Electric fan conversion and cooling system overhaul

I was informed by Mobeck that he is actually working on the idle settings and will be releasing modifications soon. In his instructions Mobeck states that the ECU is programmed to raise the RPM to 1500 initially, then drop to about 1000 before settling at 750-850. I don’t know how accurate the tach is in my car, but it does follow that pattern. I will wait and see what he comes up with.

Do you have both fans turning on at the same time?

They do seem to turn on at about the same time. Mobeck’s written instructions claim that the ECU is set to turn the fan on at 86 degrees although on his YouTube video he says 88. The needle in my car points to 90 when it engages. Same as tach, I don’t know how accurate that gauge is. The setting can be changed to turn the fan at a different temperature, but so far I didn’t want to mess with the ECU settings before testing first if everything works as programmed. I think I know where you are going with your question. I also thought about staggering the engagement of the fans so that the load doesn’t come all at once but I don’t know whether I should have the bigger fan engage earlier or later than the auxilliry fan. If I set it to engage first, due to it’s power, the auxilliary fan might never need to come on, except when the AC is on. I don’t know if that would have impact on the life of the Volvo fan. I also want to make sure that the car stays in the proper temperature range and not get cooled as soon as it reaches it. I also don’t know at what temperature the auxilliary fan is set to turn on. I know that it is triggered by both temperature, via grounding on the water pump, and by turning on the AC.

You could add in a two way relay so that both fans are brought on wired in series initially, thus at half speed and current draw, then if the temperature rises the wiring switches to parallel connection so both fans blow at full speed.
The half speed setting could be used whenever A/c is used, to ensure a flow of air through the condenser.


The aux fan is directly activated when the AC is turned ‘on’ - and should be connected accordingly. There are two ways of doing this; either from the switch - or from the AC clutch connection. The latter will stop the fan when the clutch disengages - either AC turned ‘off’, or the clutch is disengaged by the Ranco thermostat.

The ‘big’ fan is normally triggered by the thermoswitch at the radiator at around 95C - which may not occur while driving. Triggered by the ECU at lower operating temp; it will run more frequently. There is no reason for avoiding the higher temp, engines will actually run better at 95C, or 86C - which is not overheating as such…:slight_smile:

It is the thermostat that controls coolant temp; if the radiator works properly it will maintain thermostat temp, and will not let engine temp drop below. Overheating is only caused by a failed thermostat or insufficient cooling capacity - and a too cool running is just a failed thermostat - the engine produces heaps of heat. Fans running at full bore will not overcool the engine - and controlled by temp they will turn off as soon as temp drops below trigger points…

There is no need to worry about the Volvo fan - it’s dimensioned to run continuously for years…

xj6 85 Sov Europe (UK/NZ)

I don’t know if that would have impact on the life of the Volvo fan. I also want to make sure that the car stays in the proper temperature range and not get cooled as soon as it reaches it. I also don’t know at what temperature the auxilliary fan is set to turn on. I know that it is triggered by both temperature, via grounding on the water pump, and by turning on the AC.

There are controllers out there that will take the signal from the ECU for the fans to turn on and turn both fans on sequentially - the time interval is set on the controller - up to 10 secs apart.

Also remember that at least one of the fans will need to be running when the A/C is on.

Also the fans do not really be controlled by the ECU - they can be controlled by the controller mentioned above.

I have twin fans and no engine driven on my Motec controlled XJ12 and there is a few RPM increase when they engage, similar to when the A/c kicks in - I dont have any of the issues being encounted here.

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I connected the new auxiliary fan exactly as the factory AUX fan was. It is independent of the Volvo fan actuation and not adjustable.
If I change the setting for the Volvo fan to trigger at higher temp, the AUX fan will work almost all of the time when the temperature reaches it’s triggering temperature, irrespective of the AC. Only if it can’t handle the cooling and the temperature rises further will the Volvo fan engage.
I could disconnect the two wires on the water pump which would, in theory, only trigger the AUX fan when the AC is on and leave the Volvo fan to handle most of the engine cooling duties. However, since the shroud is divided, it wouldn’t pull air from the AUX side.
As far as the two way relay is concerned, I wanted to leave the AUX fan as it was from the factory and not tied to the new fan. Maybe not the best solution but I wanted it to be able to engage when the AC is on, as well.

