Bad DME or reference/speed sensor?

[josh565]

Okay so about 2 days ago i go out to start my car, it runs for about 10 seconds then died immediately, there was no stumbling just flat out died.

Things I've seen when I try to start it:
My tachometer doesn't bounce when I try to start the car
I can hear the starter spin the engine but it will never turn over on its own
I can hear the fuel pump work when the key is in the on state

Things ive done to try and fix it:
New battery
Changed oil and filter
Changed fuel filter
Checked if there's fuel in the fuel rail and there is with psi of about 35
Found a broken O2 sensor connector, check the link->
All fuses are okay
tried to wiggle all the relays to see if that would help (it didn't)
Checked all the spark plugs and they are clean

Where should I go to now? I am about to check if there is a spark to the spark plugs by grounding the plug and turning the key. I have not jumped any of the relays. I'm honestly at a loss right now and some guidance would be appreciated.

Cheers

[JonA]

Simple first step, change out the dme relay or install the infamous jumper. Instructions for the jumper are at Clarke's Garage.

Jon

[josh565]

Okay which one is the DME relay, and where can I find this 3 wire piece for jumping?

[josh565]

So I went to check the relays under the wheel, and actually took them all off because I have no idea which one was which, dont worry i labeled them. I did find one however that had its top plastic cover broken off. The copper wires were all exposed. Here is what it looked like
sorry for the hard photo but thats it circled in red. I still dont know which one I need to test but at least I found that right?

[JonA]

Clark's Garage Home Page

Go to Clark's Garage. Page down to DME Relay It's all there......

[Arizona_928]

Start by putting your relays back in and finding the dme relay.
Usually when they go bad it's obvious on the under side of the relay. Either way i would jump the terminals as explained in Clark's before rushing to buy a new relay as that might not be your problem.
You're wasting time by taking out all the relays due to the fact half the relays are circuits that have nothing to do with the car running or therefore lack of.

[Rasta Monsta]

What year is the car? Early cars can have flakey ignition switches that cause the symptoms you are describing.

[josh565]

Its an early 1984 model.

[josh565]

So Im not getting any spark at all. I took a spark plug out and plugged it back into the wire and grounded, yet no spark. So I took of the distributor cap to see how the contacts were looking like. They were slightly corroded, so I cleaned them. I then tested to see if i was getting voltage through the distributor cap. From the center wire that goes to the cylinder by the windshield washer reservoir, it had a voltage of 12 volts when the key was in the on state. Then checking the rotor to the distributor cap that was off, I wasnt getting any voltage. Does that mean I need a new rotor or distributor cap? To also note, the spring loaded piece of carbon in the distributor cap seemed to be in good shape.

I also was wondering when a spark plug is removed and you turn the engine on, should you be able to see/smell gasoline? I couldn't see or smell any gas.

[nynor]

make a mark where the rotor is at and crank the engine. verify that the rotor is turning.

[JonA]

You need to back up a little. The DME will not tell the coil to fire unless you have tach bounce. You will not get tach bounce unless the ref and speed sensors are good and the dme relay is good. There is a procedure to test the sensors at Clark's Garage and if you install the jumper for the relay you can eliminate all these as the source of your problem.
Take a minute and do a search of this forum. There are many threads with lots of useful information on this subject. It is a common problem.
Keep updating us on this thread. There are many very talented owners here who are very willing to help.

Jon

[notMyScreenName]

Jon, this problem is actually spread across a couple of threads... In the other thread the OP says that he sees fuel flow when he turns the key so the DME(FP) relay is working correctly - no need to jumper the relay at this point.

josh565: I assume that by "the cylinder by the windshield washer reservoir" you mean the ignition coil? When the key is turned to the correct position it supplies +12V to one of the two small terminals on either side of the coil (I don't remember which). The other small terminal is connected to the DME (the computer that controls the engine) and is momentarily pulled low to cause a spark. When not causing a spark the coil in internally wired such that you will see 12V on all three terminals. With all the wires connected to the coil, remove the center wire from the distributer, switch on the key (but don't crank the engine) and check that end of the wire is at +12V. If you don't see +12V there remove the wire entirely and check it for continuity. If it is bad buy a new one. If it is good then I am almost certain that there is reason that you shouldn't also be seeing +12 on the rotor. Check the continuity between the center terminal on the distributer cap and the rotor. If it's bad get a new cap. If everything is good then maybe just reseating everything will work. Report back with your results.

