www.jec.org.uk

As per title. Both low and high speed fans running at max from whenever ignition is set to II or when engine is on, regardless of coolant temp.

Car is a 1999 XKR 4.0 with 62k miles.

Fans both shut off straight away when ignition is off.

Turning AC on or off makes no difference.

Coolant temperature is in normal range according to my OBD scanner, and coolant level is fine.

Battery voltage is fine.

No diagnostic codes are stored on the system. No warning lights or other tell tales.

Fan pack has recently been out as part of an alternator replacement, and I'm sure it didn't do this before.

My thoughts are that the fan is failsafing to on in the absence of data. Perhaps relay? However, I'd expect a diagnostic code to be logged in this circumstance.

Any advice welcome!

I wonder if the electrical design of the 1999 vintage XK8 is similar to the XJ8 of the same age? I had a similar problem with my 1999 X308/XJ8 which appeared to be a problem with the fan control board. I no longer have the electrical diagrams for the X308, but from memory there are some relays and diodes on this board (if you can find where it is located!) In the case of my XJ the problem started as not running on slow speed at all but would run at full speed only. Although the fan control board appeared to be at fault it actually was caused by some short circuited wires, which I was surprised to find were routed through the left hand wheel arch and thus exposed to all the road rubbish.

The fans' operating logic is described in JTIS as follows

Cooling Fans

The two radiator cooling fans are mounted in a cowl assembly within the cooling module. The fans are controlled by the ECM dependent on the demand from the cooling system (temperature) or the air conditioning system (pressure). Outputs from the ECM control the radiator fans control module, located behind the left hand side of the bumper, to operate the fans in the series mode (slow), parallel mode (Fast) or Off modes. Hysteresis in the temperature and pressure switching values prevents 'hunting' between modes. Under hot operating conditions, the fans may continue to operate for some time after the engine has been switched off, but will stop automatically when the coolant temperature has been sufficiently reduced.

The relevant circuitry of the module is here
The white and white/blue wires going off top left, both go to the ECM, to be switched to ground as necessary for the conditions at any given time.
The white/blue wire labelled 46 is a 12v supply from the ECM Fuse box, F12,10A.
The brown/green wire labelled 71 and the brown/light green wire labelled 70 are both 12v permanent battery supplies

Although the module looks complex in the way it organises power to operate the fans in parallel or series, its construction of only three relays and two diodes makes it pretty simple to check each of them if you can gain access, but as Brian says, don't omit checking the wiring as far as possible.

Looking on ebay shows a couple of early modules for less than a tenner! Maybe worth a try? I've found AutoReserve to be reliable for used parts.

As it started after removal of the fan pack I wonder if a connector is just loose? As the fan pack is easy to remove I would start with that and if all seems connected then as suggested I would swap the control module. Agree regarding autoreserve, they are always reliable.

Chaps

Thanks for the responses. I have checked the connections that I can reach, and they seem secure. Both fans power down when the relevant fuse is removed (as to be expected.)

The relay behind the bumper looks like a devil to get to - I can't see it from inside the nearside wheel arch and can only just about see it from underneath at a rather challenging angle. I will probably need to remove the undertray that connects to the front bumper to do so, and even that looks like its attached using some kind of rivet system!

At least they are defaulting on, and haven't just stopped working!