CUCV alternator symptoms, diagnosis and fix.

[Barrman]

I am new to the CUCV world. Not the MV world or this site though. I have been reading for years about CUCV starter, glow plug and alternator troubles. So much that I was really hesitant to get one for myself. So, now that I have one I want to pass on a few things I have learned about the charging system to maybe help others.

Our resident CUCV wiring expert, Warthog, assembled my M1009 out of 3 or 4 donors. The wires were all cut off for the alternators and the alternators were not on the truck. One of the reasons I got the truck was that he had new or rebuilt ones on it. I get it home and realize ALT 2 isn't charging.

Now, having read about 20 post a day for the past 5 years about CUCV alternators. I "knew" if the red ALT 2 light wasn't coming on before engine start, the bulb must be burned up which causes the alternator to not work. Mine wasn't coming on, ever, so that must be the problem. I got the truck with no dash or cover on it. I of course didn't figure out the alternator wasn't working until I installed all that stuff.

I called up Joe. He told me to do what I would have done with any other vehicle besides a CUCV. Check voltage at the alternator with the key off and with the key on.

Each alternator is a 12 Volt unit. The big red wire that is bolted on has 12 volts from the battery. The little white plug has 2 wires on it. The thicker is the voltage out of the alternator back to the battery. The smaller wire is the exciter wire.

With the engine off, the big red wire should have battery voltage, so should the bigger wire on the plug and with the plug off, key off, the exciter wire should have nothing.

Turn the key on and the plug off, the exciter wire will have battery voltage along with the other 2.

The above should be your first check of a suspected bad alternator. This tells you if the problem is in the alternator or in the truck. After making sure all wires are hooked up and not grounding out somewhere that is.

My truck side was working properly. I had a problem in the alternator. I called Joe up and asked about the red ALT 2 light. I "knew" it had to be part of the problem because I had read about it here on SS for so long. He kept telling me to ignore the red light. I kept telling him I couldn't, even though it didn't make sense. He told me to look up the -30 procedures for testing the alternator inards and to replace what is bad.

I pulled the alternator off, looked up stuff in the -34 and was lost. They wanted me to build a variable voltage machine just to check out the voltage regulator. I don't have a variable voltage source, so I just threw in a known good regulator and put it back on the truck. I didn't turn the page in the manual to check out all the other components. I took care of the first thing listed in the manual.

The alternator didn't work and the silly ALT 2 light still wasn't coming on. I just knew that ALT 2 light was part of the problem in some way. I still drove the truck though.

Maybe I should cover how you can drive a truck around with a bad alternator and it not be a problem. CUCV's are 12 volt Chevy trucks at heart. The wire for the engine to run, the lights, the blinkers and all that are all 12 volt. The drivers side alternator, ALT1, runs that part of the truck along with the front battery. As long as ALT 1 is good, the truck will drive until you run out of fuel.

The passenger side alternator, ALT 2, only has to charge the rear battery up after the truck has been started. The starter is the only part of the truck that uses the rear battery. I had probably 30 starts on the truck without the battery getting charged. The volt meter was at the top of the yellow zone and the starter turned over at the same speed every time. That was 2 weeks of in town driving without ALT 2.

Since I didn't have the variable voltage thingy machine to test out the alternator, I broke down and paid $44.50 for a complete 27si rebuild kit. All new everything inside the alternator. I put it on and turned on the key. The ALT 2 light lit up. I fired up the engine and the voltage gauge was in the green for the first time since I bought the truck. A volt meter at each battery showed 14.4 and 28.8 across both. It was working.

But, what was wrong? I still had a suspicion that I had hit the dash or something making the light come on and therefore the entire system work. Even though Joe and my test showed the trouble was in the alternator.

I put all the parts I pulled off in a bag and went looking on the Delco Remy site for their version of a test. I posted a thread about the manuals there. They word the test different and I actually turned the page. The Diode Trio and the rectifier bridge can both be tested with a simple Ohm meter. I put new of each in the alternator.

