bad ground to fan selenoid switch (2024)

have freightliner century 98 with detroit 60 and radiator fan runs continuously. The voltage to the selenoid is 0 v when harness connected and it is 13.7 v when disconnected. I'm sure I have weak ground from somewhere to the selenoid. how to fix?

Horton fan solenoid should have 12v ignition on one wire. Some trucks run this wire through a dash mounted Fan Override Switch.
The other wire grounds through the DDEC ECM to release the fan.

Are you losing the ignition supply when you connect the solenoid?

Fan overide switch is usually connected to ECM auxially input.

Fan solenoid is ground controlled. Mercedes ECMs controll + and - together only.

With ignition ON, I read 13.7 V @ unpluged wires coming to the selenoid BUT when I connect the plug to selenoid the voltage drops to 0 V. I have found that the ground breaks somewhere @ ECM or circuit breaker because when I connect a different ground the selenoid responds and the fan stops. Will check the continuity from ECM to selenoid ground tomorrow. Thanks for the replies.

use 20 wt bulb to test wiring. if you see 0v connected DVOM reads, check voltage supply. ECM controlls grownd wire and seems to me it is ok and power supply wire is broken somewhere

Sorry, but you have me confused.

To release the fan you require a circuit from the ignition switch through the solenoid and to ground through the FAN CONTROL#1 output on the ECM(usually A2). This will energise the solenoid valve and air will flow to the fan clutch to release it.
If this circuit is broken anywhere the solenoid will close and the fan will lock. Also if you loose air the fan will lock in.

As Pablo says you can use a light bulb to test the circuit.
DO NOT USE A 20WATT BULB AS IT MAY OVERLOAD THE ECM OUTPUT, USE A 5WATT BULB.
Disconnect the solenoid and fit the wires to the bulb. With the key on the bulb should be on bright. One end of the bulb will have 12v on it. The other end will be close to 0v(less than 1v).

I do test of power circuit with 20 wt bulb. I use alternative ground, not with ECM. well, to check ECM 5 wt - less risk

I believe that the DDECIII/IV outputs have a maximum current rating of 1.5Amps.
Using the old Power(Watts) = Volts X Amps formula for a 20Watt bulb you get a current of 1.7Amps.

Using an "alternative ground" allows checking of the ignition supply to the solenoid, but it does not test the wire from the solenoid to the ECM, or the ability of the ECM output to ground the solenoid.

Thanks for the replies. I removed the DDEC IV from the truck this past saturday just to see if something wrong with pins and plugs. I had chance to spray CRC electronic on the connections. Also, I checked and found continuity on the ground wire from A2/D55 @ the output of ECM to 234F @ selenoid plug. Bottom line: with key ON and 12v bulb connected in place of selenoid, the bulb lights when connected to a different ground but when connected to selenoid ground it fails. Now need help following the ground problem from somewhere to the ECM input. Any inputs?

ouh. the MOSFET of ECM outputs is damaged, look for guy with DDRS, he can reprogram fan to other output and use the other pin.