Junkyard Engine Computer Issues

I think the PCM (powertrain control module) in my 1998 Grand Voyager,
3.3L w/anti-theft is bad, and I'd like to swap in one from a junk yard.


In reading what I can find on the net, it seems like this should work.
It sounds like there are problems with the BCM (body control module) if
it does NOT have the anti-theft option and the replacement PCM DOES.
But I do have anti-theft, so it sounds like if I get a non-anti-theft
PCM it will not damage anything.

However, I'm wondering about my VIN number and odometer readings. Will
those come from the junkyard PCM? If so, how can that be corrected?

It would be worthwhile to me to verify that the problem is the PCM by
swapping in a used one, even if I have to buy another one from the
dealer or pay to have the junk one reprogrammed.

The junkyard PCM's can be had for ~$100, sounds like one from the
dealer will cost many times that.

I think my PCM is bad because it behaves erratically even when I just
turn the key on, with the ASD, fuel pump, and purge valve relays
clicking wildly.

Battery is good, cranks well and measures 13.4V after charging. Cleaned
battery cables, checked fuses, reseated power distribution and PCM
connectors.
-Paul

 

Try Ebay first. I bought a Honda Accord PCM for 29 bucks.

 

Update on this. I searched for compatible engine computers online:
http://www.car-part.com/

and found 10 or so. There was a huge variation in price, from $500 to
$65, I ordered the $65 unit from All West Coast Auto Wrecking in
Washington.

They sent me the wrong part, a transmission computer. Easy mistake to
make, that one is on the right fender, the correct part is mounted on
the left fender. I figured I'd get the run around, but they said they
had the right part, and just send back the wrong one, which I did. I
expected it to take a week for them to receive my package and send me
the correct part, but they shipped without waiting so I got it in 3
days.

I plugged it in, and it fixed the problem. So I guess I still have to
buy one from the dealer with the correct VIN programmed, but I can take
my time and won't have to pay a fortune for troubleshooting and having
the wrong parts replaced. In fact, I may take a crack at the bad
computer. I'm an electronic tech by trade, if I can get the potting
compound off I may be able to troubleshoot it. I suspect a voltage
regulator, since the problem affected all parts of the computer.

Oh, I should mention that in searching the net, I found that a lot of
people with a similar problem had fixed it by reflowing the solder on
the instrument cluster connectors. There was a ground that tended to
crack on the 1996-2000 minivans. Their van would suddenly quit and
refuse to start, and they could get it going by pounding on the dash.
So I pulled the instrument cluster, but could see no fractured solder
joints. I reflowed the solder on the cable connectors anyway, but it
didn't help in my case.

Hope this helps someone down the road with one of these problems.
-Paul

 

--

Actually if you take the back cover off, plug the PCM back into the vehicle
and wiggle the aluminum tower like thing that looks like cooler (I have no
idea what it is) the engine will probably start and run until you wiggle it
again, this is a common problem with those PCM's. Even the ones that MOPAR
Remanufactures (by a sublet company) get put back out on the market with
these problems. Try it and let me know if it starts

Glenn Beasley
Chrysler Tech

 

That's a good tip, I can see you know your stuff. I tried that already,
I found some info online that said the solder joints on the connectors
can crack, so I was in there pushing on everything thru the potting
compound while my wife worked the key. Didn't make any difference.

Something is drawing a lot more current than on the good computer,
though, so I have a chance of zeroing in on it. Most likely it is one
of the devices connected to the "silver towers", which are heat sinks.
The parts that get hot are mounted on them, so they're the ones that
tend to fail, and they're most likely voltage regulators. Peering thru
the amber potting compound, the one at lower left looks like a
switching power supply, it probably makes 5V out of 12V.

Thanks for the reply.

-Paul

 

Well, why would you get another one with your vin encoded in it? The one you
have is working, so it would seem pretty stupid to spend the money on one to
get the vin encoded on it? Especially since your vehicle is no longer under
warranty.

 

I'm in California. I'm told it won't pass my biannual smog test which
is all computerized to prevent fraud. The car's a giant paperweight if
it won't pass the smog test.

I'm going to try to fix the old one, since I'm an electronic tech.
Meantime, I can drive about a year until the next smog test is due.

I'm actually pretty happy to have it driveable again, and know for
certain what the problem is. I pictured getting ripped off for
thousands of dollars as the dealer changed part after part.

-Paul

 

I'm in Arizona and have to go for emissions every few years also. The only
thing they are worried about with the computer, is that they can plug in the
scanner to prove there are no engine codes.

 

I'll check into it, but I think they pull the VIN from the car's
computer.
We got a stricter smog test just in the last couple of years, which now
includes a sort of dyno test.
-Paul

 

Why would they bother with a switching power supply just to drop 12
volts to 5 volts? Reduce heat? Wouldn't a voltage divider or a
regulator do the same? I'm trying to remember why these were used on
computers, to reduce weight and heat, but on a car, that increases
complexity unnecessarily? So anyway the VIN is encoded into the PCM,
the EEPROM? Can you ask your department of motor vehicles what they
suggest since this must be somewhat a common problem in repairing cars?
Or only the dealer can resolve this with reprogramming the PCM,
actually, the EEPROM? Unless it's just an EPROM. I found dental tools
were good for dealing with potted plants.

 

There are several reasons- a simple series regulator would waste more
than half of the wattage- with the engine running and the alternator
putting out 14V, it would waste almost two thirds of the power consumed
(9/14ths), converting that to heat. Then you've got reliability
problems. Switchers are very efficient. A mechanical analog is a
reduction gear. You could say, "why have compound low on a 4WD? Can't
you just slip the clutch?" Yeah, you could...

Good idea. I removed some big chunks, but this is hard work. I wonder
if paint stripper or something would dissolve that stuff?
-Paul

 

A paint stripper will, but not the liquid kind. Get a heat gun. Use it
carefully. It makes that epoxy get jelly like while it's hot.

 

I've got one of those. I'll give it a try.
-Paul

 

Nope, tried it, got it hot enough to stink but it's there's no
difference. I even tried heating up a chunk I'd pulled out so I could
really tell. Didn't soften it. I'll have to see if I can get some
strong solvents. Alcohol and paint thinner had no effect.
-Paul