M151A2 voltage regulator failing

1959 - 1978, M151, M151A1, M151A2, Technical questions and discussions, regarding anything related to the M151.
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M151A2 voltage regulator failing

Post by crewchief » Tue May 29, 2012 6:42 am

I've checked the -20 and I think I've got it figured out but wanted some second opinions before I go swapping stuff out. My truck starts fine and will run with the bat/gen gauge in the green for a minute or two, then without explanation the gauge will jump up into the red, and the engine will drag down. It will run like the for about 5 minutes, then jump back into the green, and the engine will return to normal. This has been happing during the post start and warm up sequence with the choke pulled out about a quarter inch. I have been able to adjust it back into the green by adjusting the RPM using the choke. It will occasionally do it during driving while at idle and then as always return to normal. Am I on the right track with a failing voltage regulator? My system is a 28v 60A with the voltage regulator built into the generator.
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1939 BMW R71 w/sidecar


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Re: M151A2 voltage regulator failing

Post by Rickf » Tue May 29, 2012 7:56 am

Check all of your battery connections to make sure they are clean and tight. Also check the connections at the starter and auxiliary terminal in the battery box if you have it. A loose terminal will give you the same symptoms as bad regulator and an electronic regulator is usually either good or bad but does not switch back and forth.
Make sure the battery grounds are good also, including the engine to body ground.
Rick
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

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Re: M151A2 voltage regulator failing

Post by crewchief » Tue May 29, 2012 9:46 am

I'll check all of my grounds and connections before I start pulling parts. A lose connection or ground would be a lot better than pulling and swaping a generator.
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1976 AM General M151A2
1939 BMW R71 w/sidecar

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Re: M151A2 voltage regulator failing

Post by Rickf » Tue May 29, 2012 10:22 am

THAT is for dang sure! If you have to replace the alternator take out the air cleaner and brackets. Real easy to do and saves a lot of skin on the alternator job. If you want to cheat on checking the grounds take a LARGE jumper cable. 2 or 4 gauge, and clamp it from the ground of the battery to a clean spot on the engine block. If that eliminates the problem then you have a grounding issue somewhere.

Rick
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

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Re: M151A2 voltage regulator failing

Post by svramselaar » Tue May 29, 2012 11:47 pm

hello

take a good look at the small wire no 568 at the alternator
if this wire or connector is bad you have this problem
the wire give 24 volt to the alternator to start up

george
1976 ? M 151 A2 AMG MUTT lost in acsedent
1978 M 416 A1 ( parkhurst ) trailer
1970 m151 A2 FORD BUDD tub
1957 Volvo sugga radio truck
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Re: M151A2 voltage regulator failing

Post by crewchief » Thu May 31, 2012 5:02 pm

I was able to chase this problem down to an unlikely source and easily fix it. For reference I describe the steps taken and the solution. I started the truck and checked all the grounding points, generator, batteries, starter, etc.... all were good and tight. I checked the connection of plug 568 at the generator, good there. The engine was still surging and dragging as described. I then went to observe the voltage gauge and noticed the temp gauge fluctuating wildly, it was surging and dragging with the engine. I checked the gauge and it was good. I then checked the thermostat at the back of the head. plug 38 was lose, I tightened it and the engine immediately stopped surging and dragging. The temp gauge returned to normal as did the voltage gauge. So I let it run for a bit then shut it down, let it cool for a few hours, then started it back up and let it run for an hour. Problem fixed.
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1939 BMW R71 w/sidecar

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Re: M151A2 voltage regulator failing

Post by Fil Bonica » Thu May 31, 2012 5:16 pm

There is more to this than fixing a loose temp sender wire. The alternator exitation wire must also have been loose and you fixed that also.

Keep poking around to see if you can duplicate the problem.

Fil Bonica

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Re: M151A2 voltage regulator failing

Post by crewchief » Tue Jun 12, 2012 5:04 pm

Here is the final verdict, the voltage regulator was on it's way out along with a failing bearing in the generator and it finally died. The problem reoccurred a day or two later and wouldn't resolve as it had before. So I went ahead and replaced the generator today. What I had was a combination of problems, a lose ground on the water temp sensor, corrosion on the 24v + terminal, and finally a dieing generator.
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1976 AM General M151A2
1939 BMW R71 w/sidecar

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Re: M151A2 voltage regulator failing

Post by Kurt Lesser » Thu Jun 14, 2012 3:34 pm

Just wanted to add something here that most people tend to forget. The alternator output doesn't go directly to the batteries. It does electrically but it makes a connection on the starter switch which is mounted low in the vehicle and gets soaked if you ever hit deep water. If there's corrosion there you will have problems.

Just a reminder.

Kurt
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Re: M151A2 voltage regulator failing

Post by Bart1015 » Mon Jul 06, 2015 7:38 am

Hey guys (and gals), I know this is an old thread, but I am having the same issues with my 60 amp charging system. I would really like any input to help resolve this issue.

Symptoms and temporary fixes:

As I am starting the truck, it does one of two things every time:

#1: I start the truck, the amps are in the red, truck will stumble if I go above idle. After about a minute, the amps will go to the green and the truck drives fine.

#2: I start my truck, Amps are in the green. If I step up the idle it will go to red. If I let it warm up for a few minutes, the amps will stay in the green.

Driving symptoms:

Intermittently, as I am driving the truck will stumble and I will notice the amps are in the red. I pullover and wait (with the motor running) and it will eventually go back to green, then I am good to go. Sometimes shutting down the truck and restarting will cure the high amps. Other times it will not..

I have tested the amps at the batteries while the vehicle is running and this is what I am getting:

In the green it is at 27.5 volts
in the red it is at 29.5 to 30 volts

So here is what I have done so far after following this thread:

I cleaned and reconnected all grounds, battery terminals, starter terminals, terminals on the starting switch, contacts on the generator/ault, including the small wire and its connector.

