rebuilder86 wrote: 06 Jul 2019, 14:40
I say to hell with all the computer garbage. The thermostat will regulate the temperature above and beyond all other electronic and aerodynamic inputs. So just wire in a switch and relay directly to the fan to force it to full power whenever you are towing. Switch it on manually when you see the temp increasing. I've done this for a few vehicles.
Apart from acknowledging that the engine thermostat makes no contribution to cooling above its fully-open temperature, I have some sympathy with that, inasmuch as current cooling systems (not least the one under discussion here for this C5 II) don't lend themselves to easy repair or upgrading, or even to being readily fully understood!
Having cooling fans which operate automatically throughout the temperature and driving range is obviously a good idea - everything from Off for open road driving, through Low speed for mild overheating, and on to Full Blast for extreme conditions.
Single fans can do the job (more-or-less), but twin fans offer lots more scope - not least being able to operate them in series for low speed, without the need for wastefully hot dropper resistors. They also cover more of the radiator, and have huge cooling capacity when operated in parallel.
So far as I can see, the BX (some models) was probably the last PSA car to use the purely electro-mechanical set up of a bank of three relays and twin fans to achieve this series-parallel system, with everything being simply switched by a 2-stage "thermo switch" screwed into the radiator.
After that, things started to go electronic. The XM retained the twin fans and three relays, but the switching command to the relays was provided by the engine/water temperature control ECU, receiving a temperature reading from the engine's water outlet (next to the main thermostat, in most cases). This system works extraordinarily well, and does at least retain the reliable and simple 3-relay output.
With this system, fan rating is selected at factory for the expected duty (anything from 150W to 310W for each fan), and it would be easy enough to up-rate fans if this were ever required. (I've never known anyone having to do this.)
Going yet further electronic, with the maker's current aim of shaving any whisker they can from fuel consumption (i.e. alternator load), it was seen necessary to power engine fans at the minimum current possible, and hence (I assume) the move to variable-voltage or pulse-width-modulated supply for the fan(s). All well and good, but what to do if the cooling capacity of a car so fitted (which we may, or may not, have in this case) is inadequate? Either lot of technical information is needed to know whether a more powerful fan, or an extra one, can be safely added — or else a whole chunk of system control would need lifting from another similar model with larger capacity. Either way, not simple.
Even adding manual controls to this kind of directly driven (relay-less) set up is not easy. Electronic output systems do not take kindly to having supply voltages slapped on to their outputs, and it's quite likely that disconnecting the electronic output (permanently or by relay) could throw up system faults.
So, I really do sympathise with Rebuilder's point of view, and would be quite frustrated it it were my own car. I think every step should be taken to ensure that the fan system on the car is working as it should (from Low to High speed) before going any further. I'm not sure that has been done yet.
