Voltage Drop At Higher Revs
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Cross-Country
Joined: Dec 2023
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Voltage Drop At Higher Revs
1990 F150 Custom (Australian)
5.8 EFI
2WD C6 auto
Single cab
G'day, was wondering if anyone could help me out. I noticed today my factory voltmeter started to drop while driving, at above 3000rpm. If I let off the throttle it climbs back up.
I had my son rev the car in neutral while I checked with a multimeter. Holding steady at 3000rpm the voltage at the battery dropped to about 12.3v. At idle, it is in the early 14v's. There was no load (lights, stereo etc) on when I checked.
It's an almost new battery, a good brand known for their quality over here in Oz. I've been running it with no issues for a few months.
The other suspect thing is while holding the revs high, the factory tacho jumps. This leads me to believe (and hope!) it's the dashboard voltage regulator and not the alternator.
My question is, should voltage drop that much at higher revs? Maybe I just haven't noticed before? Also, the "charge" light came on for a while at one point. (Aussie F150s have the older dash, from the '85 American models, I think.) Is that triggered by the voltmeter at the dash, or direct from the battery? I'm suspecting voltmeter.
5.8 EFI
2WD C6 auto
Single cab
G'day, was wondering if anyone could help me out. I noticed today my factory voltmeter started to drop while driving, at above 3000rpm. If I let off the throttle it climbs back up.
I had my son rev the car in neutral while I checked with a multimeter. Holding steady at 3000rpm the voltage at the battery dropped to about 12.3v. At idle, it is in the early 14v's. There was no load (lights, stereo etc) on when I checked.
It's an almost new battery, a good brand known for their quality over here in Oz. I've been running it with no issues for a few months.
The other suspect thing is while holding the revs high, the factory tacho jumps. This leads me to believe (and hope!) it's the dashboard voltage regulator and not the alternator.
My question is, should voltage drop that much at higher revs? Maybe I just haven't noticed before? Also, the "charge" light came on for a while at one point. (Aussie F150s have the older dash, from the '85 American models, I think.) Is that triggered by the voltmeter at the dash, or direct from the battery? I'm suspecting voltmeter.
Logistics Pro
Joined: Nov 2013
Posts: 3,928
Likes: 1,198
From: Mojave, Ca
The factory gauge is a voltmeter. The charge light is also kinda like a volt meter.
I've seen this behavior before in an alternator. The windings inside the alternator were broken, and as the alternator spun up, centrifugal force moved the windings apart and opened the circuit, stopping the alternator from charging.
Replace the alternator.
I've seen this behavior before in an alternator. The windings inside the alternator were broken, and as the alternator spun up, centrifugal force moved the windings apart and opened the circuit, stopping the alternator from charging.
Replace the alternator.
Cargo Master
Joined: Mar 2007
Posts: 2,822
Likes: 1,029
I would check the voltage at the battery. Then perform a voltage drop test at the alt to battery [positive post] and the alternator case to battery [ground post]. If that is ok, then start to perform voltage drop tests from the battery posts to the cluster.
I too have seen alternators fail at higher RPM's.
One other thing to think about. That battery is only used to start the engine. Once the engine is running, the alternator supplies the vehicle with power. The battery does not [yes, it can on certain circumstances]. If you focus your thinking on that, you will find the problem faster. Once the engine is running with a good alternator, you really don't need the battery.
I too have seen alternators fail at higher RPM's.
One other thing to think about. That battery is only used to start the engine. Once the engine is running, the alternator supplies the vehicle with power. The battery does not [yes, it can on certain circumstances]. If you focus your thinking on that, you will find the problem faster. Once the engine is running with a good alternator, you really don't need the battery.
Lead Driver
Joined: Jun 2005
Posts: 8,465
Likes: 725
From: Calgary Canada
Yes first check is using a real multimeter at the battery. Don't use the dash gauges for diagnostics.
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Thread Starter
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Cross-Country
Joined: Dec 2023
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One other thing to think about. That battery is only used to start the engine. Once the engine is running, the alternator supplies the vehicle with power. The battery does not [yes, it can on certain circumstances]. If you focus your thinking on that, you will find the problem faster. Once the engine is running with a good alternator, you really don't need the battery.
At any rate, when i bought the car there was a used alternator sitting on the floor, with Ford V8 written on it in paint pen. I'm now assuming PO bought it to change it over, and never did. I've had the truck for two years now though. But ill swap it in today and see what happens.
Thanks for the replies!
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