Hi Cliff,

I have only ever tried to swap the DME and DME relay when the car will not start. I take the DME and DME relay independantly and swap them out. The result for both tests is that my car still will not start however the other car will start fine. Does this answer your question? I just checked pin 86 at the relay and I have 12 V there so my understandin is that the alarm should be working fine.

Although, not the cause I was going back over my 3.2 conversion wires. I noticed that I had taped up the yellow wire in the trunk (pin 4 on DME) that from the diagrams is supposed to go to the ignition switch. I believe this was suppose to be connected to a fuse that is only on during cranking; however I can't seem to find any fuse that fits that condition.

Keep in mind that the car has run before without this hooked up but I wasn't sure if I tried to wire this up if it may help me better diagnose my symptoms. I am starting to wonder if I even have fuel now when I have the no spark condition.

Also as mentioned earlier in the thread I believe my fuel pump has been wired differently so that it is always on once the key is turned. This was also suppose to go to a wire that was taped up (black wire in trunk area). Not sure if any of this will help me in getting to the fundamental problem or if I am just convoluting the problem.

Pin #4 in the 84-89 cars is used and is very important! You are correct that pin 4 needs 12v while cranking. Internally in the DME when pin #4 sees the 12v it then grounds pin #20 which in turn grounds the DME pin 85b. This activates the fuel pump relay. This is required to run/start the fuel pump while you are cranking. In the stock configuration if pin #4 does not see 12V while cranking the fuel pump will not run! You can test this circuit by simply turning the key to 'RUN' then apply 12v to pin #4 and you will hear the fuel pump run.

Now the question I have is why could you start the car without pin #4 wired? When you wired this DME into the 75 car you must be running the pump some other way. If this is the case then maybe you don't need pin #4 wired. But this also means you would not be using the Fuel Pump relay within the DME.

As for why the car starts sometimes and not others I think this is something else. When it does not start I assume the motor still cranks? Do you smell fuel while it does not start? I ask because if the injectors are running you quickly will know it as you start to smell raw fuel.

Another quick check is to test for 5v on the AirFlowMeter at the Blue/GreenStripe wire at the AFM Harness. Seeing 5v here tells you the DME is alive and that the DME relay is working. Do this with key in 'RUN'

Once we know the answer to the above tests we can decide what to test next.

Hi Guys,

So I did several things today.

I wired up pin 4 from the harness to a yellow wire coming from my key.
I also wired my fuel pump to the DME Relay wire so it only comes on, on start.

After doing this the car as usual started when cold. I drove the car around the block 2 or 3 times parked it, and once again it wouldn't start.

I changed out the reference sensor with a new one but the car would still not start.

I then followed Lorenfb's steps in another thread. Here were the steps he suggested and my results:

Tests:
1. Check spark at coil wire (Result: No spark whatsoever)
2. Use a $3 injector light to check for injector pulse (Result: I used an osciloscope and saw no signal at all, an no smell of fuel)
3. Check the main power pins; 1, 18, 35 and injector power 14, 15 (Result: Had 11.4V at each of the pins)
4. Check for grounds pins; 5, 16, 17 (Result: All grounds showed 0.6ohm)
5. Check fuel pressure (35-40 psi) (Result: Didn't know how to do this)
6. Check pickup sensors; 8/27 (2.0 VAC), 25/26 (.10 VAC) (Result: A voltmeter didn't show anything on either however when I hooked up the Oscilloscope the speed sensor showed a signwave with a voltage 8V peak to peak (why is this so high?) and the ref sensor showed about .4V peak to peak)
7. Check temp sensor; ~ 300 ohms hot, > 1500 ohms cold (Result: where am I checking this, also i put a new temp guage sensor in however it doesn't seem to be working)

Something else to add is that I still don't have the oil cooler on the car, however it is quite cold here now, and I have heard others do this without the car getting even close to temperature. I wouldn't think that after 10 min of running that the engine could even get to full temperature.

So I am looking for suggestions on where to go from here.

