General Alternator Question
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Senior User
Joined: Jul 2014
Posts: 328
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General Alternator Question
First off I have not done the 3 g upgrade yet so this question relates to a 1 wire.
Weather is turning up north, and I thought I had a bad battery. Decided to check alternator and pulled the negative terminal. Engine cut out right away like alternator was bad
Now I pulled the one wire and checked for alternator output and have 13 v
Been doing some reading, and there are a lot of posts that say do not pull the negative as it can damage the electronic VR,
Put a charger on battery, and now it runs strong again.
So question is if I have voltage output from alternator, what would cause engine not to run after pulling the battery. I double checked the grounds and everything seams good there.
I was sure it was alternator, but never had one put out voltage, and still be bad.
Maybe a diode gone bad or not putting out the amps? Will a standard volt meter let me check amp output. I might just put it away and work on it in the spring
Tks
Weather is turning up north, and I thought I had a bad battery. Decided to check alternator and pulled the negative terminal. Engine cut out right away like alternator was bad
Now I pulled the one wire and checked for alternator output and have 13 v
Been doing some reading, and there are a lot of posts that say do not pull the negative as it can damage the electronic VR,
Put a charger on battery, and now it runs strong again.
So question is if I have voltage output from alternator, what would cause engine not to run after pulling the battery. I double checked the grounds and everything seams good there.
I was sure it was alternator, but never had one put out voltage, and still be bad.
Maybe a diode gone bad or not putting out the amps? Will a standard volt meter let me check amp output. I might just put it away and work on it in the spring
Tks
Postmaster
Joined: Mar 2011
Posts: 2,698
Likes: 7
From: Taxachusetts
Yikes!
NEVER disconnect the battery whilst the engine is running! Doing so causes a voltage spike that can fry sensitive electronics....some people have done it with no negative (so they say) results while others have fried a voltage regulator...an almost guaranteed way to fry the voltage regulator on a 3g upgrade....when you get to it.
Alt/regulator full field test: Set yer DVOM on the 20VDC setting to measure battery voltage with respect to ground.
With the engine running, yank the regulator plug and jump the F (Field) wire to the BAT (+) terminal. (NOTE - don't do this for an extended period of time!).
This excites the alternator sending it to 'full field' mode. What kind of voltage do you have?
If the reading spikes or really increases to say, ~ 18VDC, the alternator is good and you have a problem with the regulator, wiring and/or dirty connections.
If she doesn't full field, the alternator probably has a bad diode or other internal problems.
NEVER disconnect the battery whilst the engine is running! Doing so causes a voltage spike that can fry sensitive electronics....some people have done it with no negative (so they say) results while others have fried a voltage regulator...an almost guaranteed way to fry the voltage regulator on a 3g upgrade....when you get to it.
Alt/regulator full field test: Set yer DVOM on the 20VDC setting to measure battery voltage with respect to ground.
With the engine running, yank the regulator plug and jump the F (Field) wire to the BAT (+) terminal. (NOTE - don't do this for an extended period of time!).
This excites the alternator sending it to 'full field' mode. What kind of voltage do you have?
If the reading spikes or really increases to say, ~ 18VDC, the alternator is good and you have a problem with the regulator, wiring and/or dirty connections.
If she doesn't full field, the alternator probably has a bad diode or other internal problems.
Logistics Pro
Joined: Jul 2005
Posts: 4,217
Likes: 154
^X2^ unless maybe it is an old points-ignition vehicle without a modern stereo. If the alternator is good, it could be the regulator. Dunno if any Dents came with the non-solid state regulator, but I had one of those go bad in a '68 Cougar once.
Postmaster
Joined: Mar 2011
Posts: 2,698
Likes: 7
From: Taxachusetts
Hahaha.....Now why would you want to go ahead and do that?
Tuned
Joined: Nov 2016
Posts: 269
Likes: 13
From: Shelbyville
I hadn't finished wiring the truck up yet but wanted to take it for a drive. So I jump started it. After I pulled the battery cutoff to stop it. Thinking it would kill the engine like every other vehicle I've owned. But it kept going.
Ended up doing it again and driving around to win a bet.
Ended up doing it again and driving around to win a bet.
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