I got out an Ohm meter and tested the Diode Trio I had removed from the alternator. Very simple instructions. Put one lead on the long leg and the other lead on any of the 3 shorter legs. Look at what you get and then switch the leads. One way should be high and the other low. If they are the same, the Diode Trio is bad. My just removed Diode Trio had the same value on all 3 legs with the leads reversed. Bad Diode Trio.

The test for the rectifier bridge is the exact same thing on it. Mine was good.

If you have read this far, you get the summary.

If you are not getting voltage out of an alternator, no light before start up or during running, the exciter wire is exciting and all the other wires are correct. Then I would suggest you spend the $4.00 or so for a new Diode Trio for a 27-si alternator, put it in and go find something else to work on. Or at least read all the pages about testing the alternator parts, test them and then figure out what is bad.

 

[niferous]

So if my light comes on now and then while driving I should check that Diode Trio?

 

[1stSarge]

Very good post !

I’m going to add this to my “show to new guy” links.

If you are getting the exciter voltage, then the bulb is good, that’s why you don’t worry about it.

Niferous:
In this case:
Belts first
Dirty/loose/worn connections (fuse/bulb/connectors) second
Diodes will work or not work.

 

[blzrgb]

Where can you buy a diode trio ?

 

[1stSarge]

Heres one: --- $2.00

http://www.aspwholesale.com/index.php?act=viewProd&productId=184

 

[Barrman]

I would look for loose or grounding wires if you have an intermittant fault. The Diode Trio is a solid state component and it pretty much a good or bad. Not a sometimes.

Does your volt meter move up or down with the light showing up or not?

I ask because the si alternators just need the exciter wire to work at start up. Which means as long as they get "turned on", and the engine stays running. They will continue to charge until you turn off the engine. Yours are working, so something somewhere on the truck side is probably either shorting out or not staying connected.

Do the test with the plug on the back of the alternator checking voltage key on and key off. That will tell you to look at the truck wiring or the insides of the alternator. I suspect you will have a loose wire somewhere in the truck harness.

 

[niferous]

I tightened up my belts and it for the most part went away. They still squeal though from time to time so I may want to try some belt dressing and see if that helps.

 

[1stSarge]

Great!
Squeal -

1. Bad belts
2. Misaligned/damaged/dirty pulleys
3. Bad batteries

 

[Barrman]

Bad batteries was my first thought as well. What kind of voltage do they have with the engine off? Check that, then start it and see how far they drop while starting. The 1/2-1 second it takes to start a 6.2 should not drop them down much below 12.0 volts. Anything below 11.0 volts while cranking means you have a battery that can't handle any load.

With the pulleys, look and see if they are shiny all the way down inside the groove. The pulleys are just stamped steel pressed together. As they get old, they sometimes flatten out and the sides of the V on the belts don't make contact anymore. The skinny bottom of the belt is what is turning the alternator. Not near as much surface area, you will get slippage and a belt that stretches and wears out fast.

 

[Amtracker]

As I muddle through the TM -20 and this post (very helpful BTW) I think I'm misunderstanding something either in the original post or the TM. The TM refers to the alternator (voltage) regulator as a component on the M1010 only. I'm trying to eliminate this as a possible problem for my M1009's own electrical gremlins.

 

[Warthog]

Tim needs to fix his original post. He list the manual as the -30 when it is really the TM 9-2320-289-34 manual.

Look in Chapter 4-2 for testing and repair. The main replacement items in the CUCV (1008/1009) alternators are: Rectifier, Regulator, Diode, Brushes and bearings

 

[Barrman]

The M1010 is a different animal when it comes to the charging system. Because of the huge 24V load the box on the back of the M1010 needs, they just put a single alternator on the truck. 24V. Don't read about M1010 stuff unless you want to be confused.