No Spark and No injector pulses usually means something is not working with the Ref and Speed sensor. 8V pp is really high for one of those, double check them when cold and then again at no start.

Just retested and it looks like I made a mistake. It is 4V peak to peak on the idle. As mentioned I replaced the reference sensor.

Did you fix the ground screw on cyl #1 intake runner? + check/clean the ground strap in front of trany.

I took the tranny ground off and soaked in vinegar and scrubbed it with vinegar. It now looks new. I then moved the gronds on the intake that was stripped over to intake 3 where the ref sensor and idle sensor mount to the intakes.

What voltage do you need at the battery for proper operation of the DME?

The battery should have 11.8 or more, if you question the battery have it tested

Could the battery cause this type of behavior?

Could bad solder joints in the DME cause this type of behavior?

And I assume you made the critical adjustment of the sensor distance to the flywheel.

I never moved the bolt for the mounting bracket, however I did try and squeeze a feeler guage in there and it appeared to be between 0.82mm and 0.88mm

When you have the no-start situation check if the DME is actually running. Simply switch the key to ON and check if the ICV vibrates. It's the silver can on the intake.

This is a very quick check to confirm that the DME gets power and actually runs.

I was under the impression you swapped your DME into another car and it ran. Why are you still questioning the DME being faulty?


Your battery voltage is too low. You want at least 12.4 Volts. If you have 11.4 volts and start to crank I bet the voltage will drop below 10V and that will cause issues.

I agree, any battery reading less than 11.8V should be questioned. Place a charger on the battery overnight. Also have the battery load tested most Auto Parts stores do it for free.

As for the ref and speed sensors, you have a scope so simply measure peak to peak volatge while cranking. The motronic ttest manual states 2.5v or greater your good to go. If the voltage is lower then adjust the gap, otherwise leave them alone.

But test them again when the failure occurs.

Also, when failure occurs measure for 5v at the AFM while cranking be sure the 5v is seen at the AFM while cranking. If it is the DME power is most likely OK.

Ischmitz,

I checked the DME by swapping out my DME with one from a 1988 3.2 NA car. In fact I didn't really want to do this because the part numbers were different for the DME. I later realized that two pins are swapped between the years of 87 and 88. My DME DID work in the other car however I didn't want to harm the other car by trying to drive around and letting it warm up so I can't be certain thats what the problem was.

I have checked what I believe is the AFM (4 pin plug), when the key is at the on position I get 4.6-4.8V on the blue/green wire, which I believe I read somewhere was the test? I don't believe I checked it while I was cranking.

The voltage on the AFM connector is from an internal voltage regulator. The fact that it is present tells you the DME sees more than 6 or 7 volts on its supply pins while not cranking. Nothing else. The same is true when you feel the ICV vibrating: You know that the internal CPU is running and that the idle control output stages are fine.

However, once you crank your car a weak battery can quickly let its output drop below 8 volts. This is why Sal is recommending a load test at an auto parts store. Charge it overnight is also a very good idea in case it simply isnt't fully charged. The low battery voltage will first fail to open the injectors. This is why you need to know your system voltage (battery) while cranking.

BTW: Different DME's can be swapped even if part numbers don't match for the 3.2. The only thing you might notice is that a newer DME results in hunting idle on an older car. But that is an adjustment issue of the AFM air bypass and won't hurt anything. You can drive the test car with your DME and let it warm up. It won't hurt a thing in the new car. IF your DME was going to hurt the test car it would have happened instantly (burned injectors or ICV in case a driver stage in yours is short). The vert fact that the test car started tells you that wasn't the case.

I recommend letting your DME fully warm up in the test car to exclude any doubt that is has an issue.

Ingo

Just searching the wiring schematics so I properly understand all sensor inputs. Can someone comment on what the microswitch does white/green (pin 2) from DME. I can't find anywhere in the Bently manual where this is discussed. Also is there anyway that a faulty NTC will make the engine thinking it is overheating and cause a no start. From much of what I have read the answer is no, but I figure I would ask.