DME Relay Failure?
 

I have been having problems with intermittent non-starts lately. I would be out for a drive and after stopping and trying to restart there would be nothing, no engine turning, not clicks. The battery was new and I keep it on a charger. The couple of times that it happened I thought it was the immobilizer so i tried starting sequence again and it started. However, this morning it would not start and it was not the immobilizer. Again nothing, no crank, no click just dash lights and radio. I replaced the DME relay and it started right away. I thought that a DME relay failure would still allow the engine to crank but it would not start. Not what I was seeing - nothing, no cranking. Is this a case that it just happened to start after replacing the DME relay or are there different modes of DME relay failure?

Maybe you didn't disable immobiliser the first time around?

pulling dme will still allow you to crank the engine,

^^^ Correct, DME pulled will not prevent starter from spinning the engine ^^^

One can always pull the cover off the old DME relay and look for the obvious cracked solder joint. If a failed joint is seen it can always be resoldered and one now has a spare relay. The replacement relay can also be checked in this way. The failed joint is rather obvious.

Andy

OP, even if DME is removed engine will normally still turn over. My suspects in order of likelihood would be:

1. Immobilizer. Any chance you clicked the remote button more than once and re-enabled instead of disabling it?

2. Ignition keyhole cylinder is dirty or going bad. Try spraying some electronic connector cleaner in there and move the key back and forth (rotate) and in/ out.

3. Bad electrical connection at starter motor. Check connections from wiring harness as well as the negative at ground.

4. Poltergeists. If you hire an exorcist to resolve this, be sure to pay them, otherwise your car may be re-possessed.

If you still think it's the DME, go ahead and pop the old one in and see if the problem can be replicated.

Good luck, let us know what the outcome is.

Thanks everyone for the thoughts. I'll try to answer everything:

1. My battery is pretty new, at least new enough to spin the motor, and it reads 12.1 volts when the car is off

2. The DME relay bypass DOES turn on the fuel pump, so I know it is getting power. However, when I plug in the new DME relay, I do not get a fuel pump buzz. I'm beginning to suspect my new DME relay is either defective out of the box, or more likely the incorrect one. It isn't a "genuine" Porsche part.

3. All the grounding points seem to be fine, nothing obviously wrong with them.

4. I was just able to start the engine with the DME relay bypassed and a little bit of starting fluid in the intake manifold. I removed the bypass and put in my new relay, and once again, no fuel pump buzz, crank but no start.

My my current thoughts are:
1. I need to buy a "genuine" Porsche DME relay
2. Something is making the engine hard to turn when the engine is cold (although I have no idea how to prove or test for that)

i I think if I solve those two issues I would have a [mostly] working 944.

Edit: I just tried one more thing. With my new DME relay plugged in, I turned the ignition on (two clicks over) and checked the voltage at the fuel pump. Zero volts. This leads me to believe that there is a problem with the ignition wiring that is preventing the ignition from running the fuel pump, since I know the fuel pump runs when I bypass the relay.

Edit 2: @ bcameron59: you mentioned the immobilizer; I don't believe my car is equipped with this. I do have the security system that uses a secondary key, but I don't have a clicker. Currently my security system is bypassed because it would honk every time I opened or shut a door.

Does anyone agree/disagree? Thanks in advance!

Just a data point for you:

Battery voltages (no load)

12.68=100% charged
12.45=75% charged
12.24= 50% charged
12.06= 25% charged
11.89 or less=Dead

These cars NEED at least 12.45 VDC to operate properly. Anything less will not guarantee that the Immobilizer or any other part of the car will function.

Lastly, if your DME relay was made by URO, its instantly suspect as those are poor quality parts.

Steve - appreciate the what not to buy tip but what is the best choice for a DME relay? Porsche OEM? a particular aftermarket brand (and if so, who sells it)? I would default to the Porsche version but suggestions cheerfully accepted

Doug, suggest you buy one from Steve......

Gordo

Immobilizer applies to many of the 993 cars but it looks like you are posting about a 944, not sure if that was an option on those cars.

Sounds like your DME relay is bad, or the wrong one. Suggest you buy one from a different manufacturer, there are a few others besides Porsche branded. But +1 on Steve W assessment of URO quality... don't go there.

Also +1 on Steve's voltage observations, you may need to boost your car, charge the batter, and/or replace it if it's not holding a charge. If not charging properly it could be alternator belt, brushes or...

The Porsche OEM relays fail simply because of bad solder joints. Other than that, there is nothing wrong with them after you rework the relay by resoldering the joints. An easy fix that lasts.

Some background information:
944 DME Relay which is the original Porsche installed.
993 DME Relay which replaced the 944 DME Relay. Looking closely at the solder joints, I doubt it is improved.

If all else fails, replace the ground cable on the battery assuming that has not already been done. The original flat type can look good but be bad. He new cables are round in cross section.