Battery Disconnect Light comes on...transmission goes into limp mode- only 5 gears

[Erik Erickson]

Hi All,
2003 M1078A1.
The battery disconnect light comes on frequently - even for a short joy ride. Once on, I see the voltage all over the place, then the Transmission light comes on. I also see that it goes into a limp-mode, only allowing 4th or 5th gear max.

The batteries are the originals and seemingly in good working order. No problems starting the truck, even in frigid New Hampshire temperatures. I checked the battery terminals to ensure they are tight. No corrosion on the terminals.

Any other ideas? Thanks in advance!

 

[NDT]

All the usual suspects. Start at the terminals on the LBCD under the spare tire, next the battery disconnect box alongside the battery box.

 

[Ronmar]

Drop to just 2 batteries, have them LOAD TESTED. Check EVERY connection between alt and battery. The LBCD samples the F- signal from the regulator to sense alternator load. When it senses the alt is being driven to full field/overloaded, it disconnects the batteries from the alt. If it is not producing stable voltage with the batteries disconnected, it is either a bad connection or it is an alt/reg issue…

The trans needs good power and will let you know when it is not happy with the power it is receiving. What codes has it stored?

 

[GeneralDisorder]

Load test the batteries. Number one priority. Disconnect them from the truck entirely for the test.

Transmission computer will be the first to complain about voltages that are out of it's range of operation. Mine did that when I had the over-voltage problem.

See my thread about the OVCO issue I experienced. Took me 8 months to figure it out:

2008 A1R with 260a charging questions

Currently has a brand new N3221 voltage regulator (2012 vintage I believe - was new in the box with papers). The truck had a 2013 regulator when I picked it up in Houston. It died about 100 miles from Portland - 2600 miles into the trip back. I put in the brand new spare and drove the last 100...

www.steelsoldiers.com

 

[Erik Erickson]

Thanks for the suggestions. I think I caused more problems now.

I checked the battery connections. Voltages seem ok. Range between 11.X - 12.X. I looked into the battery disconnect box. Clean. Measured voltages there.

I went to the LBCD. Looked ok, but there was some surface rust on the bolts. I cleaned them up a bit. The internal mating surfaces were in nice shape. With the batteries and with the top lead disconnected (labeled 24V Battery), I tested voltage to a nearby nut on the frame. It arched which I thought was weird, but ultimately read around 24V.

I put that lead back and tested the second lead down (labeled 24V Load). This is only reading 5.1V
I tested the 12V Batt and 12V Load, both were 12V.

Before I put the spare tire back and tilt the cab back in place, I used the switches mounted near the engine to start it up. It started for maybe 5 seconds, then shut off.

Since then, I can restart the engine. With the cab in place, same thing. I looked in the fuse box for anything obvious. Nothing. I tested the 12V and 24V lugs in the breaker box.
12V batt is 12V.
12V ignition = nothing
24V batt is 5.1V.
24V ignition = nothing.

Any ideas? Did I blow up the LBCD?

 

[NDT]

Lots of times on here people ASSURE us the batteries are good and then after days of back and forth, they end up NOT being good. Suggest you have them load tested before condemning expensive LBCDs etc.

 

[Erik Erickson]

I agree. That makes sense...and I am not convinced my batteries are in great shape. The only thought I had was that for the past 3 weeks, I would start the truck daily, take joy rides around with my kids, etc. There was the 'battery disconnect' issue, but that has been that way for some time. I usually would just shut it off and restart it. The warning light would disappear and I would continue down the road.

Now, I tinkered with that LBCD and I can't get a single light in the dash to turn on, no noises, etc. SOMETHING just happened. Right?

 

[MrMikey4026]

If you have batteries at 12.0 volts or less, they are either junk or discharged.

 

[NDT]

I agree. That makes sense...and I am not convinced my batteries are in great shape. The only thought I had was that for the past 3 weeks, I would start the truck daily, take joy rides around with my kids, etc. There was the 'battery disconnect' issue, but that has been that way for some time. I usually would just shut it off and restart it. The warning light would disappear and I would continue down the road.

Now, I tinkered with that LBCD and I can't get a single light in the dash to turn on, no noises, etc. SOMETHING just happened. Right?

The 5.1 volts is why everything at the dash is dead. I don't think you blew anything up with your diagnostics. I think your symptoms point to an ever-weakening battery bank.

 

[Erik Erickson]

If you have batteries at 12.0 volts or less, they are either junk or discharged.

Agreed. I think they are not optimal, but until today I could reliably start the truck anytime. I was using these sub-optimal batteries in 0 degree weather, no ether and it would start. I suspect I did something to make it so I am not getting 24V to the dash, no lights, no buzzers.

 

[Erik Erickson]

The 5.1 volts is why everything at the dash is dead. I don't think you blew anything up with your diagnostics. I think your symptoms point to an ever-weakening battery bank.

So do you think that it is coincidental? That the batteries just got so bad that it won't start at all? Each battery is between 11.X volts and 12.X volts. When I measure the voltage for the 12V battery at the LBCD it seems spot on. The 24V seems super weird. As a sanity check, I measured the voltage at the alternator lugs. There is a 12V and 24V one that I found. They are both seemingly the right voltage there...12V and 24V.

