Medium and Heavy Trucks
Have Truck Repair Questions? Ask an Expert
Hi my name is***** will be glad to help. First check the battery to starter cables including all grounds for chafed insulation or corrosion. Test all batteries with a digital tester not just for voltage but to check for an internally shorted battery or dead cells. If ok then check first for any add on equipment such as CB radios, aftermarket radio installations or any other accessories that could be drawing including any add ons that might be wired into BAT power. Next check your LVD module which is supposed to cut power to your accessories in the cab if low voltage is detected to protect the multiple ECMs on the truck from damage from voltage spikes. Some bypass these modules when they fail instead of replacing them which can create further more costly problems down the road. Have your alternator tested to be sure is is not allowing power to backfeed causing your problem. Another potential problem is a short in the harness to the PDM or corrosion in the connectors or harness. Let me know
Im not sure if you read mi post. But i wrote that no voltage draining when i turn of batterys and i put headlights on. whouldnt that say that something short or something with lights?
Are there any fault codes logged in the dash display for the Sam cab modules? These trucks are not wired old school wiring. All the lighting circuits run through the Sam Cab modules and are controlled by them. You could potentially have a failed SAM module if the harness and PDM connections check out good with no shorts. Let me know if you can scroll through the display and pull the chassis codes. So much as a dome light out, Headlight, turn signal, etc will throw an code
This is called multiplex wiring and Freightliner has diagnostic software to read these modules that is proprietary to them. However the dash display will allow you to pull your codes for the chassis. Let me know if you can get the codes from the dash.
Here is the drivers manual. See pdf page 22. let me know if you can retrieve any codes