VS radiator fan no workee, I have tried everything

[jeepster]

I had the engine changed, now the radiator fan's not working.
Only if airconditioner is on: fan will start about half a second
after the compressor clutch and turn off 5 seconds after the
clutch.

Otherwise, running the engine with car in park, the temperature
rises and rises. The overflow tank starts bubbling at about 60% on the
temperature guage, but no fan. With old engine, the fan came
on before 50% on the guage (without aircon).
I checked the temperature guage with resistors - 39 ohms sends
the needle to H, and 62 ohms to midway, so it is working.
Next inspected the wiring for damage. I pulled the panels
out to get at PCM, all the connectors were firmly in place.
I swapped the fan relays, no effect.
I shorted out the contacts which are the relay normally
open pins, and the fans run. However low and high speed
sounded very similar.
I unplugged the coolant sensor, and started the engine:
low-speed then high-speed fan. A faulty sensor should turn on the
fan as failsafe. I should have tried this first.
I shorted contacts 5 and 6 of the PCM diagnostic connector: fan
did not run. I read somewhere that fan should start in diagnostic
mode. I got no PCM error codes, just 12.
I connected 180 ohm resistor across the sensor plug: fan off.
I connected 150 ohm resistor - this gave me low-speed fan.
Connecting 100 to 130 ohms: low speed for 5 seconds, then
high speed.
The last test suggests that the fuses, relays and PCM are
okay and the coolant sensor is bad. I could not get probes
of a multimeter on the sensor contacts, so I just bought a new
unit for $62. I put it in, topped up the coolant, and bled the
air. I ran the engine until the overflow tank was boiling, but
the fan still does not come on. The radiator cap is correct
pressure rating, and is 5 months old.
I was puzzled - is the new sensor bad?
I put the old sensor in boiling water and measured it, I got
181 ohms, which means the old sensor was near enough to good
and should have triggered the fan.

Any ideas?

[Brett_jjj]

I had a similar problem with my VSV6.The cooling fan wouldnt cut in at all. It wouldnt even cut in when the coolant temp sensor wiring connector was removed from the temp sensor with the engine running.With the sensor wiring disconnected,and the engine running,the cooling fan should start rotating on low speed after a few seconds,then it should switch to its high speed.This shows that the fan system is working ok.After hours spent testing/checking relays,wiring and connectors,and testing the cooling fan motor etc,the problem ended up being the ECU's printed circuit board (PCB) .The ECU connector pin no F6 in the blue connector was burned out on the actual board itself.It even scorched black marks onto the inside of the metal ECU cover and burned a hole through the board..ECU connector F6 goes to the cooling fan relay.Its highly unlikely to be the same problem,but if you have checked out everything else,then it might be worth a look.Also,I know the cooling fan runs when you bridge the ALDL diagnostic connector pins on the VN-VP's.Ive noticed it doesnt come on though on my VS when the diagnostic pins 5 and 6 in the ALDL connector are bridged,so maybe that only happens on the earlier models.