Electrical issue - car closes throttle/stalls
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From: Cape Coral, Florida
Since something happened and made the fuel pump start operating just swap the fuel pump relay with the horn relay...that will kind of confirm if the relay is OK...next, see if you have system voltage KOEO at pin 87 (load side feed)...if good check for ground between pin 30 and a good ground...if no ground either bad ground at G104 or open in fuel pump windings...if good let’s jump pins 30 and 87...fuel pump should run continuously..if it runs OK now we might have problem on control side feed. For this we will use a 12 volt test light (if you use a multimeter you will see system voltage at all times instead of just for 2 seconds...that control feed circuit is power side switched and has a bias voltage when you remove the relay)...connect 12 volt test light to ground... touch tip of test light to pin 86...turn key on...test light comes on and goes out after 2 seconds...if good last step is to see if we have a good ground from pin 85 to ground...you can either check with multimeter or test light connected to battery positive...if ground bad the ground location is G105 at left lower engine block and the other end of the ground strap connects to right underneath the brake booster on the frame rail !!
Last edited by C5 Diag; Oct 20, 2018 at 06:44 PM.
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From: Cape Coral, Florida
Another quick thing I do is if I can’t connect with my scanner and thinking that the PCM might not be powered up is to check for a 5 volt reference on one of the sensors...I use the MAF...disconnect and see if you have 5 volts on the yellow wire with key on...if you have 5 volts the PCM is being powered !!
Last edited by C5 Diag; Oct 20, 2018 at 07:17 PM.
Thread Starter
Pro
Joined: Apr 2009
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From: Greenville SC
Just a quick update. Got a power probe tool and worked on the car last night. Finally found the problem, so I just gotta fix it this weekend hopefully! The PCM was not getting a good ground despite having previously checked all the grounds. They looked new (16k mile garage queen) but there's obviously a problem with one of them somewhere. Just need to track down which ground it is... Anyone know which grounds are tied to the PCM?
Moderator, Tech Contributor
Joined: Sep 2013
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Likes: 4,069
From: Cape Coral, Florida
Just a quick update. Got a power probe tool and worked on the car last night. Finally found the problem, so I just gotta fix it this weekend hopefully! The PCM was not getting a good ground despite having previously checked all the grounds. They looked new (16k mile garage queen) but there's obviously a problem with one of them somewhere. Just need to track down which ground it is... Anyone know which grounds are tied to the PCM?
Last edited by C5 Diag; Nov 9, 2018 at 10:43 AM.
Tech Contributor
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From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
PCM ground is G-106 Passengers side of the engine above the starter on the side of the block.. Same connection as where the main battery ground connects to the block. Its a very small black/white stripe ground wire with a crimped on eyelet. Its going to be under the main battery ground lug.
Next to the battery on top of the chassis frame, will be G-104/108. The battery negative cable has two wires. One goes to the block and the other goes to that ground stud. Its an important ground as ALL the other chassis grounds get the battery neg source from that wire/stud ground. Make sure that it clean and tight.
Bill
Next to the battery on top of the chassis frame, will be G-104/108. The battery negative cable has two wires. One goes to the block and the other goes to that ground stud. Its an important ground as ALL the other chassis grounds get the battery neg source from that wire/stud ground. Make sure that it clean and tight.
Bill
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From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
There is an identical ground on the OTHER side of the engine block G-105. There is another block ground wire on that ground point Also give that one a look and check to make sure that its clean and tight. There should also be a braded ground strap that goes from that ground to G-105 frame chassis ground.
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From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
There is an identical ground on the OTHER side of the engine block (G-105). There is another small block ground wire on that ground point Also give that one a look and check to make sure that its clean and tight. There should also be a braded ground strap that goes from that block ground point to G-105 frame chassis ground. G-105 is a mirror image location as G-104/108.
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Pro
Joined: Apr 2009
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From: Greenville SC
Thanks for the info guys. I will be giving the grounds another good look over Tomorrow.
I determined the ground issue using the power probe. Touched the case of the pcm and saw .97v or something like that. Was pretty sure the pcm case should have been grounded internally to the harness, so i found that odd. i grabbed an alligator clip and hooked one end to the pcm case and the other end onto the ground stud under the battery on the frame rail. Checked with the probe again and it was down to around .30v. Alligator clip is very small, maybe 22ga or so.
One other thing that caught my attention was the ign. switched voltage at the pcm was something like 9.8v or something like that. batt voltage at the time showed around 11.4v. This seemed like a fairly large split to me- 1.6v loss through the ign. switch/wiring/etc. But maybe that is normal? I have cleaned the contacts in the ignition switch per Bill's instructions.
Thoughts?
I determined the ground issue using the power probe. Touched the case of the pcm and saw .97v or something like that. Was pretty sure the pcm case should have been grounded internally to the harness, so i found that odd. i grabbed an alligator clip and hooked one end to the pcm case and the other end onto the ground stud under the battery on the frame rail. Checked with the probe again and it was down to around .30v. Alligator clip is very small, maybe 22ga or so.
- Fired the car up and it idled great with no tach fluctuations for several minutes.
- Disconnected the alligator clip and the car hiccuped and stalled a couple seconds later.
