DME Relay failure - Prevention and Repair

[Steve W]

I had the mysterious and intermittent non-start problem in my 3.2 a few months ago traced to the infamous DME relay. Replacing it with a new one for $45 from the dealer, I kept the old one as a spare in the glovebox until I decided to open it this weekend to analyze what causes its intermittent failure. Figuring it could only be one of two causes, either a weaking relay coil, or a break in the circuitry, I took a magnifying glass to examine the relay. First off there are two relays enclosed soldered onto a single fiberglass circuit board surrounded by a few simple components such as a resistor and diode. One is the main relay and the second is the fuel pump relay. Its function should the car be in an accident, is to shut off the fuel pump relay and thereby prevent the fuel pump from dumping raw fuel. A close look at the relays showed them in good health with no broken coil wires, overheating, or worn out contacts. However a close look at the underside of the circuit board revealed all the causes of mysterious failures. Close examination of the solder joints on the relay farthest from the plug terminals, the fuel pump relay, revealed that almost every single one of the five or so joints had a hairline crack in the solder encircling each entire joint. With a digital multimeter, I actually measured no contact at two of the joints between the relay and the circuit board trace. Those two points though were relay supports and not part of the circuitry. The other three points which were also cracked all the way around, however at the time, had measured continuity. But because they were cracked, they would cause intermittent start or no start conditions. I think these joint failures are caused by two conditions

1) poor quality soldering or solder at the manufacturer, and

2) the way the relays are mounted in the car cantilever them horizontally off the circuit board placing lots of stress on the low quality solder joints, especially with cars that have stiffer suspensions or stronger vibrations. Over time, each joint failure will place additional stress on the remaining joint, accelerating their failure

So my repair was simple. Pull out the soldering iron, and reheat all solder joints - THOROUGHLY - making sure the heavy copper relay plates are sufficiently heated to prevent a cold solder joint, and preventing any movement until the solder has naturally cooled. I also added more solder to strengthen the joint. Good and better than new, and back into the glovebox it went.

Even if your DME relay hasn't failed yet, it could be on it's way and you can prevent it. My suggestion for those that know how to use a soldering iron is to resolder and reinforce all your solder joints in your relay to avoid being stuck at the worst time. You may be surprised at what you find and will save yourself a lot of $$ and headaches in the future. And if you don't know how to use a soldering iron, well I guess carry a spare.

[ChrisBennet]

Excellent info Steve!
-Chris

[BGCarrera32]

"...revealed that almost every single one of the five or so joints had a hairline crack in the solder"

Good info. I'm not suprised considering the relay is screwed right to the floor, where the drivers door gets slammed shut a few thousand times...oh yeah...and all the road shock through the frame...I wonder if a little creative dampening of the electronics down there might be in order...

Thanks for the info

-BG

[Jdub]

Where exactly is the DME located and did this go into all model year cars? It may be time to think about remounting.

Thanks for a great thread,
John

[Rot 911]

Originally posted by Jdub
Where exactly is the DME located and did this go into all model year cars? It may be time to think about remounting.
Thanks for a great thread,
John

The DME started with the Carreras in 1984 and can be found under the driver's seat.

[Silveresrty911S]

84-89 cars with Motronic systems.
Location.

Right under your "arse"

[Wil Ferch]

I've put soft rubber washers under the four mounting studs that hold the DME in-place. Looks like a soft rubber block, or such, under the cantilevered relay might be a good thing, too !...
--Wil Ferch

[gchappel]

Another question- does the intermittent starting problem related to the dme interfere with the starter itself- or will the engine not run. I have had a very intermittent problem with the starter not turning over- we have changed the starter, checked the ignition switch, etc. It now only does it about 3 times a year, and self corrects itself immediately- which is even making it harder to find.
thanks
Gary

[Mike Feinstein]

With a bad relay it'll crank until the battery dies, just won't start.

[Dutchie]

Great write up Steve tnx for that, and Wil the rubbers under the dme and relais is a great idea as well

[Steve W]

Thanks guys. Had I known about this problem, I would have just resoldered the joints and saved myself $45 and a lot of headaches with all the times I was stranded on the road. I think the most important thing for all 3.2 owners to do with respect to the relay is to just resolder all the joints to prevent these sort of problems. Removing the existing solder and replacing it with some decent quality solder may also help, but adding a little more, and heating it right is probably most important. The relays are a relatively heavy mechanism of copper plates and wire coils, and they place a lot of stress on those solder joints mounted horizontally like that. Wil's suggestion is also a good one, can't be too careful.

[ZOA NOM]

It should be understood that "more" solder is almost NEVER better. The preferred method would be to remove the old solder by suction or wicking, and re-solder the joint with new solder. Adding solder to the existing may cause shorts across foil runs, or other unwanted currents.

[movin]

Rebuilt DME relays anyone? Don't throw it away have it rebuilt for a fraction of the cost of new.

[RazorRacer]

Steve, will you do the same analysis for a fuel pump relay?

[Siena911]

Well done Steve, and thanks for the heads up.

Cheers Jakes

[Steve W]

Originally posted by RazorRacer
Steve, will you do the same analysis for a fuel pump relay?

I wasn't aware that fuel pump relays had defect problems. Those round relays mounted on the fuse box are usually pretty reliable and don't go through the same type of mechanical stress because they are a single relay mouned onto the relay's main base. I have found one failure inside one of those relays long ago due to a fine coil wire which broke off from it's post. Resoldering it back fixed it.

[PatrickB]

While I have your ears guys...
Might my problem be related to the infamous DME Relay? I've got an '85 3.2 with an intermittent rough running problem.... Occasional rough idle, loss of power or bog around 3-4,000 RPM under light to medium throttle, and occasionally under full throttle...
Any thoughts?
Patrick

[rcaradimos]

http://forums.pelicanparts.com/showthread.php?threadid=185563&highlight=dme+relay

If you can get to this post we have some pic's on relay and testing.

[Wil Ferch]

Partial running problems?....would think it's one of the 2-3 sensors that help operate the engine( speed and reference sensors....maybe even O2 sensor if you're still running closed loop as stock).

Wil

[beamonk]

My friend who is a german car tech has been repairing solder joints in MBZ fuel and AC relays for years. I have one in my MBZ now, BTW. He sees the same thing, hairline cracks, thinks it might be from the heat, but who knows.