Coolant Temps

[Greg130]

Hi guys, just looking to pick your brains on my 1998 E39 540 coolant Temps and Aux fan activation.

Ok so purchased the car a couple of months ago, blew the top radiator hose on day 2, replaced and has been running fine since.

Replaced Aux fan as it was dead.

New fan worked with AC on but nothing else, jumped the connectors for low and high speeds and fan works. (so I assume relays and fuses are ok?)

Replaced the 91/104c temperature switch, the one on the side of radiator.

Cruising on the highway the car sits at a steady 110c, pop it into sports and temp drops to 108c, sit in traffic or let car idle at a standstill and temp climbs to 111c and at this point the aux fan switches onto low speed and temp will generally drop back down to 110c. (Never seen temp rise above this or fan switch into high speed)

So I'm a little confused, isn't the fan supposed to switch on at 91c low speed and 104c high speed?
Is 111c for fan activation a bit high?

Any ideas or is this normal?

Cheers

[edjack]

The fan has three speeds. When the A/C is engaged, the fan runs at the slowest speed, controlled by the HVAC panel. It should also come on at low speed when the dual temp sensor sees 91 deg C.

The second speed is activated a pressure sensor in the receiver/dryer senses 17.5 bar.

The third speed comes on when the dual temp switch senses 104 deg C. It is a very fast speed.

Where did you buy the dual temp switch? Where did you jumper the fan?

The first speed relay, K21, is in the E-box, under the right side cabin filter, as is the third speed relay, K201. The second speed relay, K22, is in the carrier behind the glove box.

You can test the operation of the relays by grounding the connector on the pressure and temperature switches. Depending on how the old fan failed, it may have taken out the relay contacts.

I don't know how closely the cluster temp tracks the temp at the dual temp location.

[Greg130]

Ok so my thoughts, having already blown the top radiator hose I'm perhaps a little paranoid about this lol.

So I've changed the aux fan and duel temp switch. Tested the fuses and relays by jumping the switch, both low and high speeds work when jumped.
Fan activates at low speed when AC is on and when Ktemp display reaches 111c.

So what I'm thinking is one of two things:

1. There is a problem somewhere and the fan should activate at a ktemp readout of 91/104 respectively ?

2. The Ktemp readout is taken from the temp sensor in thermostat housing and by the time the coolant reaches the aux fan temp switch (top of rad on right side) the coolant temperature has dropped below 91c, this would mean a drop in coolant temp of approx. 20c, is this feasible?

Would it be possible for someone to check at what temp their fan activates?

Cheers again guys

[XAlt]

It's fairly hot over there, I'd pick up an 88C stat. My aux didn't kick in at normal operating temp (100-108c). 111c kick in stage sounds right, and in line with its intended use as a fallback.

[JimLev]

2. The Ktemp readout is taken from the temp sensor in thermostat housing and by the time the coolant reaches the aux fan temp switch (top of rad on right side) the coolant temperature has dropped below 91c, this would mean a drop in coolant temp of approx. 20c, is this feasible?


Cheers again guys

Yes, that is very possible. The temp sensor in the water pump housing is reading the coolant temp out of the engine which cools down as it passes thru the radiator.
For piece of mind you could jump the low speed so the aux fan will run all the time.

[Greg130]

It's fairly hot over there, I'd pick up an 88C stat. My aux didn't kick in at normal operating temp (100-108c). 111c kick in stage sounds right, and in line with its intended use as a fallback.

Yes in summer it is kind of hot lol, just coming into winter so today is a mild 27c.

Ok so 111c kick in seems about right, phew and good :-)

88c stat?
I've seen some posts about these and having to modify the housing, well fit spacers or something?

Is there a direct replacement 88c stat, just plug and play without modifications? (I have searched but not found any yet)

Cheers

- - - Updated - - -

Yes, that is very possible. The temp sensor in the water pump housing is reading the coolant temp out of the engine which cools down as it passes thru the radiator.For piece of mind you could jump the low speed so the aux fan will run all the time.

Or just have the AC on?

- - - Updated - - -

Yes, that is very possible. The temp sensor in the water pump housing is reading the coolant temp out of the engine which cools down as it passes thru the radiator.
For piece of mind you could jump the low speed so the aux fan will run all the time.

Not a bad idea thanks :-)
Or just have the AC on?

[RuskiE39]

My 540 runs pretty consistent at 108-110 while cruising on the freeway, when I drop a gear it will sometimes hit as low as 106.

[Greg130]

My 540 runs pretty consistent at 108-110 while cruising on the freeway, when I drop a gear it will sometimes hit as low as 106.

Good to know thanks, I'm just a little paranoid but it appears to be operating correctly so all good :-)