1990 Corvette Electrical Question
Thread Starter
Advanced
Joined: Oct 2009
Posts: 80
Likes: 3
From: East Longmeadow MA
1990 Corvette Electrical Question
I had orginally posted this on the Chevrolet Forum and was told that I may have better luck here. I am working on a 1990 Corvette convertible with 22,000 miles on it. It has always been stored indoors and is in excellent condition except it will not stay running. I have been trouble shooting a problem that I thought was fuel related and found out the immediate problem is looking to be more electrical. The car will not run past an idle cold and not run or start at all once warm but when I was trying to determine a cause I disconnected the plug on the alternator and the stalling/not running problem went away, plugged it back in and it came right back. I also reconnected the alternator and disconnected the battery while cold and still running and problem went away again. All of the connections seem to be good, tight, free of any corrosion or rust, and the battery tested out fine and is holding a charge. I am not receiving any error codes and running a diagnostic is not showing any codes stored. The alternator is new/rebuilt and the battery is several years old but seems to be OK.
Any suggestions?
Thank you for any feedback you can provide.
Any suggestions?
Thank you for any feedback you can provide.
Burning Brakes
Joined: May 2007
Posts: 965
Likes: 13
From: turn 12 at Road America or TX
You should see 13.8 volts at the battery terminals when the engine is running. Disconnecting the alternator connector causes ??? I don't know what, but I'll bet it causes the alternator to either shut down or run at full output. I would not do that.
Anyway, you should first make sure the battery ground is solid. There should be very little voltage drop between the battery negative terminal and the chassis - less than 0.1 volts.
I don't want to send you on a wild goose chase - my best guess is a defective ECM,
but check the obvious stuff first.
Anyway, you should first make sure the battery ground is solid. There should be very little voltage drop between the battery negative terminal and the chassis - less than 0.1 volts.
I don't want to send you on a wild goose chase - my best guess is a defective ECM,
but check the obvious stuff first.
Le Mans Master
Joined: May 1999
Posts: 6,520
Likes: 1,306
From: Windermere FL
Welcome!
I have a 90 and have not experienced this symptom but wanted to chime in that disconnecting the alternator or battery with the engine running can cause damage, at least I have been told. So I would not recommend doing that.
I will tell you that the alternators can be very problematic on this year. I used to change alternators as frequently as I changed oil when I first got it. Finally installed a PowerMaster alternator and have experienced no troubles in 10 years.
My suggestion on what you found so far is to take the alternator and battery to the local parts store and they should test them both for free. If they show ok, a ground could be your problem, or possibly a defective ECM.
I am not sure about this but if you short out leads a and B on the ALDL connector which puts it in open loop, will it run and start ok? If it does, then an ECM might be bad.
I don't like replacing parts without a good indication that it defective.
I have a 90 and have not experienced this symptom but wanted to chime in that disconnecting the alternator or battery with the engine running can cause damage, at least I have been told. So I would not recommend doing that.
I will tell you that the alternators can be very problematic on this year. I used to change alternators as frequently as I changed oil when I first got it. Finally installed a PowerMaster alternator and have experienced no troubles in 10 years.
My suggestion on what you found so far is to take the alternator and battery to the local parts store and they should test them both for free. If they show ok, a ground could be your problem, or possibly a defective ECM.
I am not sure about this but if you short out leads a and B on the ALDL connector which puts it in open loop, will it run and start ok? If it does, then an ECM might be bad.
I don't like replacing parts without a good indication that it defective.
Thread Starter
Advanced
Joined: Oct 2009
Posts: 80
Likes: 3
From: East Longmeadow MA
Worked on the car this weekend
I want to thank both "gbrtng" and "Flame Red" for the suggestions and help with my problem. I did go over the car testing the grounds and all of the areas that I tested stayed with the 0.1 volts. The alternator is a new GM that I was able to find locally and the battery is also relatively new and I had both of them bench tested and they were also fine. I have done some research and found that the early 90 ECM's were not water tight and may be damaged if they get wet. So pressure washing the engine would not be a good thing which I did. I found locally the newer version which is supposed to be sealed to prevent this damage from occuring for $112.00 which I thought was fair. I will try it out this weekend and hopefully have some good news to report.
