Control Module Voltage at 17v, but battery and alt are reading 14.5v?

This is on a 1998 z28 M6 with bolt ons and BTR stage two NA cam. Do you know what would cause the control module voltage get 17v at idle when my battery and alternator are putting out about 14.5v?

I measured voltage across the +- on the battery, the negative side of the battery to the ALT post on the driver's side, and I've only logged the control module.

Some symptoms I'm seeing are high idle, fuel level sensor circuit high voltage DTC and non functioning fuel gauge, and TPS voltage in the 1.4v range with the throttle closed at idle. With the key on engine off TPS is seeing .61v and my fuel gauge works.

So far I've tested the TPS, battery, alternator, and cleaned every ground I could find. It's puzzling to me how the PCM is seeing a higher voltage than what's being put out by the battery or alternator. Any ideas?

Bad pcm

Dang, was hoping that wasn't the case. I was thinking back about what could've caused this and I remembered let the car sit and the battery died. I hooked it up to a cheap Harbor Freight battery tender while the battery was still in the car and hooked up, after I left it charging overnight the problem surfaced. Lesson learned the hard way.

Is it possible to transfer my license for this PCM to a replacement PCM?

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Originally Posted by Nate355rs

Dang, was hoping that wasn't the case. I was thinking back about what could've caused this and I remembered let the car sit and the battery died. I hooked it up to a cheap Harbor Freight battery tender while the battery was still in the car and hooked up, after I left it charging overnight the problem surfaced. Lesson learned the hard way.

Is it possible to transfer my license for this PCM to a replacement PCM?

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Nope, each PCM has to be licensed separately

Got my hands on another 98 PCM and it has the same problem. High voltage fuel sender circuit, high control module voltage, and high TPS voltage. Really stumped now. I'm not too sure what to test next. Maybe the starter is causing something weird to happen? I'm just grasping at straws at this point.

Try unplugging the alternator control wire. The alternator could be faulty and back feeding on the control line. Also set your meter to AC and recheck battery voltage with the car running. If a diode failed you will have AC voltage in the system (ripple) and the computers do not do well with that at all. 30+v AC is not uncommon to see.

I'm getting right around 30.8v AC with the car running. Is that in the acceptable range? Little dark out but I'm going to try and pull the alt wire and see if there's any change.

Nope. Your alternator is toast. You shouldn't see anything over a volt or two on the AC scale. Test it out on another car to see.

Say you have a power wire coming off the alternator and is fused with a large blade 30 amp fuse. If the top of said fuse touched a sensor ground bolt, would that cause this problem? If so, I think my wife may have fixed this problem for me.

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Originally Posted by Nate355rs

Say you have a power wire coming off the alternator and is fused with a large blade 30 amp fuse. If the top of said fuse touched a sensor ground bolt, would that cause this problem? If so, I think my wife may have fixed this problem for me.

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No that kind of AC voltage means you have a bad alternator diode and I sure as shit hope you dont have a 30 amp fuse on the battery feed to the alternator as its capable of over 100 amps. That much AC can destroy electronics in a flash too.

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Originally Posted by matty b

No that kind of AC voltage means you have a bad alternator diode and I sure as shit hope you dont have a 30 amp fuse on the battery feed to the alternator as its capable of over 100 amps. That much AC can destroy electronics in a flash too.

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No, sorry I wasn't clear. The wire goes from the alternator to some crap stereo amp the previous owner installed. I pulled the fuse that was making contact with the ground and everything is back to working normally. Idle is back where it should be, getting a solid 14v at the control module, and TPS is reading normally again. Haven't measured the AC at the battery yet to see if it's changed, but everything else seems good for now

If it fixed it, then yes that would cause it. Im curious to see how exactly it was situated and how it is/was wired as Id have a much better idea of the actual cause. Your lucky that you didnt burn up a lot of stuff with that much AC on the system. Remember, AC voltage will go everywhere that power is going and that can hurt a lot of electronics.

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Originally Posted by matty b

If it fixed it, then yes that would cause it. Im curious to see how exactly it was situated and how it is/was wired as Id have a much better idea of the actual cause. Your lucky that you didnt burn up a lot of stuff with that much AC on the system. Remember, AC voltage will go everywhere that power is going and that can hurt a lot of electronics.

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I'm thinking I probably wasn't reading the AC voltage correctly. I was using a harbor freight voltmeter and on my other cars I'm also reading around 30, so the reading is similar amoung other cars working cars. Wonder if it was showing millivolts?

