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Evening folks,

Long story short - NB8B radiator fan no workey. Suggestions on how to troubleshoot?

Long story medium - smelled a little hot as I got home the other day, but it was 35 at the time so not entirely unreasonable. Temp gauge was a little on the high side but still well within normal. Popped the bonnet for curiosity's sake and noticed the fan wasn't running. Checked the connector to the fan and the fuse, both fine. Grabbed a multimeter and no volts on the fan terminal, the car side of the connector, or the fuse socket itself. Hit the AC and the secondary fan fired up instantly. Both radiator hoses were hot so I assume thermostat is working fine. Had other things to do so parked up and left it. Hadn't thought of it again until just now.

My first thought for troubleshooting is to go searching with a multimeter, but I figured it worthwhile throwing a post up here while I go looking through some wiring diagrams to find where I should be looking. Keen to hear suggestions of how to find out what's wrong though.

Cheers,
Joe

My best guess for troubleshooting:

Start the car, let it idle and come up to operating temperature. Using an IR temperature gun, and taking readings from the black rubber hose, monitor coolant temp. If you have a thermometer you can use in the actual coolant, so much the better. You will feel the coolant heat up the hose when the thermostat opens, that in itself is a guide. At some point after that, you may be able to hear a click from the relay (c90*?).

The fan relay has power all the time from the fuse (blue/yellow) and in run/start (black/white) power wires; there are two fuses, one is the 30a fan fuse, the other is the 15a engine fuse. The yellow wire from the relay to the fan is the one to monitor with the multimeter, if it is powered the relay and coolant temperature sensor are working, and if the fan is not working that points to the fan being the problem. If the car continues to heat up with no yellow wire current either the relay is buggered, or the coolant sensor is not reading correctly (which will also affect engine operation). Replace the relay, and if the problem persists replace the coolant sensor.

Unfortunately, the relay earths through the PCM, and so will always be live when the ignition is in run/start and therefore we can't tell from the relay if there is a problem with the sensor reading - which is known to happen - ask me how I know! You could power the black/white and red/yellow terminals of the relay, which should cause the relay to operate and produce the clicking sound which should tell you the health of the relay.

I would shut the engine down if the temperature readings got much over 90*c to avoid the coolant boiling, particularly if you have the radiator cap off.

The above information is taken from a USDM MSM wiring diagram, so exercise caution with the wiring colours - if your wiring is different you may need to pull the relay and experiment to confirm the correct pins to test (some relays have the wiring arrangement on the base).

TLDR version:
It sounds like you have no power at the output side of the fuse, so the fuse must be blown - you should have power on the relay side of the fuse and the input side of the relay at all times when the ignition is in run/start.

HTH

If you don't have a spare 12v battery, remove yours from the boot and put it somewhere handy near the front of the car.
Unplug the fan connector,
Get some cable and connect + & - direct to the fan (polarity doesn't matter for the test)
If it doesn't spin the fan is dead.
If it spins the fan is fine and the problem is a fuse (but you've already checked all those right ) or elsewhere in the wiring/relays as Green Machine has advised above.

I'll also add since you have an NB8B, get a cheap OBD scanner so you can see if the ECU is getting a coolant temp reading. If no reading your sensor is dead. The NB is a dual output sensor with 3 wires connecting to it. One is ground, the other feeds the dash gauge and the 3rd feeds to the ecu for fan triggering and warm up enrichment etc.
I don't know if the sensor can fail on just one output, i.e dash output works but ecu output is kaput. Unlikely but still worth checking.

OK more troubleshooting after GM's response but before Roadrunners.

Connecting fan directly to 12v = fan works!
Hot side of the 15a fan fuse has 12v
Fuse itself has 0 ohms resistance
One leg of the relay has 12v at all times, two legs have 12v when running, other leg has 0 resistance to ground. I didn't take notes on what colour was what because I'm an idiot. Also why the crap is the fan relay hidden under the guard? Even with googling things that took far longer to find than I'd care to admit.

Sensor was my next stop but it was getting late and I couldn't be bothered messing around at the back of the head. Hadn't considered the diagnostic port though so will try that next. Any suggestions on a cheap scanner?

Thanks folks! Will report back.

Lost a post there somehow.

Basically saying OBD reader FTW. And the coolant temperature sensor can fail gradually, which makes life difficult when you are chasing a tune on the dyno ...

Hindsight is a wonderful thing right?

For those playing at home, disconnecting the coolant temp sensor at the back of the engine and jumping the plug on the other side proves everything else I managed to back-solve over the course of about an hour with a multimeter last week. The sensor has two lines plus a ground, jump the ground to the active and it's as if the sensor is hot coolant - if the fan then turns on, by definition the relay, fuse, fan, and all wiring in between is all good.

Anyway, new sensor should be here Tuesday...

Glad you have got a result.

For Science: One of those wires is the fan, the other is the ECU, were both fails, or only the fan?

Boo... not fixed. Fitted the new coolant temp sensor last week, went for a drive, got up to temperate and the passenger side fan came on. Never saw both fans working but also that was further than we got with the old sensor so I assumed that meant we were good.

Went for a drive yesterday while it was 34 ambient, got home and both fans are running but also coolant is overflowing. The drive wasn't particularly vigorous, so even with the high ambient I wouldn't have expected it to boil. I'm moving over to a new more-generic cooling issues thread as the fan troubleshooting seems to have been resolved. And there's more context that I feel has become more relevant.