I would set the Aux fan to come later than the Main fan, don’t have to run both if no need.
The Aux fan should also run with the AC.

I don’t see anything wrong with that proposition.

Ain’t all these brushless?

In mine it comes on when the coolant reaches about 95°C.
Don’t know what the exact temp for the switch though.

At what temperature is the ECU triggering the main fan?

No no, having a backup is a very wise idea.

The ECU triggers Volvo fan when the needle points at 90 but Mobeck says it is set at 86 or 88. Of course, the gauge can be off. Frank mentioned that it is OK to set the temperature higher but AUX fan is not adjustable as it is from the factory. So, either I set the Volvo fan to come at 85/86, which, according to Frank is too early or I set it higher than it is now, which means it would come on after the AUX. I am surprised that both you and him mention 95 degrees since, in my case, they both engage at about the same time(90 on the gauge and 86/88 according to Mobeck).

The gauge is not exact and can be tuned with a resistor - what’s 2° though. Depends on location.

The fans should kick in above the temperature at which the car stabilizes when it’s moving…

Think back to mechanical fans, they are working as soon as the engine is fired up, they don’t start cooling the fluid until the thermostat opens and allows fluid to circulate through the radiator , I feel you are over thinking the problem.

Not always, Mercedes is not the only one but they had a fan in the 90s that declutched by thermostat, fully, and didn’t spin much at all until a certain temperature was reached. Some new cars have blinds again, like 100 years ago, to assist warmup. E Type fans turn on when the temperature is higher than normal or the AC is on. With AC the fans on low make sense. Having the fans on all the time is a total waste of energy.

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I will take that as a compliment. Usually I am accused of not thinking enough.

Electric fans were installed to eliminate fan noise when fans were not required for cooling - the higher their trigger temp the less they run. While moving; the natural air flow through the radiator reduces the need for fans…

With electric fans; the trigger point is set fairly high, but when started, they will quickly bring the temps down. And will ensure that engine temps does not reach dangerous levels - which is way above 95C.

The aux fan on the V12 was deliberately related to the AC as the engine must then burn more petrol when ‘on’. And the aux was just a preemptive ‘remedy’ for cooling, adding constant cooling - much like an enhanced idle to maintain idle in another field. The main fan is still the one that is designed to do the cooling, the aux fan’s capacity is limited…

The gist is; a temporary increases in engine temp is harmless! If constant engine temp is (needlessly) desired; setting the fan trigger point at thermostat temp - the fans will run continuously, which defeats the purpose of electric fans.

Setting the main fan to start at lower, or the same temp as the aux, is pointless - it renders the aux fan redundant. The ‘best’ solution is to trigger the aux with the AC (or omitting it) - the main fan may still never come on whatever its trigger point…

There is no harm done by running fans continuously, that’s what engine driven fans have been doing for donkeys’ years - it’s just contrary to electric fans raison d’etre. Given ample cooling, the thermostat will maintain constant temps whatever fans fitted…

xj6 85 Sov Europe (UK/NZ)

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It turns on to increase airflow over the condensor :slightly_smiling_face:

First question is, what temp are your thermostats?
Second, what is the temp gauge reading when the thermostats open?
Third, which temp sensor does the Mobeck ECU uses?
I personally think that 86°C, and even 88°C is too low, you risk having the fans on all the time. It should be at least 90°C with a 88°C thermostat.

A very simple solution to your delima:
Connect the Main fan to the original thermostat at the water pump, thus you stabilise the temp at 90°C, perfect.
Let the Mobeck ECU control the Aux fan instead, set at something like 92°C, and also when the AC is on.

I might try the swap you suggested.
On a related note, the video is now uploaded on YouTube under 1991 Daimler D6, for those who want to hear it run.

Running a few degrees warmer or cooler doesn’t matter in the least; the temperature oscillates either way! I’d set the fan to “AC or above say 92°” and the aux to 95°, if the thermostat is 88°. An adjustable sensor would be better, and set it so that it never comes on at all when the car is in top gear.