Evening all,

I've noticed the fan on my S-Type R runs fast from the moment the ignition is turned on, with a cold engine, even before the engine is started and with the A/C turned off.

I used an iCarsoft to read the fan speed demand which is showing 90%, but coolant temp is reading 8degC (engine is cold). When the car runs, the gauge slowly warms up to the centre point and the coolant temp reads 88-92degC so I think coolant temp sensor is working.

I do not understand why the PCM is demanding 90% fan speed seemingly all of the time (which is I assume why the fan is running all the time at full speed). Anyone have any ideas what I should check?

I read general DTCs from the PCM and can't see any.

 

Does the air conditioning work normally? IIRC, there are only two basic inputs for the computer to control the fan:

Coolant temperature
AC refrigerant pressure (high side only)

Your scanner shows coolant temperature looks normal. I'd suggest checking the AC pressure sensor. When high side pressure is above a certain limit, the fan is commanded to run and help drop the temperature, and thus the pressure. I think AC operation is also inhibited above a certain pressure.

You mentioned switching off the AC has no effect. Try unplugging the connector for the AC pressure sensor. The AC won't run, but it may also take away a potentially faulty input saying the fan should be running full bore.

On my '02 V6, the sensor is on the refrigerant line to the left of the condenser, as viewed facing forward. I believe the location is the same for all models and years.

 

Hey yes thanks for the input! I read in another thread that it also runs off Transmission Oil Temp too but I read that out last night and it was also showing 8 degrees. I was thinking possibly AC Pressure. I did look at AC Pressure Sensor output (I forget exactly what it was called) using the iCarsoft and it showed a value of 4.77V. Obviously being a voltage not sure what pressure value that corresponds too and whether it's normal or not for a cold car. I'm not sure if anyone can read the value from their own vehicle to give me a reference?

If I unplug the sensor won't the car just assume a safety stance and run the fan full bore anyway? I'll try it anyhow and see what happens!

I also noticed the car has 4 DTCs stored in the PCM that I cannot see because the iCarsoft won't read the manufacturer DTCs on my ECU (it errors when I try to do a DTC read from the PCM but will read all other ECUs which I think is a common problem with the iCarsoft 930 - I can only read the generic OBDII codes out in the OBD mode). I'm trying to get an old laptop and Mangoose cable so I can use the Jag diagnostic software and read everything properly to see if those fault codes could have any bearing on it.

 

I'm not sure what voltage would be considered normal for the.AC pressure sensor. If on a 0-5V reference scale like some other sensors, that's near the end of the range. But we still don't know if high voltage corresponds to high or low pressure, so it's all just semi-educated guesswork.

Likewise, I'm not sure what happens with the sensor unplugged. My best guess is the AC is totally inhibited. With no compressor operation, pressure can't build up so there's no need to even run the fan. All you can do is unplug it and see what happens. If the fan continues to run full bore, I'm out in left field. (Remember, the fan should still run for engine cooling). But if the fan stops screaming, the AC pressure sensor was the likely culprit.

Does the AC operate normally? Have you verified the compressor clutch is engaging? If ambient temperature is low, cold air from the vents is not proof positive by itself that the AC system is working.

 

Well you win the prize - as soon as I disconnect the A/C pressure sensor the fan stops so that was triggering it. I also noticed the sensor connector was full of something wet. Looked like coolant to be honest, or could it be refrigerant? Otherwise it's just pain old water getting in via the connector.

I dried out the connector on both sides but as soon as I reconnect it, the fan starts full bore again. So next question is to start diagnosing why the sensors making it run. A/C over pressurised? Wiring corroded, pins corroded or sensor busted?

 

Any recent work on the AC system? It would have to be SERIOUSLY overfilled to trigger the high pressure limit. Actually, I’m not sure one could physically cram that much refrigerant in the system with normal servicing equipment.

If the AC has been working recently, meaning not horribly overfilled, pressure should be nowhere near the high limit when switched off. In a closed system, R134a pressure (in PSI) is fairly close to ambient temperature (in Fahrenheit). So on a 50F day, system off and engine cool, expect to see about 50PSI on the high and low sides.

Call me Mr. Vegas, but I’d be willing to gamble your hard-earned money on a new switch. IIRC, there’s an automatic shutoff valve behind the switch, so replacement is quick and easy.

 

Yes indeedy, I've ordered a new switch so let's see what happens! It does seem relatively easy to replace since it has the valve behind it.

I also ordered a kit of AC o'rings since it seems the sensors have one between them and the valve but don't come with a new one.

The system was regassed recently but it's also had a LOT of work on the car and before this the AC hadn't been gassed for a very long time (maybe 5 years) so it's more than possible the valve has failed.

 

Fingers crossed! Be sure to post an update after replacing the sensor, even if it doesn't fix the original problem.

Have the new sensor ready to install before removing the old one. That little shutoff valve likely hasn't budged in years and may not seat fully closed. If the new one is ready, you can quickly swap it in place and hopefully avoid the need to recharge the system. Wear gloves, as escaping refrigerant can be very cold.

If you have any other AC problems, this guide should help:

https://www.jaguarforums.com/forum/s...ol-how-185002/

 

Will do, thanks for the advice. Actually that pipe has been replaced as part of the work. I had a completely new coolant system built for the car bespoke and several AC lines were also replaced but the old pressure sensor was probably reused.

Incidentally the supply voltage for the AC Pressure sensor is 5V which comes from the PCM hence the output reading of 4.77V is almost the likely maximum for the sensor. Since it's activating the fan I assume that positive voltage means higher pressure so I think it's pretty likely that the sensor has failed (assuming from what you said about it being unlikely the AC Pressure is too high when it's so cold).

I just realised it was you that made that guide. I've used it a few times so this is a good opportunity to say thank-you for making it!

 

Update as promised...

New AC Pressure Sensor arrived today, fitted and fan now no longer running. Diagnostics shows Variable Speed Fan OFF, Duty Cycle 9.6% and the AC Pressure reading 1.04V, which hopefully corresponds to acceptable AC Pressure. You can read these values out of the PCM as live data using an iCarsoft 930.

Fan behaviour now seems normal again.

Pressure sensor dead easy to fit as it has the Schroeder valve underneath so just unscrew old one, screw on new one, no loss of pressure, nip up tight to 8Nm. Ideally replace the o'ring underneath the sensor when you remove it, at the same time.

 

I have an 06 str. Just checked my a/c switch voltage with my i930. Ignition on and a known good refrigerant charge at 58F. 1.15 volts. I would say you are ok if a/c performing well

 

Thanks for the update. Glad to hear all is back to normal. The AC pressure sensor doesn't seem to cause much trouble, so I'd like to add this info to the troubleshooting guide to help out the next guy. There's not much forum experience with a bad AC pressure sensor. I'm trying to narrow down what symptoms others might see if the sensor failed in a similar fashion.

@MrDeBruce Can you please clarify if your AC still worked when the fan was acting up? I'm trying to understand if the false pressure value was so high as to totally inhibit the AC compressor clutch, or merely in the range to allow AC operation but also command the fan to run at max speed.

 

Hey sorry, regarding the a/c functioning I am not sure as it's really cold here at the moment so I never noticed a/c not working. The major symptoms for me was fan running at almost full speed as soon as ignition turned to position II without engine running.