M1088 CB70 (20A) fuse keeps blowing

[ke6rwj]

This truck has had some gremlins early months, but since has been fine.. then....

It started to not want to charge, that is getting low voltage when running. I figured it was the batteries. So I was gonna take it to the shop and swap them out. On attempting to drive there, it wouldn't shift. The transmission display only showed one number. The voltage was very low, like never ever been this low.


So I limped it back home shut it down and went inside to get some tools. When I came back out and tried to turn the power on, CB70 blows.. keeps blowing.

TROUBLESHOOTING
- verified the battery hookups not touching etc
- removed all the relays in the dash - still blows
- tilted the cab and made sure no loose connections rubbing etc

Been looking at the schematics, CB70 is a main feed line so there's alot to go through..

My ask...
Are common issues that would cause this, like known parts that fail shorted etc.
Hints and tips..

My next item is to pull the panel and make sure nothing built a nest in there or something..


Thanks.

Chris

 

[Ronmar]

What year A0 or A1 truck?

on an A0 CB70 powers ignition and all the lighting. On an A1, just the lighting…

 

[ke6rwj]

It's an A0 there is no power to the ignition at all..

 

[Ronmar]

Ok, on the A0, CB70 feeds the ign switch and the main light switch. When you turn on the ignition switch it feeds 12v to the 24v ignition control power relay K2 and the brake relay K6. K2 energizes powering the 24v ignition, transmission and start circuits. So it CB70 is pulling more than 20A of 12V, and popping before you turn on the ignition switch, the light switch or some wiring between CB70 and the light switch is shorted.

if CB70 is popping after you turn on the ignition switch, it is probably something in the wiring between ignition switch and K2 or K6. Or the coils of those relays are shorted.

 

[Wingnut13]

Ronmar…. I think you’re on the verge of being a national treasure for your knowledge and willingness to share information with these trucks…..

 

[ke6rwj]

turns out... bad light switch...