LMTV Electrical Issue doesn't respond to master switch

jwesley74

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Englewood, FL
I screwed up by starting my truck with a spare battery then disconnecting it while running to hook up regular battery. Now it doesn't respond to master switch when running. Couldn't shut off unless I disconnected battery and voltage regulator. Transmission also has all lines illuminated on display and doesn't respond to any buttons pushed. I've checked all breakers and relays. Nothing tripped. I learned after the fact I could use start inhibit switch to shut off however doesn't fix no response to master off switch. Lights on dash remain illuminated regardless of master switch on or off. Clearly I screwed up something disconnecting battery while running. Please help. Don't see anything in TM. Only thing it says if won't shut off by master switch use start inhibit switch and notify maintenance.
 

Suprman

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That is a very strange problem. Could be you somehow ended up with 24 volts on the 12 volt circuit and damaged something. The trans control is 24 volt in these trucks. If the keypad is not functioning it may be damaged. If the dash switch is unresponsive pull your 24v power relay out of the dash panel. See if that shuts the truck off. More information is needed.
 

snowtrac nome

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western alaska
on the a-1 trucks the tcm is 12 volts I have to believe the a-0 trucks are the same to the trans solenoids are also 12 volt sounds like too much power went through something maybe a lot of smoked diodes.
 

jwesley74

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Location
Englewood, FL
Problem solved. I fried the ECU. I pulled it apart fried connection to computer chip. Got new ECU programmed and drives normal again. Luckily friend had ECU in left over stock from a job and programmed it for me for free... so could have been expensive fix... I lucked out in my situatuon... just beware doing what I did disconnecting battery could cost you thousands in ECU...
 

jwesley74

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Englewood, FL
Also to add, the master switch was unresponsive because I cooked the relay in the power panel. I changed relays that fixed that problem. As soon as I changed ECU rest of the issues fixed.
 

LmtvPR

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Also to add, the master switch was unresponsive because I cooked the relay in the power panel. I changed relays that fixed that problem. As soon as I changed ECU rest of the issues fixed.
Im having a similar problem with my lmtv, what relay did you switch out?
 

ChiefMinion

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Prescott, AZ
Gentlemen, I'm working to get a M1078 A0 back up on its feet and home. The current owner was kind enough to purchase new batteries (8Ds) for it. Everything electrical worked for a bit, then one of the batteries crapped out. (Batteries+ stood behind it and replaced it. Kudos to them.) Now this master switch issues has cropped up after new battery install. (Yes, I double checked that they were installed correctly.) Unfortunately, it's about 100 miles each way for me to work on it. So, I'm trying to be prepared. I'll check out the K2 relay. I've been trying to figure out what ECU was being referred to earlier (jwesley74) in this thread. There seem to be several "ECU" units in these trucks.

Can one of you tell me which one we are talking about here and where it is located?

What type of inspection, other than visual should I try to perform?
 

Ronmar

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Port angeles wa
”This master switch issue has cropped up” what exactly does that mean? In The OPs post, his wouldn't shut off, I am getting the impression that yours won’t turn on? The A0 uses 12v batt power thru Circuit Breaker 70 and the ignition switch to energize K2. K2 provides 24V ignition power to a bank of fused circuits to operate the truck. so I would start with looking for battery 12v at CB70. Remember voltage is not good untill it is tested good under load. if CB70 shows battery voltage, measure it again after you turn on the main switch. If CB70 voltage falls off when you turn on the switch, you have a bad connection somewhere back along the path to the batteries. The most likely spot is the polarity protection box between spare tire and air filter as it is out in the weather.

Take along an assortment of automotive blade fuses. Make sure they are the type with exposed metal spots on their top edge. These will plug in where the Circuit breakers fit and the little metal test points on top allow you to easily sample the circuit voltage with the fuse installed.

if CB70 voltage stays good with switch on, K2 should go click when you turn on the switch. If not look for 12v from the ignition sw at pin 86 in K2s socket. If it is good there, K2 may have a bad coil. Remember it is a 12v coil relay.

Below is a simplified/condensed A0 schematic that I drew of the ignition and start circuits for the A0.

When you get it home, scrap the 8D batteries and install a pair of group 31’s, a small equalizer and a float charger(solar or plug-in).. Like the four 6T batteries they originally came with, a pair of 8Ds is way too much battery for the 100A alt to support. Cat specced a pair of group 31s for the equipment that uses these engines, that is all it really needs… The scrappers are charging unobtanium prices for the 100A dual volt alts($2500), and oversized batteries can place that alternator at high load and high risk… the A0 also has a 12v vampire load thru CB22 to the transmission controller. That + oversized batteries + setting more than they run = failed batts and alts and a good chunk of the electrical issues these trucks experience.

Good luck…



A7424097-7CBF-4A01-AB6B-0D100892FD22.png
 

ChiefMinion

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My issue similar to the original post. The master switch doesn't turn off the truck. If the batteries are connected, everything is powered on regardless of the position of the master switch which makes me _suspect_ a relay issue. Failed in the closed position. I hope it's that simple, but then Murphy was an incurable optimist.

The automotive fuse suggestion is a great one. I hadn't thought of that. The diagram will be VERY useful, thanks! I'll print it out and take it with me. It gives me something to work through. Fortunately, I have a buddy going with me. That makes testing a lot easier. Again, thanks for the diagram!
 

FarmingSmallKubota

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Wapakoneta, Ohio
My issue similar to the original post. The master switch doesn't turn off the truck. If the batteries are connected, everything is powered on regardless of the position of the master switch which makes me _suspect_ a relay issue. Failed in the closed position. I hope it's that simple, but then Murphy was an incurable optimist.

The automotive fuse suggestion is a great one. I hadn't thought of that. The diagram will be VERY useful, thanks! I'll print it out and take it with me. It gives me something to work through. Fortunately, I have a buddy going with me. That makes testing a lot easier. Again, thanks for the diagram!
I use a cheap $89 dollar chrome book with the TM's downloaded so i can take them anywhere when i work on trucks and Generators
 

Ronmar

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Port angeles wa
I do the same thing with an Ipad. I cropped just the schematics out of one of the manual PDFs it is a smaller file size so loads and pages faster than an entire manual.

well if yours is stuck on, either K2 is stuck or something is powering it like a stuck ignition switch. Pulling CB70 will help you to decide which fairly easilly…
 
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