Dash volt meter stays zeroed, engine miss at 1,000 rpm's

Hi,

I doubt that the internal failure is causing a misfire but how do you know it is misfiring? Is it just running rough at 1,000 RPM's? What are the short term fuel trims when it is doing this?

If there are no codes, what is telling you that this is an internal module failure? Maybe send a picture of the scan tool to get us on the same page. Thanks

Thanks for your reply.

You're correct on the terminology that I used. I'm not sure if it's a misfire. It is running rough. I'm old school, so since it feels like a misfire, it is a misfire. Now I'll say it's running rough.

I didn't connect my thoughts well when I brought up the internal voltage regulator as the cause of the running rough. I'll try again. Since the digital odometer and the rough running consistently and simultaneously occur at 1000-1100rpms, my thought is that they're connected. Since the alternator output reaches 17.0-17.1 volts at that rpm, my thought is that the PCM output is jumbled at that voltage. It starts without error. Under 1000rpms, it'll rum for 30 minutes with out error.

I bought the scan tool specifically for this problem. Prior to this usage, I have no experience with one. In getting the tool screen image I noticed the big red box with "11" in it. So my earlier claim that there wasn't a DTC number could be wrong. I don't know where to find what the "11" means. Hopefully the scan tool screen image that you asked for will attach.

I have the PCM out. Hopefully I'll get it reinstalled today and try to get the short term fuel trims for you. I was checking the PCM case for cracks and corrosion. There is oxidization on the case. There are places where I don't get continuity on the case. The PCM mounting bracket is heavily corroded. I'm wondering if during the 10 months that it sat idle, that a corroded ground occurred? Prior to parking it, it ran fine. I'll fab an auxiliary ground from the PCM mounting screw to the body and see if that makes a difference.

Okay. I think you are on the right path. If you found corrosion, I would address that first.

Also, if it sat idle for a year we need to check the normal components like plugs and injectors. Make sure the fuel is good. If there was any fuel left in the tank then we may need to drain the tank completely because old fuel will cause this issue. However, it would not cause the flickering issue. We need to eliminate it because you may be right that they are connected but I have had by rear end kicked a couple times thinking an issue was related when I had two separate issues.

As I mentioned in the original post, the plugs, coil pack and injectors were changed.

I reinstalled the PCM and the aux ground. With battery out, I verified continuity of the PCM case to negative battery terminal. I put dielectric grease on all the various connector pins and reassembled everything. I connected my digital VOM across the battery terminals.

Started it, and the original symptoms of digital odometer going out over 17 volts and rough running were still there.

I let it idle until it reached normal operating temp. I shut it off and disconnected the alternator to battery output output cable.

I restarted it, and was able to rev past the 1,000 rpm's, up to approximately 3,000 rpm's, with no rough running. Odometer stayed on with engine speed increase.

So I'm fairly confident that the high alternator output is the problem. Based on internet research, there are (3) options available to me. I'm choosing the external voltage regulator path.

Do you have any experience with external voltage regulators? What are your thoughts?

Hi,

So just so I am clear, you are suggesting leaving this PCM in the vehicle and just adding an external regulator?

While I have not done this the principal is valid because it doesn't matter where the voltage is regulator as long as it is.

Here is a link to a product that is pretty common in the aftermarket. Clearly I don't endorse them, in fact I have never used it but I know others have. You may have to call them because the way I understand it, this protects the PCM regulator which means it would need to be operating for this to work. However, they state that it is the field "replacement" module which normally means it doesn't matter if the PCM regulator is operating or not. Again, this would need to be clarified from them.

https://youtu.be/utXjxwPAcHk

Thought that I’d close this thread out with a final update.
I installed an external regulator and an auxiliary control relay for it. The regulator eliminated the alternator over-voltage condition, bit it didn’t entirely solve the rough running condition. I decided that the PCM was damaged beyond what I could fix.
I bought a reconditioned PCM online. That process was really easy-fill out an online form, pay, and 3 days later the rebuild unit arrived. Installed it and the vehicle ran flawlessly. Alternator is totally controlled, no rough running at any RPM, volt meter works, no problems. I even created an engine fault to verify that the CEL works.
So my lesson here is to keep looking until enough puzzle pieces are found. I wouldn’t have ever thought a PCM would fail partially.
Thanks for your help.

Thanks a lot for the update. Glad you got it straightened out. I am sure that info will help others that visit the site. We love having the fix on these so that others can follow it all the way through.