Do I have a bad DME?

[Dan Harjung]

Hello, and happy father's day. You helped me out of a jam a few months ago with a vacuum leak problem, but now I think I have a bigger problem and I hope you can help me out again. The same car, a 94 540i, stock with about 220,000 miles. The car won't start. Here is what I know:


There is no fuel pressure, but the fuel pump, fuse, and fuel pump relay are good. If I bypass the relay with a jumper the fuel pressure goes up to normal. The car will not start with the fuel pump running.


The ground at the primary side of the fuel pump relay, which should be switched through the DME, is not making it to ground. If I make that connection directly to ground, bypassing the DME, the fuel pump runs with the ignition in the "run" position, by the car will still not start.


I haven't tested ignition, but since the DME isn't switching fuel pump relay ground, I figured that was probably an indication of a larger DME failure which was probably also causing other problems that are contributing to the no-start condition.


I opened the DME but didn't see any obvious problems. I could start testing components in the DME, but without a schematic I would be flailing. My ET skills are decent, but this will probably put me out of my depth.


So, my questions are these. Am I on the right track with the DME, or am I missing something obvious. If it is the DME, is it feasible to repair it, or should I look for a replacement. If I should replace it, what's a good source and about how much do they typically go for? This is my son's car and he is as short on cash as a typical 17 year old with a girlfriend, so the least expensive option is preferred.


Thanks in advance for any help, I do appreciate it.


Dan

[genphreak]

You are on the right track with your tests and isolation of the DME FPR signal. However this comes from a transistor switched circuit that is very reliable. I doubt the DME is at fault.

DMEs have only a few 'hard switching tasks' to do. These are done by the bank of transistors down the sides of the DME which control the rate and duration that the fuel injectors activate- and the spark (but these are simple remote current in M50 and M60 cars as they use coil on plug). These can fail as they undergo heat stress over the years- the other circuits are pretty reliable IMHO.

You could also try to confirm if the engine has spark if you have not yet done so.
Is it turning over just not firing?

If it is turning over, it won't be incorrect key. More likely low battery voltage, CPS failure, engine circuit fusible link, a failing Crash relay, and even the DWA and/or car alarm. Any one of these can cause the ECU to not power up the fuel.

Alternately your problem might be EWS. Does your car have it? '94 production is on the cusp- though I thought these technologies came in in '95. You can tell easily as all EWS cars have a 'transponder key', a key with a (uniquely coded chip in it), which is detected by the EWS unit using an antenna in a large black plastic ring (45mm dia) around the metal ignition barrel.

If your car has this ring, you should check for DWA faults using the diagnostics connector or 'stomp test' if your car has this functionality (all US production).
Such a fault could be: Incorrect key, EWS unit or wiring failure, DWA triggered. If DWA, the unit can be unplugged by locating it in a hidden location just in front of the rear power distribution box (under back seat, driver's side on LHD cars) and the EWS should then ignore it and start.

Some car alarms and the OBC anti-theft code may be able to generate an error condition that upsets EWS also, so you need to look at those too if your car has them.

[Dan Harjung]

The engine is turning over, just not firing.


The battery is fully charged.


I tested spark (sort of, not sure how to do this with coil-on-plug. I tested voltage across the primary side of the coil during cranking and it was always at 0V, I also tested voltage from ground to the low voltage side of the coil primary during cranking and it was always 12V), no spark.


I tested the CPS and the resistance was right at spec, 1.2K.


I tested the main relay and everything tested fine except no ground at terminal 30 (ground through the ECU). The Bentley manual says this may be an ECU fault.


Not sure about the engine circuit fusible link, I don't know where to find that. Same with the Crash relay.


I disabled the car alarm years ago because of a fault that prevented starting.


This car does not have EWS, as far as I can tell (no black plastic ring).


So, what next?

[genphreak]

Thoughts:

Check power to the ECU (at the large red connector wire) is >12V cranking. Bad batteries show their limitations in different ways when put under different loadings.
Swap the crash relay with another known good one- its the tall brown one in the corner of the fusebox. Usually when these fail you'll notice hazard lights flashing. Idea being that they detect a collision and shut down the fuel pump through the DME, open the door locks, turn on hazards. They may stop the wipers and radio too, not very sure though.

Spark is hard to test yes, sorry I forgot :)

I've had starting troubles like this too (twice). Turned out to be something weird with pump activation possibly after running out of fuel (not verified). I fixed it by jumpering the FPR so the pump and the car then started. Whilst running (left it for a few minutes) I removed the jumper and piut the relay back in. Car ran fine after that.

Prior to that I had swapped the relay with another but with no luck. Like you the signal was not coming from DME. But you have tried this too I think from your prev post.

[Dan Harjung]

Okay, I checked the power at the ECU, it is >12V cranking. I checked the crash relay, it's good. I did try jumpering the FPR, but it would't start.

Signs seem to be pointing towards the ECU, so I found a replacement, I should have it Wednesday or Thursday. Assuming the new one has a different firmware revision than the old one, how do I manage that? Can I just swap the EPROM?

[genphreak]

So long as it is the same silver label DME, yes- you just swap the chip over (Your old one has the old ISN in it and the ECU is programmed to check it is a suitable match for the key's transponder, prior to starting the engine).

[Bill R.]

jumper the main relay and see if it starts and runs

[Dan Harjung]

I tried a jumper on the main relay, still won't start. As near as I can tell, there is no spark, no fuel injectors (not 100% sure on that), and no fuel pump.

I got the new DME, plugged it and no change. Swapped the EPROM and tried it, still nothing.

I disconnected the DWA, still nothing.

Any ideas on where to look next?

[Tiger]

When is the last time you checked and tried swapping crank position sensor?

[Dan Harjung]

So here is a funny story. The crank position sensor was one of the first things I checked, and it was right in spec, but I tested it at the harness on top of the motor. So I decided to check it through the DME connector to make sure the entire harness was good, and I measured infinite resistance. So, I figured I was on to something. Then I tested continuity between the DME connector and the CPS connector and both wires measured infinite resistance. That didn't seem right. So a little more investigation showed that, in my infinite stupidity, I hadn't measured the CPS at all, I had been measuring the Cam position sensor. So dumb. When I tested the CPS it was infinite resistance. So, I think I found the problem. But I guess it's good to know the Cam sensor is good, and I have a spare DME. Might try a Conforti chip or something in that one for fun.

Anyway, thanks again for all of your help, and sorry to rack your collective brains for no reason. Now I will go pour a bourbon and slink back into my hole of shame and humiliation.

[shogun]

no need to slink back into your hole of shame and humiliation, good that you told us the details and we all learned out of this, as we all learn from mistakes.
Let us know how it works when you installed the new CPS.

[genphreak]

no need to slink back into your hole of shame and humiliation, good that you told us the details and we all learned out of this, as we all learn from mistakes.
Let us know how it works when you installed the new CPS.LOL! It is SO easy to mix up the CPS and the CPS as:

1. The connectors are the same
2. The cables are the same size and colour
2. the acronyms are the same

Congrats Dan, it was a good exercise we can all learn from thanks to your sharing!

Be sure to post an audio file of that engine roaring or a video of the car taking-off!

[Dan Harjung]

Okay, so here's the update. I just installed the new CPS and it started right up. Very satisfying. Thanks again for all of your help and support. Not sure I could have done it without you.