PCM Replacement?

[89SWB]

Can a PCM just go bad with age? My 94 has 280,000 miles, has had ongoing electrical issues since I bought it 2 years ago. Latest to crop up is the glow plug relay is not getting a consistent signal to activate at startup from the PCM. I can hook up ScanTech and activate the glow plug test - sometimes it works, most times not. Not getting any light to come on the dash for glow plugs warming at startup (what started the whole diagnosis). Went through the diagnostic tree in the shop manual, ended at 'bad PCM or connection.' :mad2:

With the cold weather we've been having in Georgia (30s - 60s, not as bad as lots of other places!), the engine is turning over and over, but not firing up. Confirmed that I am getting fuel to the IP through the lift pump and the engine shutoff solenoid is getting power. Thinking the cold block is really not helping. Going to try and bypass the relay to heat the glow plugs with a jumper wire for a few seconds to test my theory tonight.

How difficult is swapping out a PCM on a 94 turbo? Any good sources for new/used/rebuilt? About how much should it cost? Going to scour ebay later today as well.

 

[chrisk1500]

It is an electrical component. Motherboards in computers fry all the time.

That being said, it is a rare occurance in these trucks to have a PCM go bad (but they do).

Swapping out the PCM is easy. Open the glovebox, take out the screws holding the liner in place, look to the right and you will see the PCM. Give it a pull to the left and towards you and it will slide out.

The last time I bought a PCM I was able to get it at a local parts place. I just had to put my own EPROM in it before I installed the PCM.

 

[Turbine Doc]

:

With the cold weather we've been having in Georgia (30s - 60s, not as bad as lots of other places!), the engine is turning over and over, but not firing up. Confirmed that I am getting fuel to the IP through the lift pump and the engine shutoff solenoid is getting power. Thinking the cold block is really not helping. Going to try and bypass the relay to heat the glow plugs with a jumper wire for a few seconds to test my theory tonight.

.

Where in Ga. ??? anywhere close to Waycross ??? I'm there most days before suspecting PCM, I'd be looking at glow controller, or the ECT sensor that tells the PCM how cold engine is and needs for glows to come on, those fail more often than PCM does

 

[JMJNet]

Tim is probably correct.

How old is your GP controller and how old is your GP?

 

[micky_blue]

Its not a diesel but I had the computer on a 99 tahoe take a dive. I was a bit leary when stealership told me that the computer fried itself but I called westers rather then a stock replace and the truck runs like a top



micky

 

[89SWB]

Tim - you're deep South! Waycross is about 5 1/2 hours away from Rome, up in NW Ga. Beautiful country down there.

Is the glow plug controller the relay? ECT sensor is about 1 year old, less than 5000 miles. Glow plugs are also new, replaced last summer with only 1000 miles or so until it got laid up in October with these little ongoing issues.

Here's the process I went through yesterday, thinking I had an issue with the glow plug relay:
- plugged in GMTD Scan Tech and cycled it through the glow plug test cycle. Sometimes I got voltage readount on the feedback panel, most times not.
- confirmed good ground on all glow plugs on driver's side
- removed GP relay connector, confirmed B+ on WIF lead, good ground on the third line, per the service manual diagnostic check

At that point, I checked the line coming from the PCM to see if it was getting voltage when I cycled it through the test cycle. Nothing while I had the connector off.

All the manual says at that point if you reach is is bad PCM or connector. Not all that helpful.

Ideas?

 

[buddy]

When you swap your PCM you have to swap the E-PROM in it. Which is also easy, but if the E-PROM is your problem, most likely if its a bad PCM, then you need a new E-PROM too, which is more expensive. The PCM can be had new or remanufactured new for only $150, but the E-PROMs I dont know if you can still buy a stock one new from the dealer. Aftermarket are $275 and up.

You have to match the E-PROM with a compatable one, which depends on your trucks build date, and what model IP you have, is it the 5068 model, or has it been replaced with a 5521 compatable type, like 5079? Maybe 5067? So its not as easy to find a used one on ebay or in a junkyard, because it has to be compatable and a 94 C3500 is more likely to be mysterious than most models. You have to look on the E-PROM inside the PCM and see what code is written on it, a four letter code like BXPY.... After you pull out the glove compartment back, there is plenty of wire harness to pull the PCM out and lay it on the glove compartment door connected. I usually just take out the ECM fuse when I am messing with the E-PROM.

I would check continuity on the line first, from the connector back to the PCM harness where its supposed to be coming from. The line might be broken or shorting.

What other electrical problems have you been having?

Do you get any codes, does the SES light work?

Also check the grounds that go to the PCM, pull the harness off and test each ground on the harness to see that it is actually grounding.

 

[THEFERMANATOR]

The last EEPROM I bought from GM cost me $38 for the updated one. The original was about $100, but if you can find one from a TSB they are normally cheaper.

 

[JMJNet]

In 95, the GP controller is near the firewall on the driver side above the OPS.

It has two rubber boots to protect the connector from the battery and to the GP.

 

[gmctd]

Intermittent connections give same symptoms as intermittent modules - R&R the PCM connectors, noting any oxidation\corrosion amongst the pins - clean as necessary - if PCM replacement is required due to failing driver, use your original eprom - boneyard PCM is useable with your eprom if you match oem transmission type: auto for auto, manual for manual - PCM is flashed for manual or automatic trans - '94 1500 still used 4L60E (700R4), all 2500\3500 used 4L80E - '95 1500 used 4L80E - at this late date, WYSIWYG, but PCM must match transmission type that currently resides in your vehicle

 

[Acesneights1]

Definitely cleans all grounds. I have seen first hand the electrical nightmares a dirty or broken ground can cause in these trucks.

 

[89SWB]

Thanks for all the tips! I went through the grounds in a big way when I first started having issues a year ago, but will not hurt to check again. Going to be near Avant Salvage on Thursday, they always seem to have 6.5 parts, might just pick up a PCM while I'm there to save a trip if a good cleaning can't figure it out (and will need to beat into my head to make sure its for a manual transmission).

I think we're using the same terms on the GP controller / relay - the relay is mounted on the firewall next to the fuel filter. It has been testing good, but not getting a consistent signal from the PCM, best I can tell.

Love this truck, but hate seeing it stuck in the driveway!

 

[buddy]

Since you didnt know how to swap PCM Im sure you haven't checked all the grounds that it is supposed to have.
Like these grounds on the harness
Pin# ckt# color
D1 551 tan/white
C1 450 black/white

When ignition is on, power B+ should be on
A1 439 pink/black

The glo-plug control is
C13 505 Yellow which goes to pin B of the Glo-plug relay

With the ignition on you can test between A1 and the grounds with a multimeter to verify you have 12V across each.

 

[Rafedial1]

94 PCM

if you do find out you have a bad PCM, I have a 94 PCM from a Vin F 6.5L turbo diesel truck. The truck had the #5068 pump. I have since parted it out.
If you want to give it a try i'll send it your way.


Cody

 

[buddy]

Rafedial1, I might be interested in that PCM and E-PROM, did yours have auto or manual tranny, and if his wasnt 5068 wouldnt be compatable.

 

[Rafedial1]

buddy, yes the truck it came from had the 4L80E trans. If the OP needs it, he's got it. If not, I'll send it your way.

OP, any updates?


Cody