Replacing Leaky Caps in a 1992 Dodge Stealth

My mom drives a 1992 Dodge Stealth (Mitsubishi 3000GT). Despite the fact that the car is almost 20 years old, it still runs pretty well, with the occasional exception. For example, recently, the window regulator (on the driver’s side) broke and had to be fixed. Then, a few weeks later, the car would stall shortly after starting it, making a clicking noise in the process.

It turns out that one of the most likely causes for this failure is due to leakage of the electrolytic capacitors on the engine control module (ECM), the computer responsible for sparkplug timing. ECMs go for anywhere from $800-$1000, so if you can identify this problem and replace the capacitors, you will have saved quite a bit of money.

So my dad and I took apart the car, following these instructions and extracted the ECM. When we opened it up, we found that two of the caps had pretty heavily leaked on the board, while the other two (yes, there were four in this car — not sure why given that every guide I’ve seen has shown 3) were probably okay. We decided to clean the board replace all of them. Unfortunately, the board was covered in a conformal coating, and we didn’t remove the coating before working, which lead to having to fix things very carefully afterwards. I think that in the end, this would have been the best way to do things.

  1. Completely remove the conformal coating in the area surrounding (and under!) the capacitors using alcohol.
  2. Desolder the capacitors from the board and use a solder sucker to clear the barrels. Note that you need a fairly powerful iron — a standard microelectronics iron won’t work as effectively as a larger iron with a chisel tip.
  3. Clean the board with more alcohol to remove the fluid that has leaked from the capacitors. In our case, it hadn’t actually corroded the board yet, so it wasn’t necessary to repair the underlying PCB. But, it was important to ensure that all of the electrolyte was removed.
  4. Ensure that the barrels are okay. It might even be a good idea to very gently sand them to reveal a fresh layer of copper.
  5. Solder new capacitors onto the board. On our board, there were two 47uF @ 50V caps, one 100uF @ 16V, and one 22uF @ 50V. We used caps that were rated up to 105C. Use lots of flux.
  6. Ensure that all electrical connections are good. Then, inspect to make sure that the caps are mechanically sound.
  7. Clean off the flux with alcohol.
  8. Apply a new conformal coating. We used acrylic coating, but maybe urethane or silicon coating would be better. As you can see below, I inspected the coating with a blu-ray laser (UV light works well too). I chose not to coat the capacitors entirely, but that probably would have been a good idea.

Removing the conformal coating before doing anything else really is a necessary first step. I thought it wasn’t, and that heat from my iron would lift the coating easily, but it didn’t. Otherwise, we were able to create pretty decent solder joints.

Here are a few pictures of the process. Unfortunately, I forgot to take more in order to really illustrate the removal of the conformal coating, but these are sufficient. In the first two pictures, we see the leakage of the electrolyte. Next, we see the result of cleaning the area around the capacitor (again, I didn’t remove the conformal coating before desoldering like I should have). Finally, in the last two pictures, the new capacitors are safely in place and the board is ready to be installed back into the car.

4 Responses to “Replacing Leaky Caps in a 1992 Dodge Stealth”

  • Did you bleed the caps first? Are they dangerous to work on if you don’t bleed them first?

  • They’re low voltage, low capacitance caps, but the car had been off for several days anyways. Their main job is to provide voltage filtering. That is, they remove ripple and noise from the supply voltage to prevent the logic on the board from acting strangely.

  • I didn’t see where you told people to make sure that they have them in the correct way, if not, they like to blow up and make a bigger mess!!

  • Good point, Peter! Electrolytic caps are polarized, so it’s important to get the pin ordering correct. The stripe on an electrolytic cap points to the negative pin. The replacements should be oriented in the same direction as the original ones that were removed.

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