Voltage Surge?

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AdamH
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Joined: Wed Mar 15, 2017 6:38 pm

Voltage Surge?

Post by AdamH » Mon Sep 16, 2019 8:23 am

Hello All,
I'm hoping to get some insight into an electrical gremlin that's popped up recently.
I have a Volt-meter gauge installed. It normally reads about 26-28v while running.
One day it just pegged up to 38v (as high as it goes). I thought it. might just be the gauge being flakey as it only lasted for 5 minutes and went back to normal.
Then I was driving at night and it happened again- volt meter pegged - and all the lights suddenly got very bright (it was actually kinda nice!)
Nothing seemed to blow, but this can't be good for the electrical system.
Any ideas what the cause is? I'm hoping its just a bad voltage regulator or something replaceable.

Thanks!

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rmel
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Location: Woodside, CA
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Re: Voltage Surge?

Post by rmel » Mon Sep 16, 2019 10:37 am

This condition is a sign of a Alternator/Regulator problem. If it persists you can
overcharge your batteries and boil the electrolyte making one ugly mess.

Check that all the Alternator connects are solid. Measure the B+, D+ and DF voltages.
D+ should be a copy of the B+ (Battery Out) voltage. DF is driven by the regulator as
a control signal, driving DF above ground until the Output is at a preset trip point near
28V. When the over-voltage condition occurs, DF may be pegged too high, which means
that the regulator thinks the Alt. Output is too low and thus over-driving the system.

Again check for good solid connections at the Alt and all connectors, the Molex connector
is a waek link in the chain as it is far from hermetic. Could also be a regulator that is on
the verge of complete failure.
Puller: 71' 710K 2.7L EFI aka Mozo
Follower: Sankey MK 3, 3/4 Tonne
Rescue Pinz: 73' 712M

Driver: Ron // KO0Q

Texas710
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Location: San Antonio, TX

Re: Voltage Surge?

Post by Texas710 » Wed Sep 18, 2019 11:37 am

This is the reason a volt meter on these trucks is a great idea!

AdamH
Posts: 36
Joined: Wed Mar 15, 2017 6:38 pm

Re: Voltage Surge?

Post by AdamH » Wed Sep 18, 2019 4:07 pm

Thanks for the great info!
I'll start by replacing the voltage regulator since its a fairly easy and inexpensive job. It looks to be original so its 45 years old at this point and wouldn't surprise me in the least that it needs replacing.
I have very thankful that I have the volt meter installed- without it I probably would not have noticed these voltage surges and ended up frying my batteries or worse!

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rmel
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Location: Woodside, CA
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Re: Voltage Surge?

Post by rmel » Thu Sep 19, 2019 10:38 am

I'd bet on a intermittent oxidized connection right at the Mloex connector first :o
The regulators are solid state, they typically fail hard and don't go intermittent.
Hate to see you replace the reg then find it didn't fix the problem. Then again it's
good to have a spare regulator.
Puller: 71' 710K 2.7L EFI aka Mozo
Follower: Sankey MK 3, 3/4 Tonne
Rescue Pinz: 73' 712M

Driver: Ron // KO0Q

Bumber
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Joined: Sun Aug 10, 2014 9:36 am

Re: Voltage Surge?

Post by Bumber » Tue Oct 15, 2019 1:27 pm

Agree. Test all the molex connections. Then pull the alternator and take it to a local shop. It only costs about $120 to rebuild a Denso and about $110 to rebuild the Bosch. All standard parts and any shop can do it.

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