Odd Voltmeter Reading

[cavetech]

My M1078 has developed a generator problem I think. When I start it up the voltmeter reads about 27 volts. After about 3/4 mile of travel the voltmeter drops down to about half.

Can this be normal?

Can the truck be driven like this?

What should I trouble shoot?

I checked the battery connections and found two loose terminal connections which I cleaned and tightened. Still have the issue.

Thanks, Steve

 

[Suprman]

Use a multimeter. Does it drop to say 24 volts or 14 volts? Less than 24 volts you will know it. The trans won't shift properly or sometimes at all. You may be loosing the excite/regulation connector to the alt regulator. Or it could be a bad connection somewhere.

 

[NDT]

Sounds like the alternator is intermittent. When it is faulting, check for excite voltage on the regulator terminal. If not, do the "suprman mod" (run a new excite wire from the fuel solenoid) or start troubleshooting. A relay in the pdp controls the excite. I would say you have 10 minutes of driving before the Allison faults out on low voltage.

 

[cavetech]

I am really glad you told me about the transmission and the voltage. I had no idea the trans needed voltage. Hopefully, tomorrow I can try and test the voltage. The volt meter on the dashboard dropped to half which is why I noticed the problem.

 

[mechanicjim]

If the alternator you have is a Niehoff they have trouble shooting guides on their website "http://www.ceniehoff.com" under the service and support tab.

 

[Suprman]

None of the troubleshooting guides work well for intermittent electrical issues especially when you cant get the gremlins to show themselves when you are doing the troubleshooting.

 

[cavetech]

OK. I've made some progress. The problem is no longer intermittent it is full time. No 28 volts showing on volt meter, only 14 volts. Circuit breaker CB-20 was not tripped and seems OK.

Cab is up (storm coming).

I've looked over the alternator for corroded connections and all looked well.

I've been through the Operator's Manual as well as service manuals 1 - 5 and can't find instructions on how to test the output.

I've been to the Niehoff website and can't find my exact alternator which is N1506. Found one # N1505-1, but can't find a picture of it or trouble shooting instructions.

Right now can anyone tell me how to test the alternator for 14 & 28 volt output? I have volt meter and multi meter.

Thanks, Steve

EDIT: I have just found a Niehoff troubleshooting guide and I'm reading it.

EDIT: DMM broken. Will buy a new one tomorrow.

 

[cavetech]

I found the alternators manufacturer. Here is a link to trouble shoot their alternator:

http://www.ceniehoff.com/Documents/Ctrl_Hyperlink/TG0018A_uid12172009221392.pdf

Steve

 

[cavetech]

Well, I finally have some more info with my pretty new DMM.

The alternator puts out both 12 volts and 24 volts according to the DMM. I take it that the alternator is working correctly. What should I check next and how should it be done?

Thanks, Steve

 

[Suprman]

If you are only measuring 12 and 24 its low. The alt outputs should be 14 and 27+. Measure with engine off and then with it running.

 

[cavetech]

If you are only measuring 12 and 24 its low. The alt outputs should be 14 and 27+. Measure with engine off and then with it running.

With engine running and measurements taken on alternator I read 14 volts and 28.5 volts.

With the engine off and reading at the alternator I read 12.8 volts and 25.8 volts.

The dashboard voltage meter is reading again. The needle is vibrating/bouncing between 26 - 27 volts. Kind of hard to read with cab tilted forward. What is my next step?

Thanks, Steve

 

[Suprman]

So the alt is working. I would check all the connections at the batteries and protective box behind the cab. Could be a bad gage or bad connection somewhere. What year is the truck?

 

[cavetech]

So the alt is working. I would check all the connections at the batteries and protective box behind the cab. Could be a bad gage or bad connection somewhere. What year is the truck?

What a relief my alternator is working.

My truck is a 1997.

I've already checked the battery connections. Are there other connections in between the alternator and the battery box?

By the way, thank you for helping. It is really appreciated.

Steve

 

[cavetech]

Voltmeter

I opened up the dashboard where I could get at the back of the voltmeter and started the truck.

I used the DMM to read the voltage at the terminals.

Engine running caused the voltmeter to read about 23 volts.

I touched every combination of terminals with the DMM and couldn't get any reading.

What am I doing wrong?

Here is a drawing of the back of the voltmeter in case it makes sense to anyone.

 

[Suprman]

Start by checking the alternator to frame ground strap, cab to frame ground strap you can see it from the passenger side looking into the front grille. Swap out the 24v power relay. I think its a 12v relay. Add a ground strap to the dash panel. Measure the voltage at the relay panel left side. If all the other stuff dosnt fix the problem then there may be a short somewhere in one circuit loading you down or a bad connection at the panel or harness.

 

[cavetech]

The Solution

I finally gave up trying to determine what was wrong much less to fix the problem. I followed all advise plus what I knew to try. No good. So I took the truck to my local Ring Power Caterpillar dealer. I got luck because one mechanic worked on military vehicles when he was in the service.

It took them 8 days to get around to my truck, but it turned out to be a bad military battery post. The post was almost half gone.

I had tightened that battery terminal, but didn't see the eroded post.

So any how I bought a new battery and installed it. Voltmeter worked fine. Problem solved.

What I took away from this experience was learning more about my new truck mainly from advice freely given here. I learned more diagnostics, etc. I also made some friends at Caterpillar. Ring Power didn't charge me anything since I put my own battery in. Now I have local experts to call for help if I break down on the road.

I have a happy truck and I am enjoying owning such a wonderful vehicle.

Thanks to everyone,

Steve

 

[NDT]

We could not help you pinpoint this because in two of your posts you told us you checked cleaned and tightened the battery connections. To me that means remove eight battery terminals, wire brush the posts and lugs, wire brush the bolts and cable ends, and reassemble. This has to be done periodically on all vehicles. Glad you solved it.

 

[Suprman]

I was just thinking the same thing

 

[cavetech]

NUTS! The same low voltage problem is back and it is not intermittent.

Now I have removed every connection and meticulously cleaned all connections in battery box. It did no good. Now I will disconnect battery by battery and test the voltage. Maybe I have another bad battery.

Steve

 

[Suprman]

Original batts or did you get all new ones? There polarity box under the spare tire check for loose connections there. And they have been known to fail. Cab power goes thru that box