Cooling System and Radiator Fan Quirks
 

I am hoping this is not a fan controller giving up its ghost. I have begun to notice lately after two outings in stop and go traffic my car's temperature needle sits pegged at 3/4s (or the second mark before the red). At speed, any speed above 35, the needle goes down to half-way, and even just above the first mark (1/4).

Today, I almost overheated with the needle exceeding the 3/4 mark but still not quite at the red. I could not hear the fans running. So far, I have observed the following:

- A/C Button does not turn on the radiator fans (I also have no refrigerant in the system)

- Any type of load applied to the engine lowers the needle ever so slightly when it is running hot (so if the needle exceeds 3/4s, revving the motor will lower it exactly to 3/4)

- Water pump is working and is only 3 years old (less than 2,000 miles on it)

- Original fan controller in the car

- Unknown time on radiator fan switch

- Original intake temp sensor on top of my intake manifold

- I have smacked the dash to see if the temperature gauge is wrong, but it does not change position, however the gas needle does, and goes bananas.

You should also know that the fuse has been pulled for the cooling air flaps, so they are now open all the time on the car.

Based on some searching, it seems like the following criteria must be met for the fans to come on:

- Coolant temperature sensor triggers fans

- A/C button powered 'on' triggers fans (how does this work with no freon?)

- Hood must be closed (my hood safety sensor is missing but has never caused any fan issues like this before)

- Intake temp sensor at the top of the intake manifold functioning


Of course I have to drive it home today in the same type of traffic, so if it gets hot I am pulling over and letting the car cool down a little.

During some backroad drives, the fan did come on when I came back to the garage.

My worry is intermittent fans which may be due to a fan controller failing. Any suggestions on what could be causing this mystery cooling issue? The car has never been overheated before I know that. I also have never seen this sucker rise to 3/4s and above.

Andy,

You should differentiate between a real overheating issue and an event where the indicated temperature is a little higher than previous experience on your 928 suggests.

As a general statement the fans are only required when stationary or moving very slowly in heavy traffic. Beyond that the air blast quickly exceeds the amount of air the fans can move.

If you do see a change in display behaviour by all means investigate to ensure nothing untoward is going on. My 928 generally sits short of the last white line in our very hot climate and can go above that on extremely hot days even with the various mods I have done. The time to worry is when you get to the red line and more so if the alarm triggers and at that point for sure you must shut off the motor until the problem is resolved.

Trust you have checked to ensure you have not lost any coolant. You can also check to ensure nothing is blocking air flow through the radiator.

The a/c relay opens a circuit to power both fans when the a/c is selected to be "running".

If the temperature is genuinely rising the fans should come on and be running. It helps if you can take a direct reading of the coolant temperature with an IR gun.

Regarding causes check the coolant level [the alarm switches fail regularly now], and if the fans are not running determine why. Fan controllers can fail and such seem to be a rare occurrence but doubtless more will fail with age. If you have the original fan motors they will also be on borrowed time. You can also check if the fan switch on the bottom driver side front of the radiator is working.

Good luck

Fred

I left my IR gun at home today, but will bring it the next time I am driving. I can confirm no coolant loss whatsoever with the car. This issue has popped up in the past two drives, and never before.

The fans definitely work and ran fine before. I don't think both fan motors would fail at the same time.

Mongo,

Not sure if this will help, but I had a similar issue and it turned out to be battery leads. I was getting proper voltage at the fans, just not enough current. With direction from Roger, DrBob, MrMerlin, FredR, & others, I cleaned, cut and remade the leads at the terminal and SHAZZAM, fans. See link below.

https://rennlist.com/forums/928-foru...ans-toast.html

Good luck. :cheers:

Saucy I was just back there. I remember reading your thread too. I will check my connections on lunch today.

Connections looked pretty clean. I could have sworn I did these when I replaced the battery.

Andy,

If you still have the original earth strap you might do well to carefully check that at both ends. When I changed mine out recently it transformed starting. Doubt the effect on the fans would be quite so pronounced but...?

Rgds

Fred

I put a ground cable in it already from the chassis to the battery.

I checked the CE Panel and can confirm also that the fuses 28 and 29 (30 amp) are intact. There are no signs of burning or melting.

I have been told to disconnect and then reconnect the final stage fan controller at the very front of the hood to see if it works. I take it the connector is self-cleaning?

Also, can anyone confirm if an AC system with no freon in it will prevent the radiator fans from turning on once the A/C button is depressed???

How do the wires look under the elec tape on the leads? My connectors looked fine, but the wires were corroded.

I would disassemble, clean and tighten these connections just because...

You were in there when you "changed the battery", and if that was recently then because you worked on it recently. If that was a while ago, then do it because it's been a while. Just because it's a so commonly overlooked yet primary cause of fan non-op.

You can test basic fan function by shorting the two switches at the intake manifold switch together. With engine off, this is the same as "intake temp high", and will run the fans at low speed. If you have a controller problem, the fans won't run.

If they run with those wires jumpered but not on warm engine, investigate the temp sensor and wiring at the bottom front of the radiator left (driver's side on US cars...) side, under the AC condenser. Failure mode should have fans running if engine is running and wires to sensor are disconnected or shorted. verify sensor from the little controller module next to the passenger seat by the audio amplifier. WSM has connections and troubleshooting steps.

The thermostat needs to open to allow water flow through the radiator as the coolant temp in the water bridge goes up. With low or no flow, the fans won't see any hot coolant and the fans won't run. Someplace recently I shared the whole spellbinding saga about my denial of thermostat issues, only to be easily solved with a new thermostat and seals. Some initial symptoms are backwards from yours, but still worth considering. Without good thermostat/radiator flow, the return hose from the bottom of the radiator to the thermostat was not very hot, while gauge showed 3/4 mark driving. My fans were running but coolant wasn't flowing well enough.

I did a little bit of night troubleshooting and the fans are running again!

I unplugged the connector at the output fan module and cleaned it up with some Deoxit. I also found a loose fuse 28. Lastly, I removed the harnesses to the fan controller by the passenger seat, but found no corrosion. After reassembly, fans are working now, but it looks like gauge shows 3/4 no matter what. I think the gauge sensor needs replacement. My fuel needle is still going bonkers!

Dr Bob, I neglected to mention last night the car has a new thermostat and seals within the housing as well. It is a low-temp thermostat, albeit the same I had in there before. It was installed when I did the water pump back in 2013.

IIRC the fan controller uses the coolant temp sensor in the bottom of the radiator. The temp gauge uses a different one - so if the bottom of the rad sensor is fubar you will still see rising temps on the gauge but no fans.

As you have no refrigerant in the AC system switchiing on the AC will not turn the cooling fans on. However if you bridge the two connections on the top of the pressure switch on the receiver dryer the fans will come on with the AC button.

I don't recall if this also energises the AC compressor clutch, which would be best avoided if there is refrigerant in the system.

Not with certainty- but I can attest to the fact pattern....diminished cooling..then a few drives where the fans were late or I took a break early on a precautionary basis...followed by no A/C whatsoever presumed permeated lines. A qualified party had the car and confirmed low a/c switch or actual low A/C charge was in play- and the switch is nearly new (as is the compressor, sigh). Parked the car at that point, but did buy a new controller as well, in case.

Fans should trigger off of the radiator sensor, the gauge gets its signal off of the sensor in the manifold. Though if S4 there may be other redundancies or forced on conditions- I thought the A/C switch was one when all is operating properly. Good point on completing the circuit- though I'd start with connecting the wires at the rad sensor depending on how your controller is wired.