Dead Batteries Caused by Bad Voltage Regulator - Diesel Forum -
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post #1 of 11 (permalink) Old 05-06-2009, 07:26 PM Thread Starter
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Dead Batteries Caused by Bad Voltage Regulator

Hi Folks,

I just wanted to pass on the experience that I had dead batteries caused by a bad voltage regulator. The symtoms went like this. Truck running perfectly on Saturday after pulling a 10000 lb boat. Truck sits on Sunday. Monday morning when I go to work, after turning the key, all I hear is the chatter of the solenoid. Batteries were very low.

So I take the truck to AutoZone for a diagnosis. Batteries and alternator are good, batteries are just low. So I buy a charger for $53 from AutoZone (every house needs a good battery charger anyway). Next day, when I get ready to go to work, again no crank and I hear the wonderful noise of the chattering solenoid. Get the jumper cables out again and this time I drive to Ford so they can trouble shoot the electrical system.

Two hours later I get a call from Ford and the service scheduler tells me that the mechanic told her that the voltage regulator in the alternator went bad. I thought to myself, that does not sound right because the voltage regulator is on the fender well. I tell her that the voltage regulator is a $16 dollar part, how much would it take for you to fix the truck? She said $360 with labor. I said, hmmm, that sounds kind of expensive and she said well you have to replace the alternator. I said, well, I can buy an alternator for much cheaper than that and replace it myself, how much is the Ford remanufactured and she said $165. She said the charge now is $45 and we are charging up your batteries. I said please give me my truck back and thanks for finding the problem.

When I get to Ford, I ask to speak to the mechanic and we both go back to see the truck. The alternator is unhooked so it does not drain the batteries. He tells me your voltage regulator in the alternator is bad so you will need a new alternator. I told him, the voltage regulator is external from the alternator and here it is on the fenderwell. He then gets an embarrased look on his face and he said, you are right. I told him I am headed to the Zone next for a $16 voltage regulator.

I head home first to verify what this mechanic is telling me because he did not score a lot of points saying the voltage regulator was internal to the alternator. I got my multimeter and measured the voltage of the battery with the alternator clipped on and off and did measure a voltage drop with the alternator clipped. I ran to the zone, bought the voltage regulator, changed it in the parking lot of the zone, and shazam, everything is working a lot better. It has been two days now and no dead batteries!

The old motorcraft voltage regulator was original from 1991 and it did its duty. So moral of the story, if you have that fantom current draw when the truck is parked, replace the voltage regulator after doing a current check on your alternator.

Cost to fix = $53 + 45 + 16 = $114

If I would have read the diesel stop first, I might have saved $98! Good luck and keep that old truck.

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1991 F-250, XLT, 7.3 Liter IDI Diesel, 4x4, automatic w/ overdrive, 3.55 rear end, Tow Package/controller, Power windows, AC, Cruise Control, 209K Miles, Miraculously Everything Works! Upgraded from a 1999 Ranger.
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post #2 of 11 (permalink) Old 05-06-2009, 08:57 PM
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Good fix.

I wish all vehicles still came with external regulators.

Electronics are not best suited in constant heat such as found in alternator housings...

1994 Ford F250 4x4: 7.3L IDI factory turbo, supercab longbed, E4OD *DEAD*. ZF5 swap,Dana 60 swap, GM 14 bolt rear w/ 4.10s, 4" exhaust from stock downpipe back to a 5" stack, IP maxed=1100 degrees max pulling uphill, "modified" wastegate=11psi max,99+ F450SD cab/chassis leaf springs in rear
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post #3 of 11 (permalink) Old 05-08-2009, 07:42 PM
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Yeah, my 91' did the same thing a while back. It was after I replaced the battery cables, my voltage regulator was trying to overcharge my batteries while running and then draining the batteries while sitting cold.

It has been three years since I replaced my volt. regulator, batteries and battery cables:

Replace those 15 year old battery cables!

The newer trucks have regulator/alternator combined, but not our older trucks

Gotta love those do it yourself repairs that are cheep and also solve the problem.


