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ECM diagnostic terminal


popeyesteve
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My 1982 Avanti II (RQB3492) has been running extremely rich, black smoke, loading up, poor fuel economy, etc. The previous owner pulled the bulb out of the "check engine" light, annoying I guess, but I put one back in to attempt to extract trouble codes from the computer. All of the pollution control equipment is intact, with apparently an LG4 305 and electronic Quadrajet (except maybe the O2 sensor was removed when he changed to dual exhaust and eliminated the cat converter) so the system should throw come codes. These LG4 305s are no powerhouses, but are normally smooth, economical engines for everyday use. I think it is running open loop, some cripple-home default setting, so it will get around but doesn't run like it should. My problem is locating the diagnostic connector. They are usually are under the driver's side dash (like my El Camino) and ther is an odd connector under there, but it is not like the "normal" ones that GM used 1982 and later, so my scan tool plug does not fit it. Am I looking in the wrong place or did they put some kind of earlier or oddball connector in these Avantis?

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My 1982 Avanti II (RQB3492) has been running extremely rich, black smoke, loading up, poor fuel economy, etc. The previous owner pulled the bulb out of the "check engine" light, annoying I guess, but I put one back in to attempt to extract trouble codes from the computer. All of the pollution control equipment is intact, with apparently an LG4 305 and electronic Quadrajet (except maybe the O2 sensor was removed when he changed to dual exhaust and eliminated the cat converter) so the system should throw come codes. These LG4 305s are no powerhouses, but are normally smooth, economical engines for everyday use. I think it is running open loop, some cripple-home default setting, so it will get around but doesn't run like it should. My problem is locating the diagnostic connector. They are usually are under the driver's side dash (like my El Camino) and ther is an odd connector under there, but it is not like the "normal" ones that GM used 1982 and later, so my scan tool plug does not fit it. Am I looking in the wrong place or did they put some kind of earlier or oddball connector in these Avantis?

Steve

I'll assume that the 82 was like my 83 and it had an ALDL (OBD1, just like GM) connector under the drivers dash. Now, the bad news is I yanked mine out and went to Holley EFI so I have no reference point.

What I do remember is that the emissions control harness was almost completely separate from the Avanti harness. So my suggestion is to find the check engine wire from a GM ECM of the same type (Part #) and follow it back to the ALDL connector which the previous owner may have stuffed some where. when I pulled my computer It was just an OEM GM unit.

You are correct about running in open loop and we both realize that the O2 sensor is required. If the owner cut out the ALDL connector you can still ground the appropriate wire to check the codes. I can't tell you which wire as I'd need the GM manual but most libraries have them.

Good luck, Bob

Edited by Avanti83
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Thanks, Bob.

I will crawl under the dash and see if I can ferret out the wiring. I don't think the previous owner would have cut off the connector since everything else seems to be intact. That odd connector is a six plug thing and open on one side, so I don't know what the heck it is. The OBD1 connector should be a 12 pin unit and is pretty distinctive. These cars are like mystery novels anyway, since who knows if the 1982 models had 1982 mechanicals or something leftover from 1981??? I just figured that since it was meant to be a diagnostic tool, it wouldn't be hidden.

Also, since the car has a 180 degree thermostat and a re-cored radiator, it runs pretty cool. That said, the torque convertor clutch rarely locks up, so that could be related to this same issue. The computer may want more heat (195 degree thermostat?) or it just may be the open loop situation. I am tempted to trash all of it and get another distributor and non-feedback Quadrajet, but that wouldn't really improve performance much with a stock 305...and would cost more than it is worth if I can get the stock system working. Put in a 383 stroker and headers and then the story changes a lot...

Steve

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It can be difficult to say how old some of the equipment was that went into Avanti production. They supposedly purchased engines in a six-month supply quantities to get volume discounts, so depending on build date of the car, any particular engine could actually be previous year equipment, though it would still have had to pass current emissions standards.

The Corvette got the computer controlled Quadrajet in 1981...it was usually the first GM car to get any particular technology...the rest of the GM line would get it the following year. Since your car is a 305 and not a Corvette 350, it sounds like your engine is '82 production.

As far as finding the computer plug...remember that the Avanti was never designed for computer controls back in the early '60s...and that same platform and dash layout was used through 1985. Avanti Motors did a lot of adapting to stay current. My own belief is that many added on pieces never really had a standardized location...each assembly person installed some items where he preferred...another assembly person might do those same things different. As an example...the electric trunk release button on my '70 is under the left side of the dash...I know of others who have said their release button was under the dash on the right side of the steering wheel. Later cars the release button was located in the glove box.

