LMTV Electrical Issue doesn't respond to master switch

[Maddog Driver]

It was the K-02 relay. When tested, it was faulty. Replaced it. Problems solved. Thanks everyone for the help and feedback!

I had a dead master sw and found the K2 dead...temp substituted the same relay from BO Drive slot. I have not been able to figure out spec for spares based on crossing the two sets of vendor numbers on the panel cover. I am not much electrician, but willing to be placed in remedial learning, I see y'all prefer Bosch. I'd like to lay in some spares for the four types, but haven't figured out how to spec them...appreciate your counsel.

 

[Ronmar]

You only need 2 types. 5 pins can be used everywhere, so you only need to stock 5 pin relays in 12 and 24v coils...

 

[Maddog Driver]

You only need 2 types. 5 pins can be used everywhere, so you only need to stock 5 pin relays in 12 and 24v coils...

What is the correct amp rating at 12v and 24v ?

 

[Ronmar]

Those standard auto size relays are 20-30A typically.

 

[Sully LMTV]

”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…



View attachment 884904

install a pair of group 31’s

Ronmar, you suggest two 12v group 31 batteries, not four group 31 batteries if replacing the stock batteries in my A0?

Thanks!
Sully

Disregard - I looked up the NAPA 7256 part number and see the batteries are 120 Ah 12V. So four (4) group 31 batteries at ~125 Ah should be fine.

https://www.napaonline.com/en/p/BAT7256

 

[Ronmar]

Ronmar, you suggest two 12v group 31 batteries, not four group 31 batteries if replacing the stock batteries in my A0?

Thanks!
Sully

Disregard - I looked up the NAPA 7256 part number and see the batteries are 120 Ah 12V. So four (4) group 31 batteries at ~125 Ah should be fine.

https://www.napaonline.com/en/p/BAT7256

No, it wont be just fine. The use of 4EA 120AH batteries was gross milspec overkill that was poorly engineered/applied. It IS the main reason these trucks have so many alternator and battery issues!

They only provided a 100A dual volt alt(basically two 50A alts) to feed that massive battery. The batteries have trouble charging with that small input and low run hours, so the batteries fail faster. Constant undercharge = constant overload for the alternator, so the alts also fail a lot sooner than they have to. Priced a replacement dual volt alt lately? A pair of 120s in parallel = 240AH. A below full 240AH battery is looking for 60A out of the alt, plus truck loads(AGM batts would be looking for 110A). This is worse on the 12v side as most of the electrical load(25-30A of lights) is on that side, so the 12v side is 150% loaded… the remaining output on the 24v side is overloaded just feeding the batteries…

Cat specced a pair of group 31s for all the power equipment that uses these engines. That or similar is also what is found in all the medium trucks on the road that use these engines(chevy Kodiak, freightliner ect). Anything more than that is just asking fir constant battery and alternator issues…

The 100AH wet cell is looking for 25A out of the alt, so with everything turned on, the alt will be worst case at 100% load until the battery tops off, which will take far less time than the originally specced monster…

 

[Sully LMTV]

No, it wont be just fine. The use of 4EA 120AH batteries was gross milspec overkill that was poorly engineered/applied. It IS the main reason these trucks have so many alternator and battery issues!

They only provided a 100A dual volt alt(basically two 50A alts) to feed that massive battery. The batteries have trouble charging with that small input and low run hours, so the batteries fail faster. Constant undercharge = constant overload for the alternator, so the alts also fail a lot sooner than they have to. Priced a replacement dual volt alt lately? A pair of 120s in parallel = 240AH. A below full 240AH battery is looking for 60A out of the alt, plus truck loads(AGM batts would be looking for 110A). This is worse on the 12v side as most of the electrical load(25-30A of lights) is on that side, so the 12v side is 150% loaded… the remaining output on the 24v side is overloaded just feeding the batteries…

Cat specced a pair of group 31s for all the power equipment that uses these engines. That or similar is also what is found in all the medium trucks on the road that use these engines(chevy Kodiak, freightliner ect). Anything more than that is just asking fir constant battery and alternator issues…

The 100AH wet cell is looking for 25A out of the alt, so with everything turned on, the alt will be worst case at 100% load until the battery tops off, which will take far less time than the originally specced monster…

Excellent info and thank you! I was looking for this type of breakdown and the toll newer battery tech can have on 90's charging technology. This will give me room on the battery tray to house batteries that will be charged via solar (and grid power if needed at times). This battery bank will be for projects not related to the truck's electrical system.

 

[Ronmar]

Excellent info and thank you! I was looking for this type of breakdown and the toll newer battery tech can have on 90's charging technology. This will give me room on the battery tray to house batteries that will be charged via solar (and grid power if needed at times). This battery bank will be for projects not related to the truck's electrical system.

I am planning on the same thing. 2 for truck service batteries and up to 6 more for house batteries(there is room for 8 group 31 batts in the box). I should add that the group 31s are not plug and play. The pockets in the box lid interfere with their placement due to battery height. I replaced the tray and am modifying the lid(removing the pockets and glassing in the holes). IF i recall correctly a group 27 at nearly the same AH and CCA specs will set in the existing battery tray…