1991 F250 XLT, 2WD, 7.3L IDI, 5 speed, 168K.

Last edited by waterdude; 05-08-2009 at 07:51 PM. Reason: add battery cable info
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post #4 of 11 (permalink) Old 08-12-2009, 08:46 PM
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I have been dealing with this for 3 years, and I finaly did a search.......

94 IDI turbo crew cab 4X4 BTS Trany, ATS turbo up grade, 3" exhaust
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post #5 of 11 (permalink) Old 08-13-2009, 09:48 AM
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ya had the same problem on one of my other trucks with a internal regulator. Bought a rebuild kit and fought with it for 2 hours rebuilding it. then teh next day digging throught the glove box i found the receipt for the alt from teh previous owner. Darn thing was only 6 months old and still under warranty before i bought the parts..
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post #6 of 11 (permalink) Old 09-20-2010, 06:20 PM
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Thanks for your investigative post!

I've been chasing a dead battery problem for about 2 months now.. But it only kills one of my batteries. Luckily I "designed" and implemented my own dual battery set-up using a couple heavy load solenoids to isolate each battery, and to control current flow during a engine off, engine cranking, and engine running condition. It's this system that kept one battery always fresh, and using an emergency jumper I am able to get the truck + all systems functional off the isolated (2nd) battery. But all this is neither here nor there.

With the "original"/normal battery always being drained after about a week of sitting, today I decided to see if I could figure out why. I started unplugging connections and poking around with the multimeter trying to see what pulled the voltage down. Originally I observed the battery @ 3.5v when it wasn't hooked up to anything, but with the Alternator in-line it would drop to 1.3v

I'll look into getting a replacement voltage regulator. Hopefully that solves my problem too!

"Clyde": 1986 F250 6.9L 4x4 Hi-boy Sngl-cab long box
he & I have a love-hate relationship
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Last edited by quattro.pilot; 09-20-2010 at 06:23 PM.
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post #7 of 11 (permalink) Old 09-20-2010, 07:05 PM
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I have a 1993 non turbo diesel pickup and whenever i try to start it it wont start it turns over fine but then drains the batteries and i have to pull it to get it started. i have replaced the drivers side battery because the passenger side was replaced not long thinking it might be an electrical problem
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post #8 of 11 (permalink) Old 09-20-2010, 07:16 PM
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A quick test is to disconnect the plug at the regulator, jumper between A & F on the plug, then read voltage at "BAT" on the alternator and B-. This shorts the field and makes the alternator put out it's max voltage. If it's not at least 15V, replace the alternator.

I just had to replace the original Motorcraft alternator on mine Friday. 23 years is long enough for me!


Check my lists of common part numbers in my photos! Updated often!

1991 F350 Lariat XLT, 7.3l, 5-speed, crew cab, SRW, 3.55, 2WD.

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post #9 of 11 (permalink) Old 09-20-2010, 09:12 PM
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What year does the volt. reg. go from ext. to int? I'm Having the same type of problem on my '94. Some mornings I have to jump the bats. Once I get it running, I can start it all day. Some mornings it will start without charging. Any facts or opinions? Thanks guys, Tim

'94 F250, 4X4, 7.3, E40D, 4.10, 147K
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post #10 of 11 (permalink) Old 09-21-2010, 06:06 AM
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Pretty sure you have an external regulator, you can tell because some of the wiring from the front loom goes to a small box (usu. grey, poss. black) on the passenger fender, which has four wires. At least two of the wires are green and yellow.

1992 F250 XLT 4x4 Super Cab 7.3 IDI E40D 3.55 ATS 088 4"R.C 360k
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post #11 of 11 (permalink) Old 09-23-2010, 07:02 PM
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After getting over my laziness, I popped my hood and found it is an internal regulator. But, last night I was shutting down the truck. While turning off the lights, the knob was just a hair from turning the interior lights on and then was acting and felt funny. I played with it and I could get the knob to lock in the on position without the lights turning on. I then turned the knob as far as possible clockwise and away from turning on the interior lights. The truck started without hesitation this morning. Tomorrow I will investigate further but I believe I have found my battery draw problem. I will post my findings tomorrow evening. Thanks.

'94 F250, 4X4, 7.3, E40D, 4.10, 147K
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