 07:00 PM

So, if the DME is clicking twice as it is supposed to, that inidcates there is no problem with the Motronic Brain? Is that correct?? What troubleshooting should I do next then?

ischmitz · ischmitz's Garage

I think Rick nailed it. In your first post you said that you had to jumper the fuel pump to make it start but were able to drive it without the jumper being in place. This indicated Term 50 signal not being present or processed correctly by the DME.

Now you say you have a new DME relay (why?) and you find that it enegizes the second stage when you crank. Have you confirmed that when the second stage of the relay clicks it switches your fuel pump on. I ask this even though your prior findings imply this. Have you checked that you still need to put a jumper on the fuel pump fuse to start but can run the engine without it?

And I second Ricks comment. There is no reason to mess with the DME computer at this point. Nothing seems to be wrong with it.

Ingo

seafood10 · 09-24-2008, 07:09 PM

I bought the new relay after reading a tech article on the Pelican site and a few other Google searches. So I have learned that the DME is in fact operating correctly and that must indicate that the Motronic is working fine, OK, I will put that back in tomorrow.

I have tried starting the car and listening for the pump but no success.
So we now know that the Motronic is fine, the DME is fine, and the car will start and drive but I have to jump the fuel pump fuse to start it up.
What should I look at now, I am a little lost but I believe that what I have learned about the DME working and the Motronic working as well as the pump working if jumped, but just need a little help with that to do next.
thanks