Mike
Mike
Thread Starter
Advanced
Joined: Oct 2009
Posts: 80
Likes: 3
From: East Longmeadow MA
The problem still persists
Changed the ECM and no change.
I am lost and do not have a direction to go.
I will have to go thru everything and see if I can find something.
If you could suggest any area to look first I would really appreciate it.
I am lost and do not have a direction to go.
I will have to go thru everything and see if I can find something.
If you could suggest any area to look first I would really appreciate it.
Team Owner
Joined: Mar 2001
Posts: 54,124
Likes: 30
From: Cincinnati, Oh USA
You should see 13.8 volts at the battery terminals when the engine is running. Disconnecting the alternator connector causes ??? I don't know what, but I'll bet it causes the alternator to either shut down or run at full output. I would not do that.
Anyway, you should first make sure the battery ground is solid. There should be very little voltage drop between the battery negative terminal and the chassis - less than 0.1 volts.
I don't want to send you on a wild goose chase - my best guess is a defective ECM,
but check the obvious stuff first.
Anyway, you should first make sure the battery ground is solid. There should be very little voltage drop between the battery negative terminal and the chassis - less than 0.1 volts.
I don't want to send you on a wild goose chase - my best guess is a defective ECM,
but check the obvious stuff first.
I find it odd that the engine runs ok with either the alternator only or the battery only but not with both. I would suspect a defective battery that loads down the alternator excessively. Measuring the voltage at the battery terminals will find this problem. Engine off, you can tell the state of charge in the battery by measuring its terminal voltage. 12.0 volts and below, discharged, charge the battery up overnight. 12.9 volts and above, fully charged and linear in between.
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Thread Starter
Advanced
Joined: Oct 2009
Posts: 80
Likes: 3
From: East Longmeadow MA
I am lost for a cause and repair also
I have checked the battery and even swapped it out for another one with no change. The control wiring for the alternator compared to the rest of the car is simple. It has a battery connection, and an excite circuit connected to main power with the key on and when the A/C is on. That is it and they are reading the correct voltage when they are suppose to have it. The thought that in August I drove it to my garage and it ran fine, parked it, and it has not been the same since is driving me crazy. Unfortunately I will have to review everything and trouble shoot every circuit to determine what happened. I will most likely start with the alternator wiring and work my way from there.
Thanks for the input and advise. It helps to have another thought process that has knowledge of our passion.
Thanks for the input and advise. It helps to have another thought process that has knowledge of our passion.
Melting Slicks
Joined: Dec 2008
Posts: 2,772
Likes: 64
From: Mystic CT
I have had cars with a bad altenator cause all sorts of misfires skips and poor performance. If the rectifier diodes go bad you end up with ac voltages on the 12 volt dc line the computer systems will not like it. I have never run an engine with no battery connected so have no idea how that effects anything in any way. For laughs set your voltmeter on ac rather than dc if you can measure any you have found the problem and my bet is the regulator inside the altenator.
Instructor
Joined: Feb 2008
Posts: 177
Likes: 5
From: Toronto Ontario
A quick test to isolate the alternator is to remove the serpentine belt and then start the car. If the car behaves normally then you have determined that the ECM and all other fuel and ignition parts are OK and that the problem lies with the alternator. Don't run the car very long because the water pump is not turning.
Pro
Joined: May 2006
Posts: 637
Likes: 0
A quick test to isolate the alternator is to remove the serpentine belt and then start the car. If the car behaves normally then you have determined that the ECM and all other fuel and ignition parts are OK and that the problem lies with the alternator. Don't run the car very long because the water pump is not turning.
Check if the service engine soon light lights up when you turn on the ignition.
If yes, turn off ignition, connect a piece of wire between teerminals A and B on the ALDL (two upper right pins).
Turn on ignition, don´t start, and see if the SES light starts blinking.
If it does, your ECM probably is OK.
Something to read:http://www.misterfixit.com/alterntr.htm
