I've read RAVE, I've looked at the confusing high speed/low speed fuse box relay circuit and still have no idea why my low speed fans don't run. If I jump 13/14/18 relays both fans run, if I use Nanocom (GEMs ECU Output Function) to run low speed AC fans they both run. On a 40degC days here stuck in traffic my high speed fans run but never the low speed. My AC works well but even with a new fan viscous coupling the performance drops at idle. As far as I can tell there needs to be an HVAC low speed fan request, a GEMs fan request (for 12V and Ground) and the the two way/low speed AC pressure sensor needs to be closed. Suspecting the HVAC controller output but does anyone know when the HVAC is supposed to request low speed fans so I can check the output? Thanks - going crazy on this one.
They aren't controlled by the HEVAC, but by the engine ECU. The diagram isn't the clearest but they should run at low speed whenever the engine ECU sends a AC grant signal (which it should unless it is getting too hot). Power comes from fuse 31 and goes directly to the LH fan motor. The other side of the motor goes to centre contact on changeover relay 13 which is not operated unless the AC refrigerant pressure exceeds 21 bar (which it won't unless the AC is having to work really hard). From the normally closed contact at relay 13, it goes to the contact in relay 18 which should be closed by the AC grant signal from the ECU. The other side of that relay contact goes to the normally closed contact in relay 14, another changeover relay that operates in parallel with relay 13, and from there to one side of the RH fan motor with the other side grounded so the two fans operate in series so at low speed.
If the AC refrigerant pressure exceeds 21 bar, both relays 13 and 14 are operated so the connection from the LH fan goes via the relay contact in relay 13 directly to ground and power from fuse 36 goes via the now closed contact in relay 14 to the RH fan motor so they both run at high speed.
Are you sure both run at high speed? I noticed that mine didn't seem to run at low speed and that was down to one of the fan motors having burnt out so it was open circuit. Like that they won't run at low speed but the remaining one will run at high speed.
Hi, thanks for your reply - that description helps a lot with relay operation - I really struggled with that and couldn't figure out the basic operation - thank you! So now I know the LHS motor should run low speed as soon as there's an AC Request HVAC to GEMs, and the RHS motor should run low speed too if the X315 switch goes over 21 Bar, I know I've got to check the HVAC panel plug C244 pin 8 blue/black wire (condenser fan motor control). This is the HVAC request to the GEMs ECU for fan motor control. If I drive the fans from the Output Options on Nanocom both fans run at low speed - not sure why both run (faulty close X315 pressure switch?) but anyway at least I know they both work! Now I can measure HVAC C244 pin8 to see if it's high or low and if it's high (no request) with AC On I know the HVAC panel is faulty. I can even feed a 0v to pin8 wire and see if the LHS fan runs. Thank you! Hopefully this sounds sensible!
PaulD4 wrote:
So now I know the LHS motor should run low speed as soon as there's an AC Request HVAC to GEMs,
No, they should both run low speed when there is an AC grant from the GEMS ECU.
and the RHS motor should run low speed too if the X315 switch goes over 21 Bar,
No, the RHS should run at low speed in series with the LHS fan. If the X815 switch goes over 21 bar, then both relays 13 and 14 operate and they both run at high speed.
If I drive the fans from the Output Options on Nanocom both fans run at low speed - not sure why both run
They should, they are in series. You need to check if relay 18 is operating and that is controlled by the GEMS ECU.
To be quite honest, when running at low speed they do naff all so I wouldn't worry too much about them not running then. It is only important that they run at high speed when needed.
Thanks, I'll check Relay18 but confused. On Nanocom you can run low speed fans from GEMs (both run low speed so relay 18 must work?) and you can also check the AC Request from HVAC which also works - I have both except no low speed fans AC on. Guess I check the relays anyway.
Turns out it's the HVAC Controller C244 pin8 (output) not sending 0V to GEMs C507 pin30 (input) when AC on engine running. The GEMs should then send C505 pin3 (output) to pin2 of Relay18 running the fans in low speed. I get no earth on Relay 18 pin2 AC on engine running but if I ground externally the fans run low speed. The GEMs ECU is working and will drive the low speed fans via a NanoCom request. Also whatever the NanoCom check is for AC Request is, it's misleading - it doesn't seem to relate to the output of the HVAC Controller as I get a 'Yes' for AC request but no ground out to GEMs.
I'd like a working HVAC Controller but the easy fix is to put a Relay18 pin2 ground switch in. The fans will still run in high speed like this for AC.
I'm intrigued now about how engine cooling fan requests work. Low speed would be the same as AC low speed but does anyone know how High Speed engine cooling works since AC Dual Pressure switch won't be in play?
If they're just AC fans (No engine cooling requests) a dual position switch could earth pin2 RLY 18 (for low speed) and then pin2 on RLY13/14 to run high speed.
If by engine cooling fan, you mean the big one on the front of the water pump, it has a viscous coupling. The coupling is filled with a silicone fluid that gets thicker as it get hot. So when cold the fan is just free-wheeling but as it heats up, the fluid in it causes the fan to be driven at engine speed. It will also engage when the engine is initially started from cold until the fluid is spread around inside the coupling. You can usually hear the roar from the fan which gets quieter as the coupling releases the drive.
Yes but do the AC Condenser fans also run if the engine water temp is too high? I don't think they could run in high speed AC off so I think the electric fans are AC only.
A lot of the AC Cooling fan operation descriptions and diagrams don't seem to be correct or very clear in Rave. Only that fuse box relay wiring schematic gives you the true operation (thank you). Pretty sure the electric fans can only run high for AC because you need the pressure switch over 21Bar. Do you think that's correct?
So a manual two 'speed' switch means they could be used in low and high rather than an aftermarket fan unit.
They aren't there to cool the engine but to cool the AC condenser, so only run at high speed when the AC refrigerant pressure gets high. There is no connection between them and coolant temperature as the viscous fan can shift far more air than they ever will. The AC on a P38 runs all the time, even in cold weather. If you look at the evaporator temperature on the Nanocom, you will see that it always sits at around 5-6 degrees C and the compressor clutch engages if it starts to get higher than that. Obviously, in very cold weather it won't cut in as the evaporator is already down at that temperature but ordinarily it always runs.