ALDL location 94 chevy half-ton

I have a 1994 Chevy 1500 half-ton Z71 4x4, with the 6.5L turbo-diesel engine. On a recent road trip, my "service engine soon" light came on. It was hot (>100F) and the light was speed-dependent. Uphill over 70mph, light comes on. Crest hill, light goes off. Reduce speed to 65mph, no light. I was not towing anything, nor was the truck heavily loaded. Guages indicated no problems.

My specific questions are:

  1. Is this vehicle OBD-I?
  2. If it's OBD-1, where's the 12-pin ALDL connector? On my old Bronco, it was conveniently located under the hood attached to the air intake... easy access. Not so on this truck.
  3. Which pins do I short to dump the codes?

I had previously found a news post detailing the procedure to evaluate the codes, but I seem to have lost it. I have searched for 45 minutes on google with every combination of "obd-1, 1994 chevy truck, aldl connector", etc. etc. etc. to no avail.

Any replies greatly appreciated.

Thanks

Dave

Reply to
Dave
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It's not really a top-post if I don't have anything you have to read below.

Okay, I found an article on the web, found the ALDL connector, shorted pins

1 and 2, turned on the key. first off, I thought I'd see a series of 12's... but I didn't. I just got 78 78 78 78 78 78 78.... kept repeating forever. Now, I have a confession to make. My truck has an aftermarket power chip in it. I'm going to ask the question "does this screw up the reporting of error codes" but I'm pretty sure the answer will be "maybe so, maybe not, no way to tell, really". I'll throw the stock one back in when I have time.

In the meantime, anybody know what code 78 means?

Reply to
Dave

I put the original chip back in, PCM was disconnected for >5minutes so should clear codes. I still get code 78 repeated forever. ???

Reply to
Dave

Can't really address your "78" code problem. Are you sure you are shorting the correct pins? You can go to

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on the home page, click on "Tech Tips" and then click on "62/65 Tips" and there is not only a clear description off how to get the codes, but also a list of OBS-I codes for your truck.

Check it out and let us know?

George Vigneron

Reply to
George

I haven't found a GM specific code 78, but code 79 is for transmission over temp , which kind of fits your scenario.

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might be of use. Whitelightning

Reply to
Whitelightning

DTC 78 - Turbo Wastegate Solenoid Fault

Reply to
George

not sure where you got 1 & 2.....a and b is more like it. Check out this link....

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report back with the code....over.....

Reply to
ShoeSalesman

Okay, boys, after I put back the original chip and drove around a bit (man oh man, what a difference that chip makes!) I also have code 33 which corresponds to low vacuum.

So, unless I was really BAD in a previous life, I've got a simple vacuum leak. Will let you know.

BTW, my PCM doesn't seem to reset itself... I had the damn thing completely unhooked from the wiring harness to swap chips for at least 5 minutes but the codes were still there. I took out the "ECM/IGN" 10A fuse for more than an hour but the codes persist. Guess I'll have to try the battery, maybe they're stored someplace other than the aluminum PCM box... I prefer the Ford procedure: disconnect the lead shorting the diagnostic pin to ground any time during the self-test and the codes are cleared. I'm also sort of surprised there's not more diagnostics in the PCM... my Ford, which was 3 years older than the Chevy, had a comprehensive set of self-tests, both key-off and key-on which tested operation of all sorts of components which otherwise might not show up in normal driving conditions.

As an additional note, every procedure I've seen for reading the codes say that I should see code 12 repeated three times at the beginning and end of my codes... that doesn't happen for me. Now I get 33-78-33-78-33-78... and so on.

Thanks for the help

Dave

Reply to
Dave

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