The rest of the Chevy CUCV series uses the dual 12 volt alternators to make 24 volts. Those alternators are Delco-Remy 27si internal regulator units. Basically, the same internally as any GM vehicle sold from 1972 until 1990 or so. Just the outside size of the case and the diode trio change as you go from a 10si, to a 12si and up to a 27si.

Actually, only the diode trio with a longer leg is the only difference inside between what is on a CUCV and what came on any Chevy truck of the year range listed above. The regulators, contacts, rectifier and such are all the same size and will bolt on. However, the rectifier is basically what sets the amp rating of the alternator. You can use any of them in a pinch, but getting the proper one should be a priority.

Since I started this thread to educate people about chasing down problems. I might as well go over the isolated ground stuff too.

The drivers side alternator is ALT 1 and feeds just the front battery. This battery is what runs all the systems on the truck except for the starter, stock glow plug power supply and the slave cable circuit. Alt 2 is on the passenger and feeds just the rear battery.

The rear and front battery combined work the starter, slave cable and the glow plug resistor with 24V.

The two seperate 12V systems need to stay seperate. To do that, Alt 1 which runs the truck uses the truck as its ground. Alt 2 can't do that and has to be grounded back to the rear battery only. How do you mount an aluminum cased alternator to an engine and not have it grounded to the engine? You isolate it. Little fiber washers go between the rectifier inside the alternator and the alternator case. The recitfier itself is grounded to the battery. All the wires just go to the rear battery.

Only Alt 2 needs to be isolated. However, to make things simple. The military ordered and kept isolated ground alternators on both sides. So, a non isolated ground alternator from a mid 1980's Cadillac or Olds big boat car will bolt right on to both sides. It will only work on the drivers side though. Install it on the passenger side and sparks will fly why you plug in the batteries. Why tell you this. So, if your alt 2 goes out, you can remove it, put Alt 1 on the passenger side and then by a new readily available replacement from your local auto parts store. Giving you a vehicle to drive while you get the parts to rebuilt the bad alternator. Then put the civi car alternator in the back as a spare once you get both isolated ground alternators back on the truck and working.

Actually, getting the proper kit for amperage and the isolated ground wafers will let you swap the civi one over to an isolated ground unit rated the same as the military units. Presto chango, one new isolated ground spare ready to be installed on either side of the truck.


Now that I have gone on and on about the basic parts and what works where. Start your engine. Use a digital volt ohm meter to measure voltage on each battery terminals. 14.4 volts should be what you are getting on each battery. It should be very constant too. Not bouncing around. Sure, it might be at 14.5 volts right after starting the engine and then go down to 14.4 volts after a minute or so. But, it will then stay there. If it is bouncing around, then you could have a loose wire somewhere, a wire shorting out or a regulator about to die. However, most of the ones I have had go bad normally pick too low or too high and stay that way.

 

[Barrman]

Thanks Joe. I was thinking M35 manuals when I wrote the first post. Sorry about any confusion.

 

[niferous]

Ok I'm having what I think is alternator trouble and I'm a little lost. Automotive electrical work is not my strongest point so I'm hoping I can get a little help.

I recently started having a little trouble with my voltage and I'm not sure where to start. I had the passenger side alternator rebuilt, and had the batteries tested and they passed. When I got the truck the GEN 2 light would flash on and off. Also when the light would come on the voltage would drop. I tightened the belts as best I could and that, for the most part, went away. Now every once in a while the light comes on but the voltage does not drop.

However up until about two weeks ago the voltage always stayed about half way into the green but for some reason now the voltage always sits on the line between green and yellow. The truck runs and starts fine. But it just seems odd that all of a sudden it would do this.

I tested both batteries and off and on they read 12 volts. I'm not sure where to test on the back of the alternators and while I know it's been covered in the post so far, is there anyway someone can post some kind of picture and explain what voltage I should be getting at each point?

 

[Warthog]

What type of voltmeter are you using? Digital or analog (needle type)?

You can test the alternator output at the batteries. Barrman summed it up at the end of his last post.

