Rebuilt my PCM at home for three bucks

[eljefino]

So my 91 F150 had a phantom miss, particularly when cold and just going off idle. It felt like all 8 cylinders dropping out for a split second. I tried a rebuilt distributor with the old (NAPA) TFI module, then a new TFI module. No better. The CEL would blink in concert with the dropouts.

Apparently the capacitors dry out and leak on these EEC-IV computers, but the good news is, they're stupidly simple inside and only have three caps!

So I took the above picture just to keep the "negative stripes" on record so the new parts would go in the same way.

Here's a nice ugly close-up:

And here's the mess it left on the board:

So I visited my friendly local Radio Shack (it still exists!) and they didn't have the values I needed. (two 47uf and one 10uf. The 10 was particularly bad off.) Instead, I mail ordered them from the internet's #1 auction site. Took all my trash to the dump in case this truck was crippled, but the surgery took an hour and was a success. Idle skip went away and it shifts more softly, too. For whatever reason it up-shifted its E4OD early and threw the TCC on shortly after shifting. It still does this, I drive like an old man, but it behaves better now.

According to my reserach there are worse computers out there... cars not running at all, fuel pump relays priming for more than one second, etc. But the internet scuttlebutt is if you have one of these in your vehicle, you should change them out before the capacitor juice etches your circuit board into uselessness. Glad I got mine in time. Cleaned the surfaces with alcohol hand gel and q-tips, but there are probably other things that work too.

 

[Owen Lucas]

That's some awesome work. The gumption and knowledge to take this unit apart and change the cap, an old school figure it out fix instead of buying reman parts.

I would have been worried of the cap juice causing damage but I guess if it hasn't spread or lifted the traces it's fine.

I am amazed you still have a Radio Shack! Do they still have RC cars, Tandy Computers and Realistic home stereo equipment inside for sale?

 

[truckramrod]

This is great. I wonder if this is the issues with my brothers 91 5.0 foxbody. It Runs rich and backfires.

 

[truckramrod]

This is great. I wonder if this is the issues with my brothers 91 5.0 foxbody. It Runs rich and backfires. I've completed many small cheap repairs of obvious issues. I need to start at square 1 I'm sure.

 

[ripcord]

Awesome info!! My OBS truck does the same thing. It's had a new distributor (cam shaft position sensor) and a new TFS module in the past several years, but still has a random dropout type miss at idle every once and a while. I guess I need to check this. Thanks!

 

[SubLGT]

Is eBay known for providing high quality capacitors?

 

[supton]

Nice work. Not sure what cleans up exploded capacitor goo, but hopefully with the bad cap gone, the damage will be limited.

 

[supton]

Is eBay known for providing high quality capacitors?

No, but use them until they dry out, then replace. I do get some from there, for hobbyist stuff, but never for anything important--job like this, I would dig into my stash for the good ones. [I make a parts order from Digikey or Mouser periodically.]

Ebay has some nice NOS stuff but NOS and electrolytics don't go together really well...

 

[JTK]

You high tech computer fixin' G-ru! That's awesome!

 

[oldhp]

eljefino, That's outstanding!!! Watch Wes Work did the same thing to fix a ECU mess up!​

 

[SubLGT]

I did a R&R on capacitors in a Mitsubishi ECU about 20 years ago and I remember it being a challenge to remove the solder from the capacitors. It was a double or triple layer circuit board as I recall, and getting enough heat on the old solder, without damaging the board, was a challenge. I did not have the specialized equipment that makes component removal easy. I only had a simple suction ball to remove the molten solder.

 

[eljefino]

eljefino, That's outstanding!!! Watch Wes Work did the same thing to fix a ECU mess up!​

 

[sloinker]

Oh Yeah, I remember the Weak-IV from back in the day.

 

[paulo57509]

Similar to the same vintage Toyota/Lexus ECU capacitors - leakage.

All my crazy Lexus issues SOLVED!! (ECU-leaking capacitor) - ClubLexus - Lexus Forum Discussion

LS - 1st and 2nd Gen (1990-2000) - All my crazy Lexus issues SOLVED!! (ECU-leaking capacitor) - 430230 *photo courtesy of distinguished Japanese ECU engineer Yamae 430231 IMPORTANT: The capacitors may look perfectly fine and yet be bad, they look great at first, then deteriorate and cause damage...

www.clublexus.com

 

[meep]

I did a R&R on capacitors in a Mitsubishi ECU about 20 years ago and I remember it being a challenge to remove the solder from the capacitors. It was a double or triple layer circuit board as I recall, and getting enough heat on the old solder, without damaging the board, was a challenge. I did not have the specialized equipment that makes component removal easy. I only had a simple suction ball to remove the molten solder.

i ran into this with an EV plate amp in a powered speaker repair and got lucky - the failed parts fell off and left their pins in place - and I just soldered to the stubs instead of trying to get them out. Sometimes you get lucky. I think I dabbed some hot melt glue in between to support it mechanically, as vibration would be an issue.

 

[researcher]

yup the 90's era Hondas have this problem too. Capacitors (electrolytic type) the seals dry out and the electrolytic fluid oozes onto the circuit board..and only have a lifespan of about 20 years! You do have to catch the bad capacitors before they etch the board and destroy the traces! Half the time it's the conformal coating on the board that the soldering iron has to be hot enough to melt through.. Best to use a conformal coating removal gel, the type of conformal coating is usually acrylic with the hondas. And of course, re-apply an acrylic conformal coating after you're done soldering, keeps the board protected in the automotive environment.

 

[Number_35]

Good work! I need to be the pedantic annoying guy here - besides the three polarized electrolytic caps you changed, I also see three tantalum caps. I presume they don't degrade as badly with age.

 

[supton]

Good work! I need to be the pedantic annoying guy here - besides the three polarized electrolytic caps you changed, I also see three tantalum caps. I presume they don't degrade as badly with age.

I don't recall them aging badly. They just tend to go when abused any amount. I haven't used them for a long time, not since mono's got much better. For a while, had to be careful with them around LDO's but I think the latest LDO's don't need the ESR to be stable.

 

[ripcord]

Good work! I need to be the pedantic annoying guy here - besides the three polarized electrolytic caps you changed, I also see three tantalum caps. I presume they don't degrade as badly with age.

Tantalum caps don't degrade over time in the same way. They can blow from voltage spikes.

 

[eljefino]

Good work! I need to be the pedantic annoying guy here - besides the three polarized electrolytic caps you changed, I also see three tantalum caps. I presume they don't degrade as badly with age.

You're technically correct, the best kind of correct!