I have a 1999 Freightliner FLD120. The temp and oil light indicators/buzzer came on and the fan is running constantly and the heater on the mirrors won't work. The engine is not running hot. Have checked grounds, replaced fan solenoid, changed coolant temp sensor, installed new EDM, changed ground from alternator to the starter, new ignition relay and mirror heat relay. The buzzer went off and fan was running fine for a few days and then randomly came back on while I was driving and haven't been able to get the problem to stop. Truck isn't throwing active codes. Any help would be appreciated.
Mines doing same thing.Fan comes on for no reason 01 Fld . This is such a common problem I forget it’s even there!!! Probably my fan switch.Sounds like you did everything you should have. As long as A/C is off (including defroster) and it still does it. You may need to check the wiring harness to fan selinoid you say you already replaced (on firewall near steering column) My last Truck 99 Fld had a bulliton on it from factory. No one seemed to know anything about it. Till I asked Freightliner in Youngstown about it while buying some parts. They were all over it. Finally someone heard of this!!! They sold me a new harness that included a pigtail splice of some sort and explained how to wire it in even loaned me special expensive crimping tool. I fixed it on their lot within a half hour. Truck had that problem for years up until then that fixed it. That’s all I got Take it from there. Good Luck
Check all wires and any sensors related to the cooling system. Williams Detroit found that once a week problem we had. Truck was out of town. I don't remember exactly what it did other warning light. It was the harness mentioned.
Traced the electrical wires from the solenoid back to the firewall. Found a small break in the positive wire from the starter to the alternator. The exposed metal on the positive wire was pressing up against the valve cover. The valve cover had worn a hole in both the positive and negative wire (I had already changed the negative wire). I wrapped it in electrical tape and the buzzer and warning lights are now off. Thanks everyone for all of your help and suggestions.