"Now that I have gone on and on about the basic parts and what works where. Start your engine. Use a digital volt ohm meter to measure voltage on each battery terminals. 14.4 volts should be what you are getting on each battery. It should be very constant too. Not bouncing around. Sure, it might be at 14.5 volts right after starting the engine and then go down to 14.4 volts after a minute or so. But, it will then stay there. If it is bouncing around, then you could have a loose wire somewhere, a wire shorting out or a regulator about to die. However, most of the ones I have had go bad normally pick too low or too high and stay that way. "

Use the volt meter to check each battery. Black lead to negative terminal and red lead to the postive terminal.

A fully charged battery with the truck not running should read ~12.7v. With the truck running it should read ~14.5v.

Add the two readings together and you will have ~25.4v off and ~29.0v running.

Anything not close to the above readings will indicate something wrong with one or both of the alternators.

Here is a diagram that AMPHI made for another project (sorry for butchering it). It show where to attach the voltmeter leads.

 

[niferous]

Ok I'm about to go to work but I'll check it tomorrow. Thanks!

 

[Barrman]

Joe summed up what wasn't already mentioned real well. I will add this. If your volt meter on the truck is just staying at the yellow/green divide. You probably only have 1 working alternator. Do your checks and tell us what you find.

 

[Wagner]

I am a relative M1009 newbie with alternator #1 problems. The gen #1 is not comming on at all. An alernator specialist that I went to said he thought the belt needed to be replaced. I replaced the belt but the Alt #1 light still does not come on either when staring or running. The alternator gauge jumps around on starting and settles dead center between the yellow and green. The Alternat specialist did jump the excitor and the Alt # 1 when on for a short while but no longer does so.

I am trying to decide whether or not to just replace alternator #1 and see it that corrects the problem. I would appreciate any thoughts on what else might be the issue.

 

[Warthog]

I am a relative M1009 newbie with alternator #1 problems. The gen #1 is not comming on at all. An alernator specialist that I went to said he thought the belt needed to be replaced. I replaced the belt but the Alt #1 light still does not come on either when staring or running. The alternator gauge jumps around on starting and settles dead center between the yellow and green. The Alternat specialist did jump the excitor and the Alt # 1 when on for a short while but no longer does so.

I am trying to decide whether or not to just replace alternator #1 and see it that corrects the problem. I would appreciate any thoughts on what else might be the issue.

The CUCV alternators are different that Civvy units in that they use "isolated ground" units. This means the internal electricals are "isolated" from the case by fiber washers. Also the idiot lights must be working for the alternators to excite.

Has your mechanic ever worked on CUCVs before? Many of us have had to learn ourselfs and teach our mechanics.

You can test the wiring to see if it is in working order. It has been covered many times.

We will cover it again.

1. First you must make sure your batteries are fully charged and can handle a load. Autozone can load test them.

2. Unplug the two-pin plug from the alternators

3. With the key OFF check for voltage at the small RED wires. You should have a constant 12v on the GEN1 (driver side) and 24v at the GEN2 (passenger). You will not see any voltage at the brown wires.

4. If you do not have voltage at the RED wires you have to trace the circuit and find the problem

5. With the key ON check for voltage at the small BROWN wires. You should have a constant 12v on the GEN1 (driver side) and 24v at the GEN2 (passenger)

6. If you do not have voltage at the BROWN wires you have to trace the circuit and find the problem

7. If the above test works, then you need to remove the alternator and have it tested/rebuilt.

Check the wiring and then report back.

 

[Wagner]

Thank you Warhog for the great advise. My M1009 is based in Western Arkansas and I live in Connecticut and I mostly will not get to work on it until after the first of the year however when I do I will go through your test protocol and "report back". The fellow that looked at my M1009 had not seen one before though he specialized in alternator service and repair.

I was wondering where the needle position should rest on the batt/gen instrument if the system is working properly....mine rests dead center on the line between the yellow and green?

Thank you again.

Wagner