81 ecm prob?

[TAZ 76]

Hey there I'm new here, and new to diag. probs on my vette. The story goes, got car road worthy and drove for 2 weeks. On the last drive, after I got back in , the batt was dead. I have batt leads to the back of the car (because of shaved handles). + goes to the pos batt, - goes to the bracket that holds the ecm behind the batt. My friend hooked his end (jump vehicle) up backwards, only for a quick spark. Then hooked up correctly. after letting batt take a charge, went to start and would fire but not run. If you give some throttle it will miss, backfire ect. Towed home. Now if I disconnect the 4 wire harness that goes from the dist to the car, it will run and sounds close to norm. My question is do I need a new ecm? If so does anyone have a used one f/s. Or could it be something else. Big thanks for any help Aaron

 

[Peer81]

Hello Taz,

Sounds like a weird thing that happend.
My question is, when you have your key on contact, do the generator and check engine light work? The "check engine" light will pass along any errors the ecm might encounter so this would give you a good clue.

Groeten Peter.

 

[bill81vette]

Welcome to CAC!!!,Taz!!Although your problem does sound different...I'm sure we will get you thru it!!!!If you haven't done so please check out the L-81 registry and register your vette if you haven't done so yet!!enjoy the vette and the forums!!!!!!!

 

[smithjw]

Taz, I had a similar problem when I first got my 81. Needed to jump start the car because the battery died. I tried to jump the car using the usual safety precautions. Connect positive battery to positive battery, then connect from ground on car to ground on vette. Of course, finding a ground on the vette that was close to the positive terminal on the battery wasn't easy, so I ended up using the metal frame that the ECU is mounted in. Now unlike you, I had it connected correcty as far as polarity goes, but still, the amount of juice that flowed through the jump was enough to overload the ECU circuit board. I could smell that electrical burning smell. Had to replace the ECU board. The chip from the old board was still good, so I just transferred it from old to new. Lesson learned, don't ground to the ECU bracket during a jump.

 

[Peer81]

@Jeff, why didn't you just ground on the negative terminal? If you didn't want to do this with the gass building up the ECU board in the same place is even worse! A lot of current has to go trough a little piece of metal to go to the negative terminal so more connections in between and more changes of a spark. If you don't want to connect it to the neg. terminal connect it to the bolt where the negavite wire from the terminal is connected to your chassis. This is a little to the right from your forward diff mount.

Groeten Peter.

 

[smithjw]

Peter, your exactly right, I just didn't think of that at the time this happened. It was about 6 years ago. I just out of habit did not connect to the terminal on the battery because of the possibility of the battery fumes igniting. Did not realize that I would still be connecting to ground in the same confined area! DOH! I also didn't think that I would be passing all that current practically right through the ECU board. I know better now.

 

[TAZ 76]

didn't think of that, was just convenient to ground to a bolt inside. now i'm wishing I would have done differently.
Can I swap in an hei dist and normal carb, and disconnect plugs to the ecu to get it to run.

Aaron

 

[TAZ 76]

Peter The gen light is VERY dim, no engine light. Will check bulb tues after work.
Aaron

 

[Peer81]

No problem Jeff, 6 years ago I didn't even have a drivers license

Aaron, If I remember correctly the ECM only controles the "forward" spark controle. And your car is running like it should without ECM (not as smooth but running) so if you could change the ECM somewhere maybe it could help locating your problem. I can't remember if the ECM has a fuse? Maybe that is blown?

Groeten Peter.

 

[Yoda]

No problem Jeff, 6 years ago I didn't even have a drivers license

Aaron, If I remember correctly the ECM only controles the "forward" spark controle. And your car is running like it should without ECM (not as smooth but running) so if you could change the ECM somewhere maybe it could help locating your problem. I can't remember if the ECM has a fuse? Maybe that is blown?

Groeten Peter.

