Voltage Spikes - What causes them?
Thread Starter
Racer
Joined: Oct 2007
Posts: 270
Likes: 0
From: Chapel Hill NC
Voltage Spikes - What causes them?
1977 manual, SB.
The issue: About a week after installing a new alternator... my voltage will go from 14.7 to 16.9+ while accelerating/driving. At idle it will read 14.7.
The volt meter on the dash is correct and working properly. The needle goes over to the RED.
I wonder where the cause is?
***************************************
1. Same results on 3 brand new alternators, different brands from different sources. All are pre-tested and bench tested after installation.
2. Brand new battery.
3. NO ACCESSORIES. (no radio or lights, etc. are used)
Is there a relay somewhere... something that might be the cause of the spikes? What would cause the alternator to generate nearly 17 volts during driving conditions only?
The issue: About a week after installing a new alternator... my voltage will go from 14.7 to 16.9+ while accelerating/driving. At idle it will read 14.7.
The volt meter on the dash is correct and working properly. The needle goes over to the RED.
I wonder where the cause is?
***************************************
1. Same results on 3 brand new alternators, different brands from different sources. All are pre-tested and bench tested after installation.
2. Brand new battery.
3. NO ACCESSORIES. (no radio or lights, etc. are used)
Is there a relay somewhere... something that might be the cause of the spikes? What would cause the alternator to generate nearly 17 volts during driving conditions only?
Melting Slicks
Joined: Apr 2008
Posts: 2,627
Likes: 242
From: Russell Ontario
St. Jude Donor '09
The voltage regulator is a factor , but I would not trust the car gauge , put a good multimeter with an AMP clamp on it and see what it does , it will go up and down . Ron
Race Director
Joined: May 2006
Posts: 16,528
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From: Dayton, Ohio
Bad connections can fool the regulator causing it to go to full output sorta like flicking on and off the headlights.
The brown wire exciter circuit can be bypassed to see if it is causing a problem.
The brown wire exciter circuit can be bypassed to see if it is causing a problem.
Thread Starter
Racer
Joined: Oct 2007
Posts: 270
Likes: 0
From: Chapel Hill NC
I have a voltage tester that plugs into the cigarette lighter receptacle for testing while driving, and it reads the same voltage spikes as the in-dash voltage meter.
The voltage regulators built-in to the alternators are all good.
The voltage regulators built-in to the alternators are all good.
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Sounds like you have an intermittent spike. I would try moving wires and bumping and flexing the center cluster while watching the voltmeter and see if you can see a spike.
The printed circuit on your 77 can be a problem.
The printed circuit on your 77 can be a problem.
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
If you decide to do some troubleshooting ,you can isolate the brown wire exciter circuit with a simple test light like in this photo. If this stops the spikes then you will know you have a problem most likely in the dash area. Let me know if you need help.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Roger is in the correct 'ballpark', but I think your problem is with the exciter wire and/or connector. You may have a bad connection in the plug-in connector...or there may be a fatique separation somewhere inside that wire. From your symptoms, that 'signal' is not getting fed back to the alternator continually, so it isn't getting the correct signal on when to charge and when NOT to charge.
Insert the + probe of your test voltmeter into the exciter connector so that it makes electrical connection with it. Then connect - to a ground point (engine block, ground connection on back of alternator, etc.). Start the car and watch the voltmeter dial. Now grab the exciter wire and move it around a bit to test for bad electrical connection. If the voltage jumps around, there is a problem with that wiring.
Insert the + probe of your test voltmeter into the exciter connector so that it makes electrical connection with it. Then connect - to a ground point (engine block, ground connection on back of alternator, etc.). Start the car and watch the voltmeter dial. Now grab the exciter wire and move it around a bit to test for bad electrical connection. If the voltage jumps around, there is a problem with that wiring.
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Thread Starter
Racer
Joined: Oct 2007
Posts: 270
Likes: 0
From: Chapel Hill NC
Thanks for all the responses!!!
Got me pointed in the right direction!
Turns out it was the brand new electrical connector on the alternator.
I could not get a consistent reading from it during troubleshooting... so I put the old on back on.
So far - no more issues.
Turns out it was the brand new electrical connector on the alternator.
I could not get a consistent reading from it during troubleshooting... so I put the old on back on.
So far - no more issues.
Racer
Joined: Aug 2017
Posts: 352
Likes: 47
From: Bham, Alabama
Resurrecting
I’m resurrecting an old post as I have a similar issue. My alternator is spiking and causing my ignition modules to fail. I mistakenly reverse polarity in my battery a while back and caused some issues. I found the fusible link at the starter and the two fusible links at the Horn relay were bad. I replaced all those and thought I had the problem solved.
I was test driving it this evening with the headlights on and went for WOT. At that point, I lost spark and noticed the ammeter was spiking. Aside from what I’ve already replaced, does anyone have any thoughts on what could be causing the issue? I replaced the alternator before I discovered the two burnt fusible links at the Horn relay. The alternator was replaced after I reversed polarity. So I don’t think it’s the alternator, but I think it’s wiring going to the alternator that’s not telling the regulator to function properly.
thanks for your help.
I was test driving it this evening with the headlights on and went for WOT. At that point, I lost spark and noticed the ammeter was spiking. Aside from what I’ve already replaced, does anyone have any thoughts on what could be causing the issue? I replaced the alternator before I discovered the two burnt fusible links at the Horn relay. The alternator was replaced after I reversed polarity. So I don’t think it’s the alternator, but I think it’s wiring going to the alternator that’s not telling the regulator to function properly.
thanks for your help.
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14 year old thread? You can start a new one e with your question.
Racer
Joined: Aug 2017
Posts: 352
Likes: 47
From: Bham, Alabama
Or I can continue this one on a unique topic to make it easier for the next person searching for "Voltage spikes” before starting another thread on the same topic.
Keith
Keith
Last edited by Hig116; Oct 15, 2025 at 10:24 AM.
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