im chasing an unknown code.

95 Silverado 2wd 5.7 tbi 4l60e

When i jump the aldl with a paperclip, i get code 12 like I'm supposed to.

After the check engine light is tripped, jump the pins and i don't get a code. I get 10 seconds of no light, 5 seconds of solid light. The pattern repeats itself as long as you leave it jumped. Do not get the code 12 like im supposed to.

Any ideas? Anyone seen this before? My gut says bad ecm or something, but that doesn't seem right because its fine otherwise.

additionally, i had thought i was done with tbi stuff and got rid of my aldl cables, chip burners, etc. Who makes a good usb to aldl cable these days, and which version?

Moates.net extreme aldl is a good setup. Sounds like you have a bad ecm.

Wait...you have jumped the pins in the ALDL to put the ECM into diagnostic mode.

What is this second jumping of pins that you mention?

Tuned: He says he's working on a '95. Did the trucks get OBD 1.5 in that year? I know some cars did. If so, would that impact the way you retrieve codes? I'm aware of 1.5, but don't really have much experience with it.

I think your right I forgot about 1.5

I just don't know which platforms got that for '95...a few, some, all? I know FWD A-bodies did. But that doesn't help much here.

I think some platforms in 95 where still a 12 pin connector.

For clarification:

no check engine light: jump a&b pins, put computer into diagnostics mode: code 12 blinks properly.

after check engine light is set ( seems to be set by light throttle, torque converter locked, 60+mph) if i jump the pins, no code 12. Just the long cycle of on-off light. Not blinking any cofe, even 12 signifying diagnostics mode.

its a 12 bin 8190 baud i think. Not 100% certain, but definitely not obd1.5

If he has specs like my truck which sounds like he does, its a 7424 ECM with BJYL prom. OBD1 still.
Take it over to autozone and have them scan it with their reader. Dont buy any parts from them lol, just waste 5 minutes of their time to see if a reader picks up anything.

OK, so you're losing communication once a code is set?

Here's what I think would be the proper chart for that from my '90 FSM. Note, this is the TBI 305 page, but the steps are the same for TPI.



Not much here...circuit, or ECM. Strange that you would have communications with no code, and lose if later.

Tuned or any one else with thoughts on this?

I think it might be a bad ecm. Looks like there are several service numbers for ecms used in 95. Check your service number and search eBay or get a reman ecm.

thanks! Going to give this flow chart a try this weekend. Don't have time till then. But it is really strange behavior, and only happens after the cel is tripped via drive cycle. Almost has to be ecm, unless theres a short hat happens after a bump/frequency and corrects itself but doesn't clear the cel.

So the plot thickens a bit.

check engine light came on today. Same cel behavior with long blinks/pauses during the drive cycle. Key on, engine off, code 12. Restart truck and cel stays on, did not self clear like normal. Same long blink cycle. Repeated this test multiple times. Left truck off for about 45 minutes, code cleared. Leads me even further towards ecm being bad. Thought?

If you have a 427 ecm just transfer your memcal.

https://m.ebay.com/itm/1994-95-CHEVR...cAAOSwrlBZjieW

Ive gotta tear the dash far enough appart to figure out which service number it is. Doesn't look like its going to be easy to get to.....

You should be able to just remove the glovebox insert.

You dont even have to take it out, its labeled on top of the box. If you can read upside down.

It is a 7427.

i have a 6395 ecm from a 94 suburban in a box of tbi conversion parts. Will this interchange? Im assuming that the 7427 prom would need to be retained.

i also have an ebl, but that wont run the automatic unfortunately....

I think you can exchange the two ecms just swap out your memcal.

Are you leaving the jumper in place with the engine running?

Only long enough to observe code behavior

The slow flash with engine running in diagnostic mode indicates normal closed loop operation.

I love hanging around people who are smarter than me.

Did I say something wrong? Just trying to help.

Absolutely not. It's been so long since I've used a jumper wire on the ALDL to assess open/closed loop operation I forgot it was possible.

On the one hand, God Bless scan tools. On the other hand, God Bless guys who remember the no-tool shortcuts.

Even with that light behavior signifying normal operations, shouldn't it also display a code 12 or another code if the light was set during drive cycle?

ive finally ordered a new aldl cable too. I want to get to the bottom of this.

Key on, engine OFF.

Correct, supposedly the only difference with all the OBD 1 red/blue is whether it can do electronic multi wire EGR. I still have never seen anything different hardware wise between the 7427 and 8395. The rumor is the 1993 8395 could not drive the 454 "electronic" egr. I have disassembled many many pcm's from many years and still haven't seen a change in hardware, it would be good to know if I could find one. All I have ever seen is in the memcal.
KOEO by Schurkey is what you are seeing between memcal's and not hardware. I'd like to know if anyone else does.