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  • 2 years later...

Thanks to CaffieneRacer and IndyJimW for the information posted this week. That 1981 style connector is exactly what I have also, so obviously I have an early CCC system in my 1982 model Avanti. That is good information so I know what year Camaro parts to ask for at my friendly local auto parts store. I ran the check and found no stored codes in the ECM (12-12-12). This week, I am changing to a 195 degree thermostat to see if this thing will go into closed loop after heating up. This thing has been sitting for a while and I want to get it back on the road. This spring, I junked the mystery rear end that the PO had installed in the car and got a proper Avanti Dana 44 housing. I had to replace everything in it except the axles, but it now has an essentially new rear end with 3.54:1 ring and pinion, Detroit Eaton TruTrack differential and all new brakes, seals, bearings, bushings and everything else.

If I can get the CCC to act normal, I should have a good runner.

Steve

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Steve great to hear and glad this thread helped. If you have any more ecm or CCC related questions please ask.

I should also say that in upgrading my computer to a later 1228079 unit and forking out some proper dough on a ZZ4 prom made the car run amazing. This upgrade increases the speed at which the TCC locks up as well as it no longer locks in second gear like it would with the old computer. For me with a 2.87 rear end the lock up at such a low speed would really lug the engine. The spark curve with the zz4 prom definitely has some seat of your pants differences. I'll be curious to see how my mileage goes (if I can keep from foot from being too enthusiastic that is). If you have an early computer I would definitely recommend the upgrade to the newer computer and if you have swapped your 305 to a 350 I'd say the ZZ4 prom is definitely something to consider (it's a little pricey).

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The 195 degree thermostat made no difference except that it runs at higher temperature now. The same rich condition, black smoke and loading up at idle and no TCC lockup. No trouble codes are being set in the ECM. I found that the O2 sensor is indeed in place back in the spring of the year and replaced it with a new one just for good measure. Also back in the spring, I switched the carburetor for another (the original stock one from my 1987 El Camino LG4 305) and the car ran exactly the same, so I switched it back. This is getting old.

On a different note, after changing out the rear end from a 2.87:1 to a 3.54:1 the speedometer is now 10% fast instead of 12% slow, according to my GPS.

Steve

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The 195 degree thermostat made no difference except that it runs at higher temperature now. The same rich condition, black smoke and loading up at idle and no TCC lockup. No trouble codes are being set in the ECM. I found that the O2 sensor is indeed in place back in the spring of the year and replaced it with a new one just for good measure. Also back in the spring, I switched the carburetor for another (the original stock one from my 1987 El Camino LG4 305) and the car ran exactly the same, so I switched it back. This is getting old.

On a different note, after changing out the rear end from a 2.87:1 to a 3.54:1 the speedometer is now 10% fast instead of 12% slow, according to my GPS.

Steve

If you are running that rich and not setting a trouble code, what happens if you disconnect the ECM. Will it start and if so, does it run any different. The ECM may be faulty and you are stuck in Limp Home mode and no codes. IIRC, the computer is a complete add-on and essentially separate system from the rest of the car electronics.

I don't know the answer about the ECM but you have exhausted a lot of other causes.

I've had this type of problem with these era systems from a bad sensor to broken wires in the harness but there was always a trouble code set. It didn't provide a great deal of info but it got me started in the right direction.

Edited by Avanti83
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Have you connected a dwell meter up to the MCS to see if it is going into closed loop mode? Do you know how to do this? Check that first then post back and I'll have a couple of ideas depending on the results of that test.

Also I'm sure you've done this but make sure your base timing is set at 6 degrees advance with the 4-wire connector unplugged from the distributor.

Also it appears you have a later model ECM. The one that at least was installed on my 82 was actually from an 81 GM system. You can tell the difference because the 81s were the only year where the ecm wasn't set to dry cycle the MCS once you turned the key to on.

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  • 2 years later...