Started it this morning and let it warm up, voltage and sensors looked good. After about 5 minutes it was like a switch was flipped and the problem came back exactly as before. Shut it off and haven't really looked at it since then.

Milliamps and millivolts are totally different. Were you reading 30 mV AC or 30V AC? Try this , disconnect the battery lead to the alternator and the regulator connector and see if it does that. Ive screwed up enough helping people on the internet thinking they have good electrical systems and then see them to know they are garbage swaps with bullshit hooked up wrong at every point of the vehicle so Im not gonna assume with yours. If you dont know the difference between amps and volts and how to set a non-autoranging meter, I would suggest having a professional identify the problem as you need to have a base knowledge of certain things before trying to diagnose yourself or hope others can help when you are unable to provide proper data to them. Case in point, 30V AC and 30 mV AC ripple is literally 3 magnitudes of difference between them and if you cant get that right its unlikely anyone is going to be able to help you out.

When your car starts screwing up do your dome lights pulse? I've seen the lights get bright/dim/bright/dim in about 1/2 second intervals with a bad diode.

High voltage is normally caused by a bad/poor ground.

You need to do a A/C ripple test with a scope to properly see if the diode is bad.

I think a few people in here are confused about what they are talking about...

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Originally Posted by matty b

Milliamps and millivolts are totally different. Were you reading 30 mV AC or 30V AC? Try this , disconnect the battery lead to the alternator and the regulator connector and see if it does that. Ive screwed up enough helping people on the internet thinking they have good electrical systems and then see them to know they are garbage swaps with bullshit hooked up wrong at every point of the vehicle so Im not gonna assume with yours. If you dont know the difference between amps and volts and how to set a non-autoranging meter, I would suggest having a professional identify the problem as you need to have a base knowledge of certain things before trying to diagnose yourself or hope others can help when you are unable to provide proper data to them. Case in point, 30V AC and 30 mV AC ripple is literally 3 magnitudes of difference between them and if you cant get that right its unlikely anyone is going to be able to help you out.

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It sounds like you've made some assumptions that weren't right in the past, and you're making assumptions again so let me clear this up and be as specific as I can be. Milliamps was a simple typo, thank you for catching that, let's not get too carried away about it.

As I said, I'm using the HF multimeter. It's cheap, but it's what I have. https://manuals.harborfreight.com/ma...0999/90899.pdf

I put it in the ACV 200 range with the leads in the COM and VΩmA jacks, if you know of another way to set it up to measure AC voltage I'm all ears.

With the multimeter setup like above I got a reading of 30 at the battery with the car at idle. I assumed that meant 30V, but my Accord, Tahoe, and Subaru all read between 29-33 at idle. Considering the other cars run normally, and from what you've said about acceptable AC voltage range, I think it's actually showing mV... even though nothing in the documentation or on the device mentions auto-ranging or AC mV. Or more likely it's combining DC and AC voltage because it's a cheap DMM that can't separate the two and gives a wrong reading. Whatever it is I'm sure it's a combination of a cheap tool and my inexperience/limited knowledge about AC/DC in general.

I appreciate your help, I'll try taking the connections off the alternator and see what happens.

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Originally Posted by Jay@HAP

High voltage is normally caused by a bad/poor ground.

You need to do a A/C ripple test with a scope to properly see if the diode is bad.

I think a few people in here are confused about what they are talking about...

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I think you're right, I'll ask around and see if any of my friends have a scope to test it with. It seems like testing AC voltage, especially with my HF multimeter, isn't really going to tell me anything. Can you perform a decent ripple test with a better multimeter or is a scope really the best way to go?

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Originally Posted by Nate355rs

I think you're right, I'll ask around and see if any of my friends have a scope to test it with. It seems like testing AC voltage, especially with my HF multimeter, isn't really going to tell me anything. Can you perform a decent ripple test with a better multimeter or is a scope really the best way to go?

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You can do it with a multi meter but it has to be a certain kind. I can't remember at the moment what it needs to have. Yes you can do it with a meter but it's not as fast a scope and most meters are going to show you the average AC voltage not the min/max that you really need to see.. Anything over 500mv is considered excessive.

I don't know if it's been mentioned or not, but to do a A/C ripple test you test from the positive battery terminal to the big power wire on the alt.

edit.....

I'm pretty sure the meter needs to be RMS capable, which I think is "root mean square"

If the car is driveable bring it by an auto zone or equivalent shop and have them test the alternator. Or pull it off and bring it down there.