PICS - Failed DME Relay

Steve W · Steve W's Garage

Thought you guys might be interested in these. Most of us have heard about the infamous failing DME relays in the 3.2 Carreras, and I believe they also fail in the 944s, but have rarely seen pictures of how and why they fail. This one was pulled out of a '85 Carrera, after having been replaced in 1998. It is only 6 years old. The red arrows point to all the failing and cracked solder joints that develop over time with vibration, door slamming, and the cantilevered loads caused by the horizontal mounting angle of the heavy relays on the poorly soldered joints on the circuit board.

Though I haven't fully linked it yet, I wrote up a web page with a more details here if anyone is interested.

vash · vash's Garage

so it is repairable?

Jim Smolka · 07-07-2004, 09:52 AM

It looks like a short term fix could be to resolder the connections.

However, most relays have a designed number of times the relay contacts can be opened and closed before they fail.

Oh, and good job on the photos.

Steve W · Steve W's Garage

I think some guys have by resoldering the joints. On this particular relay though, one of the relays seemed to suffer some heat damage, because the red enamel coating on the coil wires of one relay seemed faded in relation to the other.

I've though, for example, took my perfectly new relay, removed the original solder, and resoldered it with a sufficient quantitly of quality 2%silver solder. Not sure about the solder quality used in manufacturing, but I also attrubute the cracking to a high load distribution on a relatively narrow solder profile. You can see that the cracks have propogated where the profile is narrowest. Increasing the amount of solder at the critical joints should distribute the stress more widely and decrease the probability of failure.

BGCarrera32 · 07-07-2004, 10:24 AM

Not that what I am about to say is new info or anything brilliant, but I do believe a lot of the failure is due to door slamming/closing and substantial shock (for an electronic component anyway) through the floorboard like Steve noted. I have placed some soft durometer neoprene washers under the mounting tab of both my relay and DME, while not tightening them down completely (but good enough for a solid ground). Hopefully it isolates *some* vibration and shock to the units themselves taking the edge off...

david914 · 07-07-2004, 02:05 PM

I have witnessed that kind of solder joint failure on "non-moving" (non-automotive) components many times. From my experience, the problem has more to do with components that carry an excessive amount of current. The component, and subsequently the joint, heat up thermally cycling the solder. Over time this causes the joint to separate. The permanent (or at least, longer term) fix would be a heavier relay assembly. As with the A/C circuit in these cars, the DME relay circuit is marginally sized.

tiorio · 07-07-2004, 02:45 PM

Ok. So I have some overboost protection/secondary fuel pump relay that failed on my turbo. I kept the old one 'just in case' as the problem was very intermitent.

Here's the question:

How the heck did you open that relay up so nicely? I've been more than a little wary of breaking the thing while opening it, what was the trick (or tricks)?

Thanks for the nice pics BTW, very interesting!

soup dragon · 07-07-2004, 02:48 PM

use a fine flat blade screwdriver and the black cover just unclips.

bigrubberjeep · 07-07-2004, 02:59 PM

Great pics Steve!

Steve W · Steve W's Garage

Quote:

Originally posted by bigrubberjeep
Great pics Steve!

thanks ... it's amazing what you can do with a scanner

bigrubberjeep · 07-07-2004, 08:32 PM

Quote:

Originally posted by Steve W
thanks ... it's amazing what you can do with a scanner

Scanner? Good idea!

MOMO3.2 · 07-07-2004, 09:04 PM

Thank you Steve. You are a great resource on this board.

Sincerely,

Mike

rick-l · 07-07-2004, 09:26 PM

It doesn't look like that board has plated thru holes and ferrules on the top and bottom? It looks like a single sided etched copper board.
What a poor design, expecting a 1 oz copper layer and solder to hold a relay on in a high vibration environment.
I bet if you remove the solder some of the copper foil will come off. The solder cracks are larger than the drilled hole. I would also bet thermal cycling had something to do with the solder/foil cracks.

Zendalar · 12:47 PM

One can see exactly the same kind of failed solder joints in home electronics, for example in televisions.

Thermal stress with help of current causes these kinds of problems.

If it were mechanical stress, the solder would fail at the point where 2 different metals touch each others. Or it would fail at the circuitboard where copper and "fiberglass" or whatever the circuitboard is made of, the solder rarely fails due to mechanical stress.

These faults may appear where there has been used low quality soldering tin (or the paste in the "tin"). If any of the soldering paste is inside the "joint" the soldered point will fail at some point.

just my thoughts (been fixing electronical devices all my youth and adulthood)....

...but on the other hand, it might be heavy relays