Open Road

1990 GT

So a trio of odd occurrences makes me think I have a loose ground wire somewhere.

1. I have RF interference coming from the passenger door 4" speaker. This was after I ripped out the old system and replaced HU, speakers, and amp. Note at first the radio was clear, but no squeal.

2. Engine stalled on me multiple times last night in the drive way. One time and I call it user error (even though I'm been driving manuals for 35+ years). But this happened multiple times with either clutch pushed in or giving it gas and revs not low enough to stall. Just felt random.

3. Car alarm started going off multiple times last night when it wasn't activated and door weren't locked.

New battery ground strap and new engine ground strap. I'm thinking I didn't get one of the grounds tight enough during the audio install and that nut has worked its way loose or there is another untouched ground that is loose/needs cleaning. Sound right?

Only other thing that comes to mind is the alt is dying.

First figure out how far off is your voltage gauge. With the engine warm and idling note the (approximate) indicated Volts on the gauge and then see what the voltage is at the jump post.

Now you know what the voltage gauge is actually telling you.

Is the voltage below 12 when it dies? Or ever?

With working alternator and a good battery you should never see less than ~12.75v at idle and the jump post.

To answer your question: yes, a dying alternator that's killing your battery could cause those symptoms.

But, if the behavior suddenly started after you messed with the stereo... then, check your work.

At idle the gauge is straight up and down at 13. Voltage at the hot post is 15. Battery was at 16.3.

These seem way too high to me. I need to go research if a dying alt can do that and how to test the alt.

Car didn’t stall once tonight. The alarm did trigger the first time I opened the driver’s door but didn’t trigger again. All my grounds were solid from the radio install.

JBT3 wrote: ↑Tue Jun 03, 2025 9:33 pm At idle the gauge is straight up and down at 13. Voltage at the hot post is 15. Battery was at 16.3.

That is not-good. AFAIK a failing voltage regulator on the alt can do that. And your battery may be cooked.

JBT3 wrote: ↑Tue Jun 03, 2025 9:33 pm At idle the gauge is straight up and down at 13. Voltage at the hot post is 15. Battery was at 16.3.

Don't operate the car until you get that squared. The ECUs will not tolerate 15v for long.

Thanks! At the very least it seems I need a new voltage regulator. Is it likely the alt is toast too? Wonder if I can get it rebuilt locally. Roger’s uber alt is tempting.

JBT3 wrote: ↑Wed Jun 04, 2025 7:21 am Is it likely the alt is toast too?

Not because of a failed voltage regulator. Bearings might be 30-ish years old?

JBT3 wrote: ↑Wed Jun 04, 2025 7:21 am Wonder if I can get it rebuilt locally.

Depends upon the locality.

JBT,

Check with MrMerlin (Stan K). I think he has a favorite alternator rebuilder that's fairly local.

Gary

Thanks, Gary. Let's meet up this summer when our schedules match up.

I found a reman Bosch, so waiting for it to arrive. If that doesn't work out, I'll get my original rebuilt. Actually may do that anyway and then sell one of them.

I pulled the original alt today and it was easy as pie. The long bolt didn't give me any issues - I read that might be because my motor mounts are squashed (which they are - and yes, they are on the list.)

Rebuilt arrived. Shiny. The new Pulley diameter is 1/4” larger. Not sure it matters as long as the belt fits and it’s not too tight. If I’m wrong about that, I can put the old pulley on it. Thoughts?

JBT3 wrote: ↑Tue Jun 10, 2025 8:21 pm Rebuilt arrived. Shiny. The new Pulley diameter is 1/4” larger. Not sure it matters as long as the belt fits and it’s not too tight. If I’m wrong about that, I can put the old pulley on it. Thoughts?

I seem to recall, a long time ago, in a galaxy far, far away, that it was discovered that automatics had a smaller pulley than manuals so as to spin the alternator every-so-slightly faster at idle. Or maybe it was S4 vs. GT. The latter makes more sense since the GTs idle 100 rpm higher than S4 or GTS.

New alt is in and I’m still getting very high voltage readings at the hot post. 17V+. I quickly shut it down when I saw that. Voltage gauge was holding steady at 13V. Guess it could be my cheap ass voltage meter.

JBT3 wrote: ↑Wed Jul 09, 2025 7:32 pm New alt is in and I’m still getting very high voltage readings at the hot post. 17V+. I quickly shut it down when I saw that. Voltage gauge was holding steady at 13V. Guess it could be my cheap ass voltage meter.

When was the last time you put a battery in your meter? IIRC the battery in the meter is a reference for measuring...

Dave - you are the man! New battery and meter was reading 13.7 at a stead rate. Took the GT for a ride and it ran great (no random alarms and no stalling); however, after a bit of driving the RF whine through the radio started to return.

I'll have to do some more research on how to address that. I would swear it got worse when I hit a few bumps in the road, so maybe I have a loose ground wire somewhere, but they were all tight before the alt adventure.

JBT3 wrote: ↑Fri Jul 11, 2025 9:04 am ... after a bit of driving the RF whine through the radio started to return.

Still one speaker? Or all of them?

It’s hard to tell because the last few drives it will come and go. Sometimes getting Louder and other times it is faint. When it is whining it varies with engine speed.

JBT3 wrote: ↑Sat Aug 02, 2025 2:32 pm It’s hard to tell because the last few drives it will come and go. Sometimes getting Louder and other times it is faint. When it is whining it varies with engine speed.

So... new alternator (presumably from a reliable source) and a new battery in the VM and you get intermittent alternator whine...

Yes?

Alternator whine frequency is always a function of engine RPM and amplitude can be a function of engine load.

Alternator whine not associated with the alternator can be due to:
- bad pre-amp cables (e.g. compromised shielding, no shielding (shitty cables), internally broken due to age and/or bent past minimum bend radius, etc.)
- poorly terminated pre-amp cables (e.g. loose, not pushed in all the way, bad crimp from RCA body to conductor, etc.)
- pre-amp or speaker cables routed over and in contact with power or ground to head unit or amps
- loose ground(s)
- multiple grounds in the system connected to different ground points where ground points are very dirty or not actually ground points.

Intermittent whine is - in my experience - due to the first two. Constant whine is the last two.

Determining which speaker (or side or front/back) is a very important step in diagnosis.

Also being able to reproduce the whine "in shop" at idle is important for speeding diagnosis and repair.

Whine can also disappear after disconnecting the battery and then gradually increase in volume. Keep that in mind when messing with cabling; if you think you've solved it you'll need to drive around for an hour or so to be sure.

Not sure if the there is anything unique about the 928, however; whine frequency is related to alternator speed (hence engine speed), whine amplitude can be directly or inversely related to alternator (electrical system load).
I would assume that there is at learned, and maybe multiple capacitors in the electrical system to help filter noise of multiple types.
Capacitors do not age well.
If there are bypass capacitors in the charging circuit, the accessory only audio power circuits, or any CPU boxes, I would look there.
If the audio components are of age, I would look at any filter capacitors within them.