Mitsubishi: 90 Eclipse GSR4 repaired ECU diagnosis

Well, I was baffled by this for quite a while. The ohmmeter (one of those
business-card-sized digital units) indicated continuity (beeps) on all the
major traces, but I wasn't getting any voltages much beyond the where the
connectors enter the card, when I powered it on the bench.

It finally dawned on me that the ohmmeter will beep with some resistance,
perhaps up to 10 ohms. The resistance of the entire card (through all the
ICs and discrete circuits, etc.) is lower than that, so I wasn't measuring
along the +5V trace--I was measuring between 5V and ground.

It appears that the failure mode you observed is in play here, too. The
trace seems to have failed under the big electrolytic cap nearest the
connector, where all the brown crud was. I will put on new electrolytics.
I'm ordering high-temperature (industrial/military grade) caps with -40C
to +105C temp range, just as were on it. It doesn't make sense to use
consumer grade caps in this harsh environment. And they only cost a few
pennies more.

I'm reserving judgement until the new caps come and I do the replacement,
but it looks like your description of the failure mechanism is right--the
heavier components are cantilevered off the board, held by cement.
Ultimately vibration and bumps cause them to pull the foil from the board
and open it.

It amazes me that Mitsubishi was able to make this on a two-layer board.
I expected a four-layer board.

Larry

Hopefully thats your problem. Although if the caps aren't leaking or
bulging, you could repair the trace and fire it up for a quick test.
How would you know it was working ? Check the diagnostic output pin for
pulsed signal.

Good Luck




"Lmarks" <com> wrote in message
news:talkaboutautos.com... 

I replaced the four electrolytics today. There was brown crud under two of
them. Either the adhesive they use is corrosive (as you suggest), or the
electrolyte is leaking and corrosive.

While I had the big one out that's near the connector (50V, 47uF), I
looked further for the open circuit. I never could see it, but I located
where it had to be and repaired it. The main trace comes in with 12 volts
on two adjacent pins. This trace goes through a T-filter. The next via on
that 12-volt trace is a reverse-biased diode to ground, to protect against
reverse polarity (car battery installed backwards).

Using a real analog VOM (not the beeper) I found continuity from the 12
volt connector to through the filter, but not to the diode. It looked
like there was corrosion around the filter lead. So I took a short piece
of AWG 18 wire and bridged from the filter to the diode and suddenly 12v
and 5v appeared in the right places.

Had I thought to look here first, I would have also checked the diagnostic
output. It's back in the package now, so I'll just install it and try it
tomorrow and report back.

Thanks a lot, Nirodac.

Just to finish the story, I installed the computer today and the car
started right up and runs fine.

It is more than wishful thinking that the car actually runs better than
before the failure. I expect there may have been some noise due to caps
going bad that was affecting engine performance above 4000 RPM. It now
winds smoothly to 6000 (not something I do very often--I'm nearly 60 years
old).

Nirodac, thanks a lot. If you see this, drop a note to the Hotmail
account listed above.

Larry Marks