[Jazzbass]

The 944 rewire and Andy's comments got me researching DME relays. Since I also have one of those damn things in my car, I did some research into the problems.

The DME relay is really 2 relays in one. The problem is not really the relays themselves, since relays tend to be pretty trouble-free - but the printed circuit board they're attached to. All of the heat, vibration, etc, of the car takes its toll on this board and the solder joints on it start to fail. The solder job on these is pretty crappy to begin with, so it doesn't seem to take much to make them fail.

Here's a pic of the DME relay that was in my car when I bought it:



Inside - showing the relay side. You can see the two relays - the one on the left is the fuel pump relay and the one on the right is the main DME/fuel injector relay.



The other side showing the PCB that the relays are soldered to. This is typically where the problems occur.



A quick inspection shows that sure enough there is one solder joint that has failed (red arrows) and several I'm not too happy about (blue arrows). Note the solder joint that has failed has a ring around the pin - this is a crack in the solder joint. Cracks are caused by vibration of the component (the relays are heavy and then to crack their solder joints quicker than smaller components) and heat/cool cycles expanding and contracting the solder.



Here's a close up of the bad solder joint, with the red arrow pointing to the crack.



Fortunately, its an easy problem to fix with a soldering iron. Here I'm reflowing the solder on all the joints. When I do this I add a little bit of new solder, but not too much. The main idea is to completely melt the existing joint. When it cools, all the cracks will be gone. NOTE: Don't blow on the solder to cool it! This can cause the solder to cool too fast and you get what's called a "cold solder" - a joint where the solder doesn;t adhere properly to either the component, the board or both.



All the joints reflowed. Note the excess flux on the board. I could wash this off, but I'm lazy and don't feel like it. Not a big deal anyway.



There you go - DME relay repair made easy!

[boardman]

jeez chris most of those looked like cold joints before ya fixed em!! im glad to see ya popped it open so everyone could see how easy it is to fix someone elses crap solder work! i see this junk every day in the audio business

[Jazzbass]

Originally Posted by boardman

jeez chris most of those looked like cold joints before ya fixed em!! im glad to see ya popped it open so everyone could see how easy it is to fix someone elses crap solder work! i see this junk every day in the audio business

My first real job in high school was repairing TVs (16 years ago). 50% of the work I did was reflowing cold/broken solder joints, the other 50% was replacing rectifier bridge diodes in the main power supply after people left them plugged in during a thunderstorm. I made $5/hr, they made $50/hr in labor .

[Gator_86_951]

I dig the soldering skills. Can you do surface mount parts? Guess it depends on the packages sizes. I was never very good at soldering, actually I am horrible with a capital H. I had to bribe the soldering lady at work with star bucks when I let the magic smoke out.

[Andy Clements]

Nice photo tutorial Jazz! I've fixed a few of those over the years myself. I always tell guys to save the old ones 'cause they can usually be repaired as a spare. Why is it that the 911 part has to be all fancy and use the round pins instead of the conventional blade prongs us lowly FEWC's get!

I knew the DME relay was basically just a dual relay in one can....but I was not aware of the explanation you found from Wong regarding engine RPM and the safety shut-off for the fuel pump. You can see why I thought a combo of toggle switches could replace the relay, but it's a bit more complex than that. I guess it would be good practice to open and inspect the relay annually for pending joint cracks that might ruin a race.

So any further verification on the kill switch function via disabling the 12V+ (pin 86) on the 944 relay......Mr. EE???

Nice research Chris!

[Lupin..the..3rd]

Thanks for the writeup and nice photos Chris!

[William Miller]

Nice!
Andy, BTW, the DME relay for the 964 and 993 also have the blade style connectors as do most of the other relays in the car.
The tiney terminals are for the coils and the larger (normal size) terminals are for the load that is switched.

Chris, I tried, but failed to re-melt the solder on a curcuit board that held a connector on the toy train just before x-mass.
To say the least the board was bad or I screwed it up. I may need your services when the parts come in!

[}{arlequin]

crap.... looks like a need to own a soldering iron... nice post jazz

[Jazzbass]

Originally Posted by }{arlequin

crap.... looks like a need to own a soldering iron... nice post jazz

I use a Weller WTCPT. Its one of those things that's on the pricy side, but worth every penny.

[}{arlequin]

LMAO.... i knew i could count on you on saving me from 'wasting' my $11 at radio snack