The ECM is fused with a 10amp fuse. Here is a picture of the fuse panel. The ECM fuse is located on the right side about 1/4 the way down just above the 20amp fuse for the stop/hazard lights

Bud

 

[TAZ 76]

The fuse is good. I don't know of any more 81's in the area to try out another ecu. At this point does it seem like I need an ecu? I would really hate to spend $250 and not fix the problem. Any other tips/ideas?
Could I totally remove the ecu and still get the car to run decent? (as decent as when I disconnect just the dist pigtail)There is a club show this weekend, and since I just finished building I would love to make the show.

Thanks, Aaron

 

[smithjw]

Aaron, it doesn't cost that much for a replacement. The $250 price includes a core charge. You get like $150 back when you return the old one to them. You can do away with the computer altogether if you change your distributor and carb I think, but that's much more expensive than just getting the new ECM.

 

[TAZ 76]

I might of found the problem. I opened the ecm and found a burned curcit near the lower wire harness (the very bottom wire). I think I will order an ecm and hope that is all that is wrong.
Thanks, Aaron

 

[Yoda]

Aaron,

Check with your local NAPPA Store. I don't remember exactly, but as I recall I replaced mine back in 1997 with one from NAPPA for around $95 or maybe it was $149 I'm sorry I just don't remember for sure.

But NAPPA had one, that was in Modesto, CA.

Good Luck and KNOCK 'em dead at the car show this weekend

Bud

 

[TAZ 76]

Thanks, I didn't think to try napa. Over here they are hard to deal with, if it's not an air filter or oil they are lost. But i'll try them anyways. I might have to just push the car to the show if it's not running. (12 miles)

 

[TAZ 76]

Thanks for the help everyone!!! Went to napa online and found a reman ecm. Took the part # in and had it overnight, only $91.50. They didn't even ask for a core or tack on a core charge. Put it in late last night and took for a spin tonite, works as good as new!!!!! Will try to get some pics of the show tomorrow.
Thanks again, Aaron

 

[Yoda]

Aaron

Car show time bring home the Gold.... '81s Rock

Bud

 

[WONKABOY]

Thanks for the help everyone!!! Went to napa online and found a reman ecm. Took the part # in and had it overnight, only $91.50. They didn't even ask for a core or tack on a core charge. Put it in late last night and took for a spin tonite, works as good as new!!!!! Will try to get some pics of the show tomorrow.
Thanks again, Aaron

Hi, can you describe the problem leading up to replacing the ecm? my 81 is acting up MOST of the time the acceleration is smooth as a whistle but it does not last for long , i get a miss when accelerating and you can hear it cruising along , if you punch the gas it will pop at high rmp , well today it was running with the miss , so i got mad and punch the gas to the floor ( maybe bad gas ) and when i did it it had it's usual pop Thu the carb then all of a sudden it had the smooth acceleration again ( felt normal and sounded normal ) then 1 minute later it was back to misfiring sound though the exhaust, this all starting happing about a month ago , i know my check engine lite works because if it sits there for a period of time idling ( fueling it ) it will come on, as soon a i take off it goes off and never comes on again , maybe my points? do the 81 have points like the old cars? maybe spark plug wires? i know i never see smoke coming out of the exhaust so my plugs should not be fouled out.

PLEASE RESPOND ANYBODY WITH SUGGESTIONS.

AND WELCOME TO THE CLUB!!

Rick

 

[TAZ 76]

Hey there Rick, mine wouldn't even idle at all with the bad ecm.If I disconnected the harness(eletronic advance) from the dist. I could get it to run decent, but not great. The 81 dosen't have points. To me it dosen't sound like an ecm, sounds like possibly spark, fuel or timing. Sorry i'm not much help with engine diag. but there are alot of smart engine guys here that might be more help.
Aaron

 

[TAZ 76]

The car show on sat went well, rained lightly on and off. The 4 cars I painted were all lined up. ( 2vettes, '68 camaro, lifted chev 4x4). Just over 100 cars showed up. I was giddy when I found out I took home 1st place in late model. My pics are too large file size to upload and I don't know if there is any way to downsize them (722kb to 83 kb). If anyone has any ideas on how to get the size to be uploadable i'll post them.
Thanks Aaron