1993 GMC C1500 - ECM commands?

**stevenh512** · 05-02-2022

I have a 1993 GMC C1500 with a 4.3L engine, TBI and a 4L60E transmission, California emissions. I have not pulled the ECM to read any numbers on the label or the chip, but I've found that code mask $E6 works for this truck. TunerPro and ALDLDroid both identify the PROM ID as 24AF.

I've used an FTDI USB to 5v serial cable with tx and rx tied together to connect to the ALDL connector and it's nice to be able to get some realtime data out of it along with being able to check for trouble codes without having to do the paperclip thing. But I'm wondering what else I can do now that I have a working ALDL interface. Has anyone done any reverse-engineering and perhaps figured out any of the mode 4 (or other) commands that can be sent to the ECM to perform certain diagnostic or maintenance/repair tasks? I'm specifically interested in TPS and IAC relearn and bleeding or flushing the ABS module. If the truck were just a few years newer I'm sure I could do all of these things with a bluetooth OBD2 adapter and the right app, but I haven't found a lot of information on these OBD1 systems.

**NomakeWan** · 05-02-2022

Where are you located? I have a '94 and '95 Corvette, and have dabbled with those sorts of commands. I don't think the ABS on my cars is the same as what's in your C1500, however, so the commands may be totally different. That said, if you're reasonably local to me, I have a way we could work out what the commands are for your car if no one else has already.

**stevenh512** · 05-02-2022

Unfortunately I'm nowhere near Orange, I'm in the Mojave desert near Joshua Tree and Twentynine Palms.

**tayto** · 05-02-2022

your truck never had tps or iac relearn by a scantool AFAIK. i also don't think the C/K trucks went to ”real” ABS until 1994, but I could be wrong? All the stuff I've ever worked on had RWAL, which I usually remove. A knock off tech2 or older snapon tool is probably the route I'd go here... how often do you need to bleed abs?

**stevenh512** · 05-03-2022

I don't anticipate needing to bleed abs too often, but I do need to flush the system as thoroughly as possible at least once. I've had this truck for about a year and the brake fluid is really dirty, it looks like it's probably been in there since 1993. If I can do that without sending any ECM commands, then I can go ahead and do that.

I recall working on a similar truck (with an appropriate scan tool, I think it was a Tech 1) in my high school auto shop, but that was almost 30 years ago and I don't remember which commands were available. Since it does have a TPS and IAC I'd assume there has to be a way to relearn them when you replace them with a new part. If there aren't ECM commands for that, do I just disconnect the battery?

**NomakeWan** · 05-03-2022

Again, I don't know about your truck, but the Corvette didn't get the ability to bleed ABS with a scantool until 1995 with the introduction of the Bosch 5 ABS unit. All previous units had an actual bleed valve on the unit itself that you used to bleed the ABS with.

TPS relearn, however, was a thing as far back as 1992; it wasn't for the ECM though, it was for the ABS/ASR system.

**tayto** · 05-04-2022

I set IAC steps to 5-15 counts, done. the older ECMs you could jumper A&B and stroke the IAC closed, then unplug IAC,start engine and set base idle. TPS most ECMs do a key on reset AFAIK. if you want to get fussy about it you can drill the holes bigger and set the closed throttle voltage to 0.45Vdc.
ABS I have always bled brakes manually and neve6f had an issue. only time i've used scan tool was installing new units. FWIW