So, after getting tired of seeing my car sitting in my shop and going nowhere fun, I decided to delve back into this rich-running situation and found some interesting conclusions. First, the ECM has indeed been replaced at some point, with a factory remanufactured unit, part number T1226025 for what that is worth. Second, while fooling around with this the last time, I decided to check all vacuum sources for leaks and replaced all the suspect vacuum lines for good measure. I figured that a bad vacuum leak would trigger the "system" to load more fuel, causing the symptoms experienced. Nothing changed. Then I went back to sleep until yesterday. I decided to check all the vacuum stuff again, including the very visible and intact 3/8" line to the vacuum booster. But this time I plugged off the power brake booster line before changing it out to rule out possibility that the booster itself was the culprit and eureka, the engine starts and runs completely normally. The booster diaphragm apparently has an internal leak, which is adding too much air to the fuel mixture. I haven't had a chance to drive it yet to go back to the TCC lockup issue, but I think this will fix the rich running issue.

Now, I have a new problem. I can't seem to locate a ready local source for a new brake booster, its part number or what it interchanges with. I can't believe they designed a unique one for this application. I am not averse to sending it out for rebuilding, but don't have any experience with any reputable source for that either.

Any suggestions?

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  • 1 month later...

Okay, so I sent the booster off to Booster Dewey, put it back on and now I have better brake modulation. Unfortunately, I am disappointed to admit that ABSOLUTELY NOTHING has changed with the way the car runs. Still runs very rich with lots of black gas smoke. BUT, I ended up with a few new issues in the mean time.

First, the passenger window went halfway down and the motor stopped. I took the door apart, removed the motor and scissors assembly. Things were pretty gummed up. I took everything apart, cleaned it all up re-lubed the tracks, scissors assembly. I disassembled the motor and screw drive and cleaned all the hardened grease and stuff, re-lubed everything and put the door back together. Done, the window works again.

Second, I removed the intermittently functioning aftermarket security system and the rat's nest of wiring related to that when I replaced the carpeting last year. What I found during that process was quite a bit of fooling around behind the dash by either the installer of that system or some previous owner that I have been cleaning up to "factory" condition. There were some loose, unconnected or dead wires for the gages present, but I have straightened those out and fixed a couple of other issues I found during the process. The "choke" light in the dash had had the bulb removed. I put a bulb in it but it wouldn't go out after engine start. I replaced the choke heater relay and fixed that problem.

Third, my battery was old so I bought a new one. After I put it in, I started the car and wonderful, the charging system is dead. I took the alternator off and had it tested. Its bearings screamed on the tester and it was not charging at all, kaput. I got a rebuilt alternator and put that on. The system is still dead. I put on the spare (new) alternator I carry in my motorhome. The system is still dead. I took that one off and had it tested. It checks fine off the car. I have chased every wire I can think to check and all check ok. The Chevrolet service manuals all have diagnosis charts, but every one I have relies in a "Charge" light to excite the alternator. My car does not have a "Charge" light, just an ammeter, so I am a little lost figuring out what is suppose to "excite" (turn on) the alternator. I am finding nothing in any of those circuits which appear altered from the factory installation.

Suggestions???

Fourth, during my many hours of Avanti yoga under the dashboard, I have had to remove and reinstall the tachometer and some other gages to get to some of the wiring just to access it for straightening up. I have that all completed and all the gages are functioning now. There is a mystery part under there that was partially broken and taped together and fragile. It is some kind of coiled wire contraption (resistor?)  that was mounted between the fuse box and dash on the steel brace along with the ALDL diagnosis plug. It plugs in with a spade connector on each end and is made of some fiber insulation board with a coiled wire between the plugs. It is now quite broken an requires a replacement. Does anyone know what it is, does and where to get a replacement?

This car has sat for too long and I want to be driving it this summer, but it is fighting back mightily to keep its parking space.

 

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Also, while I was at it, I took the old Blaupunkt cassette radio out to access wiring a little easier. It only works on the rear speakers and the cassette does not work. What suggestions do you have for a replacement stereo radio?

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  • 2 weeks later...

After I cleaned up the under-dash wiring and under-hood wiring, I found a main hot connector through the firewall that worked intermittently. With that fixed, I have a charging system again. Still loads of black smoke, running rich, but making progress.

I still don't know what that resistor is for and no one I have consulted (NAPA, Bumper to Bumper, O'Reilly's, Pep Boys, etc, etc) has found it in any catalog. The wires that connect to it are dead, key on or off, so I guess it an Avanti appendix. 

Has anyone used the Custom AutoSound or RetroSound radios? 

Comments...preferences?

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Custom Autosound makes crap.  It will fit and work but has little ability to filter interference.  The sound quality is inferior as well.  RetroSound I know nothing about.

If you do a search of reviews of Custom Autosound it probably won't be pretty.

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