1981 Corvette Won't Start - - Help!!
Thread Starter
Drifting
Joined: Jun 2011
Posts: 1,704
Likes: 27
1981 Corvette Won't Start - - Help!!
I've been working on my 1981 Corvette replacing the electric antenna, so I've been messin alittle with the wiring, also been around the battery area. Now the car won't start, will turn over, but I checked and I have no spark. I thought at first maybe I tripped the security system, but did alittle research and found that in that case the starter shouldn't even engage. The only other thing I could think of is maybe I fried the ECM when I was working around the battery.
Any ideas why I am not getting any spark?
Any ideas why I am not getting any spark?
Drifting
Joined: Aug 2005
Posts: 1,682
Likes: 3
From: League City TX
Check to make sure you have voltage to the distributor Bat terminal.
If you do, you should be able to unplug the computer (4 wire?) wiring connector to see if it is having an impact on the spark.
Its been a long time since I messed with an '81, but I think it should still run with the computer connector unplugged.
If I'm wrong about that I'm sure somebody will come along shortly and correct me.
If you have spark with it disconnected, you have a computer issue. If you still don't have spark, its probably your module.
If you do, you should be able to unplug the computer (4 wire?) wiring connector to see if it is having an impact on the spark.
Its been a long time since I messed with an '81, but I think it should still run with the computer connector unplugged.
If I'm wrong about that I'm sure somebody will come along shortly and correct me.
If you have spark with it disconnected, you have a computer issue. If you still don't have spark, its probably your module.
Thread Starter
Drifting
Joined: Jun 2011
Posts: 1,704
Likes: 27
I disconnected the 4-wire plug going to the distributor, it runs now. 
Plug it back in, and it doesn't run. ECM fried? I've been wanting to move forward, and really don't want to spend money on a new ECM. Maybe its time to convert...
Plug it back in, and it doesn't run. ECM fried? I've been wanting to move forward, and really don't want to spend money on a new ECM. Maybe its time to convert...
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Thread Starter
Drifting
Joined: Jun 2011
Posts: 1,704
Likes: 27
Thread Starter
Drifting
Joined: Jun 2011
Posts: 1,704
Likes: 27
I got a reburbished ECM from O'Reilly's today for $82 w/ exchange. Car runs now, but doesn't run nearly as good, and the check engine light stays on. I didn't do anything special, just plugged it in and started it up. Do I need to reset the timing? Or is it just a matter of the ECM relearning the engine?
Instructor
Joined: Oct 2009
Posts: 165
Likes: 1
From: Stanley NC
ECM needs a few miles to set the system but that should happen in less than 20 miles. On the check engine light, check the codes by the light flashing and counting. You use a screwdriver to connect the 2 prongs in the 5 prong plug under the ashtray. The number of flashes will let you know what system problem is if you have the manual to cross reference. You should get a 12 at first to show that system is operating properly and problem code after that. If light comes on and goes off probably an O2 code but if the light stays on and sets then code will tell you even if it is the prom unit.
Melting Slicks
Joined: Nov 2005
Posts: 3,323
Likes: 142
From: Green Valley Arizona
I assume you did take the PROM out of the old ECM and put into the new one, right?
These PROMs are old and factory ones are no longer available.
Make sure you did not knock the vacuum line to the MAP sensor out. Engine will run...just not real good.
Pull the codes and let us know.
Also suggest the L81 Corvette Registry for questions...all 81 owners there.
These PROMs are old and factory ones are no longer available.
Make sure you did not knock the vacuum line to the MAP sensor out. Engine will run...just not real good.
Pull the codes and let us know.
Also suggest the L81 Corvette Registry for questions...all 81 owners there.
Last edited by LannyL81; Oct 16, 2011 at 08:12 PM.
Thread Starter
Drifting
Joined: Jun 2011
Posts: 1,704
Likes: 27
I assume you did take the PROM out of the old ECM and put into the new one, right?
These PROMs are old and factory ones are no longer available.
Make sure you did not know the vacuum line to the MAP sensor out. Engine will run...just not real good.
Pull the codes and let us know.
Also suggest the L81 Corvette Registry for questions...all 81 owners there.
These PROMs are old and factory ones are no longer available.
Make sure you did not know the vacuum line to the MAP sensor out. Engine will run...just not real good.
Pull the codes and let us know.
Also suggest the L81 Corvette Registry for questions...all 81 owners there.
Melting Slicks
Joined: Nov 2005
Posts: 3,323
Likes: 142
From: Green Valley Arizona
The engine will run with a bad PROM...as mine did..just not really well. The Check Engine Lite will stay on though.
Even just running the engine and varying the throttle to simulate going through the gears will also get the ECM to "learn"....not the best as there is no load...but it helps.
And I meant to type....be sure you did not KNOCK out the MAP sensor vacuum line....gotta with my glasses on anymore.....
Even just running the engine and varying the throttle to simulate going through the gears will also get the ECM to "learn"....not the best as there is no load...but it helps.
And I meant to type....be sure you did not KNOCK out the MAP sensor vacuum line....gotta with my glasses on anymore.....
Thread Starter
Drifting
Joined: Jun 2011
Posts: 1,704
Likes: 27
Well, I drove the car around for about 40 miles, and check engine light is still on. Then I grounded the diagnostic terminal, and the check engine light, for some reason, would not flash at all. I know this works because a friend that has an '82 did it and was able to retrieve codes.
I will check the vacuum line for the MAP sensor, that very well could be a legit problem. Although it wasn't a problem before I burned up the ECM...just sayin...
And when they took the PROM out, and put it into the new ECM, I swore I saw it spring back up just a tad. The guy swore that it seated properly. I guess I could just pull the ECM out again and take a look.
so...now I got:
1) possible vacuum line to MAP sensor out
2) PROM not seated properly
I'll check these out tomorrow, as we all probably have the same problem, there is just not enough time to do all this stuff....
I will check the vacuum line for the MAP sensor, that very well could be a legit problem. Although it wasn't a problem before I burned up the ECM...just sayin...
And when they took the PROM out, and put it into the new ECM, I swore I saw it spring back up just a tad. The guy swore that it seated properly. I guess I could just pull the ECM out again and take a look.
so...now I got:
1) possible vacuum line to MAP sensor out
2) PROM not seated properly
I'll check these out tomorrow, as we all probably have the same problem, there is just not enough time to do all this stuff....
Thread Starter
Drifting
Joined: Jun 2011
Posts: 1,704
Likes: 27
Also, at WOT, the car sometimes backfires through the carb, spits and sputters. At cruising speed, it runs ok. Idles ok. The timing advance seems to be all off. But then again, the ECM could just be in limp mode. It does run bad, feels like about 150 hp.
Instructor
Joined: Aug 2010
Posts: 233
Likes: 18
From: Monrovia Md
Check the codes before you do anything . If the prom is bad the computer will tell you. To many people try to debug 81's without looking at the codes first -- That is like a doctor operating without asking the patient what the symptoms are. Once you get the codes there are a lot of people that can walk you through the rest. Do you have a service manual -- great help in debugging codes.
The 81 computer does not "learn" and needs no startup time. The check engine light only goes off if the problem goes away.
If you fried the boards on the computer, there is a real possibility you also took out the prom. But as I said, the computer will tell you that the prom is not working correctly.
The 81 computer does not "learn" and needs no startup time. The check engine light only goes off if the problem goes away.
If you fried the boards on the computer, there is a real possibility you also took out the prom. But as I said, the computer will tell you that the prom is not working correctly.
Last edited by wajulia; Oct 17, 2011 at 08:08 AM.