I'm having some trouble diagnosing this issue. The voltage on the wire harness is 12.4v, but as soon as I hook up the radiator fan, it drops straight to zero, and no power goes to the fan. Anybody have any ideas?
a failed fan
Fan works just fine. I hooked it up directly to the battery, spins as it should.
that would be helpful information to know. Please provide the whole story. What else did you try?
If you're talking about the wire going to the fan having 12.4 volts when you check it with your multimeter, but after you put that wire under load (connecting it to the fan) it reads 0 volts, then you have a problem with the wire or in its connectors (corrosion or a damaged wire most likely).
The reason your mulimeter reads 12.4 volts is because your multimeter isn't pulling any load through that wire/circuit, it's just reading the voltage under no load.
Corrosion has high electrical resistance that can cause the voltage to drop, which in turn causes the amount of current flowing to fall.
By using a temporary jumper wire from the battery to the fan and having the circuit/fan work, you've already confirmed this.
So 1st, temporarily swap the fan relay with another relay to make sure that isn't the problem.
Then check the fan's ground wire. You didn't mention if you used the fan's existing ground wiring when you tested it when you used a jumper wire from battery + to the fan, or if you provided power & ground to it.
If you used the existing ground, then the ground is good.
So start backtracking. Pull the fan relay and use your multimeter, or if the relay has a pin map on it, to determine which of the relay's sockets is battery voltage going to the relay to make sure the problem isn't there.
So in the case of the diagram below, it's pin socket #30
Instead of hooking your jumper wire to the battery, like you did to test the fan earlier, hook it up to that pin socket. (relay removed so you can get to it)
If the fan will work, you know the issue isn't with the wiring going to the relay. (it can support the load).
So, if the problem isn't there, you know it's between, (using the above diagram) Relay pin socket #87 and the wiring to, or the connector at the fan.
Then it's just a matter of doing the work to determine where the problem is.
Typically, when diagnosing any electrical problem (after determining that there is power to the circuit) you start at the load which in this case is the fan.
You've already tested the fan to be working, so you'd move back and examine the connector plug to the fan.
No problem there, you move back a little at a time towards the relay to see if there is damage to the wire or if there's another connector on that wire between the relay and the fan that may be corroded or compromised.