[911tracker85]

I replied to another similar thread.

in my case my coil failed. checked a LOT of stuff, but not the coil at first because I could not find my test light AND it was NEW.

I agree with notmyscreenname's post. the way I found my coil issue was to connect test light to the 'trigger lead' (just cant recall what that control connection is called) on the coil.

as he noted, with key on, you can use a volt meter to verify +12v on both leads to the coil. when cranking the engine, the voltage on the control lead should toggle. so with the test light connected to that control lead, if cranking does not cause the test light to flicker, you are not getting signals from the DME to fire the coil, so no spark. the toggling is fast enough I am not sure you will see it register on a volt meter, but using a test light worked for me as I was able to see it flicker, but nothing out of the coil lead to the distributor.

if you do not see +12V on both, and do not see the voltage toggle on the control lead when cranking, there can be many things that can cause that such as DME, crank/speed sensors, wiring harness, etc.

PS. trying to test the signals from the crank/speed sensors needs a O-scope. ironically I chased that first as my tach was not bouncing as referenced. turns out my tach just did not work. I also put my DME into a friend's car to verify it was good.

in my case I was working on an 86 951. not sure about other issues with an early NA.

good luck

[JBC]

Sounds a lot like what I had on my 83. Same symptoms. It turned out to be bad speed and reference sensors. They went bad. Replaced them and the vehicle started. The DME will not allows spark and fuel if it does not see inputs from these sensors.

[Backyardbodyman]

Learning a lot from this thread. I've never experienced this problem. Not yet. Seen a lot of no start threads that turned out to be those dam speed reference sensors. Might be a good idea to replace them. If that doesn't do it,still good to have a spare set. You never know when those are gonna go. Good luck. Let us know what happens

[josh565]

Did a little bit more work on the 2000 kilo paper weight. I checked the speed and reference sensors how clarks garage did it, heres a link to what I used as references Speed and Reference Sensors - Checking, Replacement, and Adjustment. Any ways, what I did is I checked the resistance through each of the terminals how clarks garage showed.

Speed Sensor
terminal 8 - 27 measured 300 ohms
terminal 8 - 23 measured 0.8 M ohms

Refernce Sensor
terminal 25 - 26 measure 1300 ohms
terminal 25 - 78 measure 0.6 M ohms

I wish I had an oscilloscope though to check the voltage waves.

I also grabbed a test light to check the ignition coil. When I turned the key to the on state, It lit up. When I started to crank, the light flickered lightly and not very quickly.

Does this mean that my sensors are bad, I am still going to order some new ones just in case, plus wont harm having a spare set.

Plus I dont know if this means anything but the first time i did start my car, it ran for about 3 seconds then died. Did the same thing again a couple days later but ran for 10 seconds this time.

[JonA]

Speed sensor is slightly out of range. This is another test for the sensors .

https://www.youtube.com/watch?v=zeFOOcLSk60&feature=youtu.be


When my DME relay went bad the car would occasionally start and run for a few seconds. I think a bad solder joint in the relay would occasionally conduct but then warm up and fail. I know you believe your DME relay is good but changing it out or putting in the jumper would take little effort and I'd sleep better at night.
Just sayin.......

Jon

[josh565]

just to make sure, the DME relay is actually the fuel pump relay correct?

[JonA]

Yes, it does both....

[josh565]

From what I understand though is that if the DME relay was bad, then the fuel pump would not turn on when the key is in the on state. As for my situation, it does turn on when I turn the key though...