- Reattached the alligator clip, fired it back up and ran great again for another few minutes.
- Disconnected the alligator clip once again and i was able to make my way back to the drivers seat where the tach was bouncing around again. Rough idle also.
- Reattached the alligator clip, smoothed out and no more erratic tach.
One other thing that caught my attention was the ign. switched voltage at the pcm was something like 9.8v or something like that. batt voltage at the time showed around 11.4v. This seemed like a fairly large split to me- 1.6v loss through the ign. switch/wiring/etc. But maybe that is normal? I have cleaned the contacts in the ignition switch per Bill's instructions.
Thoughts?
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Tech Contributor
Joined: Dec 1999
Posts: 32,910
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From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
Its NOT normal and is an indication that you have a fair amount of voltage loss on one of the ignition switch contacts. Check that voltage when you CRANK the engine and I bet it goes out of spec low. If it does the PCM shuts down under low voltage cutoff until the voltage rises above the low voltage cut off level..
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Joined: Sep 2013
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From: Cape Coral, Florida
Thanks for the info guys. I will be giving the grounds another good look over Tomorrow.
I determined the ground issue using the power probe. Touched the case of the pcm and saw .97v or something like that. Was pretty sure the pcm case should have been grounded internally to the harness, so i found that odd. i grabbed an alligator clip and hooked one end to the pcm case and the other end onto the ground stud under the battery on the frame rail. Checked with the probe again and it was down to around .30v. Alligator clip is very small, maybe 22ga or so.
One other thing that caught my attention was the ign. switched voltage at the pcm was something like 9.8v or something like that. batt voltage at the time showed around 11.4v. This seemed like a fairly large split to me- 1.6v loss through the ign. switch/wiring/etc. But maybe that is normal? I have cleaned the contacts in the ignition switch per Bill's instructions.
Thoughts?
I determined the ground issue using the power probe. Touched the case of the pcm and saw .97v or something like that. Was pretty sure the pcm case should have been grounded internally to the harness, so i found that odd. i grabbed an alligator clip and hooked one end to the pcm case and the other end onto the ground stud under the battery on the frame rail. Checked with the probe again and it was down to around .30v. Alligator clip is very small, maybe 22ga or so.
- Fired the car up and it idled great with no tach fluctuations for several minutes.
- Disconnected the alligator clip and the car hiccuped and stalled a couple seconds later.
- Reattached the alligator clip, fired it back up and ran great again for another few minutes.
- Disconnected the alligator clip once again and i was able to make my way back to the drivers seat where the tach was bouncing around again. Rough idle also.
- Reattached the alligator clip, smoothed out and no more erratic tach.
One other thing that caught my attention was the ign. switched voltage at the pcm was something like 9.8v or something like that. batt voltage at the time showed around 11.4v. This seemed like a fairly large split to me- 1.6v loss through the ign. switch/wiring/etc. But maybe that is normal? I have cleaned the contacts in the ignition switch per Bill's instructions.
Thoughts?
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,621
Likes: 4,069
From: Cape Coral, Florida
Its NOT normal and is an indication that you have a fair amount of voltage loss on one of the ignition switch contacts. Check that voltage when you CRANK the engine and I bet it goes out of spec low. If it does the PCM shuts down under low voltage cutoff until the voltage rises above the low voltage cut off level..
Thread Starter
Pro
Joined: Apr 2009
Posts: 673
Likes: 13
From: Greenville SC
The 9.8v reading was taken by backprobing the "ign. battery feed" pin #19 in the blue connector on the pcm with ign switch in the on position and engine not running. I am going to recheck that pin again tomorrow to verify that reading was correct.
The ~1v reading on the pcm case was during the same "on" state. If the pcm was properly grounded, it should have shown zero volts on the case but wasnt so...
The ~1v reading on the pcm case was during the same "on" state. If the pcm was properly grounded, it should have shown zero volts on the case but wasnt so...
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,621
Likes: 4,069
From: Cape Coral, Florida
The 9.8v reading was taken by backprobing the "ign. battery feed" pin #19 in the blue connector on the pcm with ign switch in the on position and engine not running. I am going to recheck that pin again tomorrow to verify that reading was correct.
The ~1v reading on the pcm case was during the same "on" state. If the pcm was properly grounded, it should have shown zero volts on the case but wasnt so...
The ~1v reading on the pcm case was during the same "on" state. If the pcm was properly grounded, it should have shown zero volts on the case but wasnt so...
P.S. I'll be fiddling with my car tomorrow so if you want to compare something let me know !!
Last edited by C5 Diag; Nov 9, 2018 at 02:51 PM.
Thread Starter
Pro
Joined: Apr 2009
Posts: 673
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From: Greenville SC
Finally getting around to an update... Someone who installed the motor in the car (previous owner) added a PCM ground location on the back of the passenger side cylinder head. It was loose! So those issues are now gone and I'm back to the Code C1232. I'm making a new thread since it's an unrelated issue to what this thread was about:
https://www.corvetteforum.com/forums...859-c1232.html
Thanks for all the help!
https://www.corvetteforum.com/forums...859-c1232.html
Thanks for all the help!