I could not locate a grounding strap on the motor anywhere. I connected a 2 gauge wire from the block to the body (I cleaned the paint to metal for a good contact)

I believe the batteries are good and I am going to the local battery store today to get them tested.

I am not sure if the regulator is failing or if I am missing a bad connection somewhere. I know this has to be a simple fix but I am at a loss to figure it out.

Anyone who leads me to the fix will receive a nice cup of coffee in the form of a Starbucks gift card!

Thank you for the advice.
1942 VEP GPW, J-2 winch, BC-659, 1919a4
1945 GPW, 1943 MBT
Sold - 1967 M151a1, 1943 MB, M416
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Re: M151A2 voltage regulator failing

Post by Rickf » Mon Jul 06, 2015 8:34 am

You could have an intermittent short inside one of the batteries, Sometimes when they sit too long they sulfate and it does not always make contact all the time. If you know someone else with a military vehicle near you and one of you has slave cables you could disconnect your batteries and plug into the other vehicle and start off of that one and see if it does the same thing. If it does then you probably have a bad regulator. You have covered all of the connections. If you do not have a slave cable you could use jumper cables but you may not be able to start from the jumper cables. You would have to start on your batteries and then disconnect the ground cable. MAKE SURE THE JUMPER CABLES ARE HOOKED UP BEFORE DOING THIS! Positive to positive battery cable and ground jumper to a good ground on the engine block. Then disconnect your ground. NEVER disconnect a battery cable while the vehicle is running with an alternator, nine times out of ten you will damage the alternator or regulator.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
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Re: M151A2 voltage regulator failing

Post by Kurt Lesser » Mon Jul 06, 2015 10:52 am

The Prestolite regulators are now made and sold by Southern Automotive. You can buy them directly or they get posted pretty regularly on eBay. Southern Automotive's website is : http://www.southernautomotive.com/catal ... cts_id/101 The most common alternator is the AMA-5104UT. Drawing link: http://www.southernautomotive.com/catal ... ucts_id/29

If you have voltage shifts it could be the regulator but is more likely the brushes not making good contact with the slip rings. They put the field excitation voltage on the armature and if they don't make good contact the result is voltage variation. If the regulator itself fails the symptoms are more likely to be extremely low or no output from the alternator (red zone). The regulator is probably the most common failure in the 60 amp alternator. There's a 1.5 ohm 50 watt resistor that blows out when they start to fail that can be replaced by itself. Available from most electronic supply houses they run about $6.00. Another common failure are the carbon slip ring brushes. They run about $5.00 and can be purchased through most alternator repair shops.

The Regulator is accessed by removing 6 screws from the rear cover and gentle prying the regulator assembly out of the housing. You'll want to blow out the space with compressed air and you should be able to see both the resistor and the brushes.

If it sounds like a meat grinder it's probably bearing failure but a good shop can change them too.

MADE SURE YOU DISCONNECT THE BATTERIES BEFORE DOING ANYTHING TO THE ALTERNATOR!

Kurt Lesser
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Last edited by Kurt Lesser on Mon Jul 06, 2015 7:41 pm, edited 1 time in total.
Kurt Lesser
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Re: M151A2 voltage regulator failing

Post by muttguru » Mon Jul 06, 2015 11:10 am

Bart,
I go with Rick on this. I reckon one (or both) batteries have a fault. If I were you, I'd get the batteries load-tested to see if there's a failing plate or short.

You could also try using your (or your wife's) auto-battery to replace first one, then the other, M151 battery to see if you notice any change in the gauge indication.
NB, if in the unlikely event that BOTH M151 batteries are faulty, this test probably won't give a definitive result.

I was concerned that you could not locate the body-to-engine bonding cable. It may well be missing...in which case it may have used the carb link and return spring as the route...... I saw this happen with an old Ford Consul MkII which had a disconnected ground cable at the bellhousing.

Your actual M151 ground cable should be easy to locate. Lift the hood and look at the horn. The horn mounting bracket attaches to the body using two bolts. The bolt nearest the engine should have the eye-terminal of the ground cable underneath the bolt head. The other end of the cable attaches to the block near to the distributor. It isn't a braided strap as you might imagine, it's a cable similar to the cables attaching to the batteries. If it isn't there, get a new one made up.

We will be interested to know what you find.

Ken
Contact address - muttguru@aol.com
Always wanted - Details and pictures of M416 Trailer data plates & M151 data plates & body-tags for my research. Thanks!

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Re: M151A2 voltage regulator failing

Post by Bart1015 » Mon Jul 06, 2015 11:15 am

Thank you for all the advice. I will start working through the suggestions in the next few days. I really hope I can track this down. I will keep you all updated on my progress.

Brett
1942 VEP GPW, J-2 winch, BC-659, 1919a4
1945 GPW, 1943 MBT
Sold - 1967 M151a1, 1943 MB, M416
www.ToThePointPolygraph.com

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Re: M151A2 voltage regulator failing

Post by Kurt Lesser » Mon Jul 06, 2015 7:50 pm

Something else I wanted to add to this thread is that the gauge in the 151 doesn't read amps, it reads volts. Most people don't understand the difference between the two but volts indicate the health of the batteries. If they won't take a charge the voltmeter will read in the yellow or red areas. An ammeter shows the amount of current flowing into and out of the batteries. This is a better indication of charging and discharging. A voltmeter will show good voltage if you have corroded terminals that will not allow enough current to flow to recharge the batteries. This is why it's necessary to keep all of your contacts and connections clean and tight.
Last edited by Kurt Lesser on Tue Jul 07, 2015 12:29 pm, edited 2 times in total.
Kurt Lesser
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