Strange right?

 

[Mullaney]

Agreed. I think they are not optimal, but until today I could reliably start the truck anytime. I was using these sub-optimal batteries in 0 degree weather, no ether and it would start. I suspect I did something to make it so I am not getting 24V to the dash, no lights, no buzzers.

.
I have a M1088. It has 4 batteries in it. 2 @ 12v and 2 @ 24v. That flippin truck couldn't charge it's way out of a wet paper bag until I upgraded the alternator. I DON"T THINK you need to do that, but with the crummy batteries that I had AND the questionable alternator I had - I had problems.

SOLUTION for me was to load up the batteries and haul them to the local parts house. THEY had a load tester and I didn't. Two of the four were not capable of a sustained load. Replacing those batteries solved my particular problem.

 

[Erik Erickson]

.
I have a M1088. It has 4 batteries in it. 2 @ 12v and 2 @ 24v. That flippin truck couldn't charge it's way out of a wet paper bag until I upgraded the alternator. I DON"T THINK you need to do that, but with the crummy batteries that I had AND the questionable alternator I had - I had problems.

SOLUTION for me was to load up the batteries and haul them to the local parts house. THEY had a load tester and I didn't. Two of the four were not capable of a sustained load. Replacing those batteries solved my particular problem.

Thanks for that. I think it would be good to get some batteries I have a bit more confidence in. I just don't know if that is the cause of my issues. Planned upgrade for sure...among many others as the budget allows.

 

[Mullaney]

Thanks for that. I think it would be good to get some batteries I have a bit more confidence in. I just don't know if that is the cause of my issues. Planned upgrade for sure...among many others as the budget allows.

.
I understand how that works!

Having confidence that you DO or DON'T need to spend the money is a worthwhile thing too. AutoZone, NAPA (in our area anyhow), Pep Boys, and Advance Auto Parts all do that testing for free. Can't hardly beat the price if you load them up and drive them there.

 

[Ronmar]

Well if you are only getting 5.1 on the 24v load terminal with the 24v batt terminal connected, the 24v diodes are not passing correctly or you have a bad 24v connection at the LBCD. the batt power runs from batteries to the 24v batt terminal(alternator 24 is also connected here) thru the diodes and back out the 24load terminal and on to the power panel.

The 12v path is the same, batt to LBCD 12v batt terminal(along with alternator12), thru the 12v diodes to the 12v load terminal and on to the power panel….

 

[GeneralDisorder]

12.6v is a fully charged battery. 10.5v is a fully discharged battery.

Anything in the 11.Xv range is EXTREMELY suspect and may have essentially zero capacity with shorted cells. See chart:

Disconnect each battery from the truck and LOAD TEST them. You can't continue the troubleshooting process till you do this. I use one of these at my shop but there are cheaper ones without the printer and there's the old carbon pile testers that worked for decades but couldn't do some of the fancier testing that can detect actual failure mode:

https://www.amazon.com/ESI-726-Digital-Electrical-Analyzer/dp/B00PSGQ37U

The LBCD diodes (between 12v batt and 12v load, and between 24v batt and 24v load) should show a 0.7v drop across them.

 

[Ronmar]

Yep, you want in the upper 12s on battery voltage to proceed. They also need to deliver a respectable amount of current when loaded to the test voltage(crank amp test).

The diodes in the LBCD should only drop the voltage 0.3-0.5V from the battery terminal to the load terminal on the 12 and 24v circuits. And as always when troubleshooting DC circuits, voltage is not considered good until it is measured good WHILE supplying it’s load…

 

[Erik Erickson]

Update:

I want to thank everyone for your comments up this point.
Yesterday I was removing some of the terminals from the LBCD, starting with the top one. I put it all back together and, as you know, nothings would start.

Today, I was able to pull the LBCD, which was a total pain to explore it more. While doing so (with the power on), I was removing the top terminal and I could hear the all the buzzers in the interior turn on!



From the prior day, I inadvertently sheared the stud on the 24V Batt terminal. This was an oversight on my part - I think this is a special stud. Notice the threads taper to a smaller diameter?



The other issue is that the nut that was epoxied into LBCD was loose and can be removed.


Here is a picture of the little bit of the stud still inside the LBCD.


So, I need some recommendations.
1. Do I attempt a repair or do I need to replace?
2. If repair, how to secure the nut into the LBCD? Epoxy, some other adhesive?
3. Do I attempt to remove the bit of stud?
4. If yes, any ideas on where I can buy a replacement?
5. If no, do you suggest a conductive paste or another solution?
6. Did I forget to ask anything?

Thanks in advance guys. You have been very helpful!

By the way, I checked all the battery voltages today. All stable at 12.4V- 12.8V

 

[coachgeo]

your not the first one to have similar issues. Search might come up with what others did.

 

[coachgeo]

here to help you get started

Advanced search link using terms LBCD fix

Search results for query: LBCD fix

one of thread titles from, above results looks interesting. Vaguely remember it

LBCD replacement due to loose studs. CARNAGE!!

When addressing loose and corroded connections at the LBCD I noted that several studs were loose in the housing. After confirming with several members that this was a clear indication of failure (the unit is no longer sealed against moisture at the very least), I procured and installed a...

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