No ignition spark, narrowed it down, need some advice on purchase
 

For the past week, my "new" 84 944 has been refusing to start at seemingly random times, although it has been happening more and more often as the days go by. It doesn't matter the time of day, and it's usually after I have been driving it. For example, after parking for a few minutes at the bank today it would not start. I am not getting any spark out of the coil, and the coil is good. I have been to Clarks garage, and I have read all of the posts on the crank sensor wires, dme relay, the dme itself, tach bounce, etc...

Here is the clincher. After testing everything I could today in the bank parking lot with basic hand tools, I decided to disconnect the battery to reset the dme. So I did and after reconnection the car fired up.

Is this a positive indication that I should replace my DME, or are all of the other variables I listed up top still a possibility? In other words, what unit on the car controls spark, and is influenced by a basic power reset?

Welcome to the board!

A no spark condition like you described is tough to diagnose because it can be a bunch of different things.

If the DME works, but the problem is intermittant, then it probably isn't the DME. It could be cold solders in the DME - that can make for intermittant no starts. A cold solder joint is a solder point on the printed circuit board (PCB) that's become "loose" over the years. A solution there is to reflow the solder.

Here's a good thread on Clark's: Clark's Garage Message Board :: View topic - DME Failure

Disconnecting the battery then reconnecting probably wouldn't do anything useful.

I'd do a visual on the guts of the DME and see how it looks. Any corrosion might increase the possibilty that that's the culprit.

Before I spent any money on a new DME (unless you can find one cheap), I'd play with the wiring harness for the speed and reference sensors - just jiggle it and see if it makes a difference.

My kid's 951 had terrible problems like yours and we screwed with it for months and months, till one day, it was refusing to start, and one of his friends jiggled the sensor wires - and the car started right up. After that, we were able to replicate the situation over and over.

The problem on his was that the wiring harness between the sensor plugs (in the engine compartment) and the DME was damaged. The cure was splicing in a new section of harness.

Another potential culprit: a failing DME relay? Try tapping on the relay next time it won't start. Burnt points in the relay?

You also might try tapping on the DME and see if that makes a difference - if it did start after tapping on the DME, that might indicate cold solders.

Speed sensor is the culprit. Since you are doing that one go ahead and replace the reference sensor also.

i know that those are a common reason for no spark, but why would those sensors, which are essentially just magnets, be affected by disconnecting the battery?

Lot of good info above. I had the same problem, mine was the DME. I just replaced it. I have yet to resolder anything, but it does look like there is chance its only solder joints in mine. I did check the things above before buying another one though.

i just checked the sensors and it looks like the previous owner already changed them at some point. they are relatively clean, the wiring is supple, not brittle, and the connection has been covered in a dab of silicone (to keep out moisture). as i mentioned, i believe these sensors are pulsing, so they work off magnets. if the wiring is ok, there is little reason for the sensor to have a fault.

as far as i know, the motronic is the only ignition related item that would be affected by a power reset. but i would love if an expert would chime in before i buy a new one or send this one out for a rebuild...

The motronic brain is not affected by disconnecting/reconnecting the battery anymore than turning the ignition off and on would affect it. I think you're chasing a false indication.

Check your Ignitor if it has one, then the electrical grounds especially in the engine and the DME.

Could have a floating ground. or a ground loop. Either one will cause Havok, even a fire.

Hope this post was helpful, Later.

I don't know that that's true. There is always a ground current flowing through the car, and the dme is full of capacitors and resistors that hold a charge after the car is off. Since I did a power reset 3 days ago the problem has yet to return. It was happening 2-4 times per day before that...

Right now I'm driving it daily with my diagnostic tools in the trunk, waiting for the problem to return so I can pinpoint it exactly.

Ok the capacitors will loose all the power in a couple of seconds. I had an issue with my car like yours. Car would start fine when cold, then after a ride it would hesitate sometimes. Replaced the Speed sensor and not more hot start issues. Also your harness might have an issue.

Ohh also BMW sensors are half the price and are the same thing except the cable is longer.