Car randomly dies. Tap DME it runs again. Not solder joints
10-10-2014, 08:37 PM
#1
Racer
Thread Starter
Car randomly dies. Tap DME it runs again. Not solder joints
I'm having an issue with my supercharged 944. The car will just randomly die. If I tap on the DME, the car will fire again, if I lightly tap on the DME while it's running the car will die.
I did a search and came up with cracked solder joints to the ignition transistor, great. I opened my DME to see perfect solder joints, so I flowed new solder in there anyway. Car still dies.
What's next in the DME that I should look at? Should I, can I replace the big silver transistor?
I did a search and came up with cracked solder joints to the ignition transistor, great. I opened my DME to see perfect solder joints, so I flowed new solder in there anyway. Car still dies.
What's next in the DME that I should look at? Should I, can I replace the big silver transistor?
10-10-2014, 10:06 PM
#3
Burning Brakes
Here's a thought:
If you can get the car running with the DME open (carefully insulate both boards from metal surfaces with thin bubble wrap or plastic) and then gently tap different areas of each board with a plastic or non-metallic rod (wood is good).
Start out with very light tapping covering all of the board. If the car doesn't die, tap a little harder until you find an area that is "sensitive" to your tapping. Then look at the solder joints around that area for cracks; if you don't find any, resolder as many as you can.
Larger and heavy parts tend to crack more. The idea is to localize the problem area as well as possible so that you don't have to resolder the entire board.
Happy hunting!
If you can get the car running with the DME open (carefully insulate both boards from metal surfaces with thin bubble wrap or plastic) and then gently tap different areas of each board with a plastic or non-metallic rod (wood is good).
Start out with very light tapping covering all of the board. If the car doesn't die, tap a little harder until you find an area that is "sensitive" to your tapping. Then look at the solder joints around that area for cracks; if you don't find any, resolder as many as you can.
Larger and heavy parts tend to crack more. The idea is to localize the problem area as well as possible so that you don't have to resolder the entire board.
Happy hunting!
10-10-2014, 10:35 PM
#4
Racer
Thread Starter
It dies when I barely tap it. A very, very light touch will cause the car to die.
The DME is in great shape internally but it did 2 two trips to Colorado via the US postal service. My guess is something is loose or broken but damned if I can find it.
The DME is in great shape internally but it did 2 two trips to Colorado via the US postal service. My guess is something is loose or broken but damned if I can find it.
10-14-2014, 12:14 AM
#7
Burning Brakes
DigiKey in Minnesota sells BU941 automotive ignition transistor for about $3.50 each. Google for other sources, but prices will be similar.
You'll need a ground wire, as the BU941 has two pins, vs. three pins on the original Bosch 1012 transistor which is probably what your old Motronic has. Bosch 1012 is out of production. Ground wire goes from the BU931 housing to the middle wire on the circuit board. Note that there is is a mica washer between the ign. transistor and the aluminum heat sink, and you need to use thermal grease to conduct heat from the ign. transistor, which runs up to 175 degrees Celsius, pretty damn hot. Grease the little alumium cubes on both sides, so they conduct heat from the heat sink to the aluminum chassis, and Locktite the screws so they stay very tight. That way, heat flows properly from the ignition transistor trigger out through the body of the box, or else the ignition transistor gets too hot and fails.
The coil works and shoots the spark to the spark plugs when the ignition transistor completes the grounding circuit. Without that, you're dead in the water.
Car should be good for another few decades with this mod, and reflow of solder joints. Note that there are two heat sinks in the Motronic, one opposite the ignition transistor, and the solder connections to these have the most thermal stress, so are the most prone to faillure, so resolder all such joints carefully. $3 soldering pen from Harbor Freight works well.
You'll need a ground wire, as the BU941 has two pins, vs. three pins on the original Bosch 1012 transistor which is probably what your old Motronic has. Bosch 1012 is out of production. Ground wire goes from the BU931 housing to the middle wire on the circuit board. Note that there is is a mica washer between the ign. transistor and the aluminum heat sink, and you need to use thermal grease to conduct heat from the ign. transistor, which runs up to 175 degrees Celsius, pretty damn hot. Grease the little alumium cubes on both sides, so they conduct heat from the heat sink to the aluminum chassis, and Locktite the screws so they stay very tight. That way, heat flows properly from the ignition transistor trigger out through the body of the box, or else the ignition transistor gets too hot and fails.
The coil works and shoots the spark to the spark plugs when the ignition transistor completes the grounding circuit. Without that, you're dead in the water.
Car should be good for another few decades with this mod, and reflow of solder joints. Note that there are two heat sinks in the Motronic, one opposite the ignition transistor, and the solder connections to these have the most thermal stress, so are the most prone to faillure, so resolder all such joints carefully. $3 soldering pen from Harbor Freight works well.