BX Fan Operation

roscoe · Post by **roscoe** » Sat Nov 19, 2005 8:51 am

Hi All. I was working on my TZD and 16V today and figured I had something wrong with the cooling fans on both of them, but I'm not sure after checking my parts car as well. What I have done is: remove plug from temp sensor switch in radiator and short from the earth to the low speed terminal - with ignition switch on both fans run in low speed - on all cars (with A/C turned off). When I short between earth and the high speed terminal, only one fan runs on high speed, the other stops - this happens on all three cars. When I do the same with the A/C turned on, the low speed operation is the same, but when I short the high speed terminal to earth, BOTH fans run in high speed - only on the 16Vs as the TZD doesn't have A/C. According to some old posts and a Peugeot cooling document, as well as the wiring diagram, I would have expected BOTH fans to run in high speed regardless of whether the A/C was on. When only the A/C is switched on, both fans on the 16V run in low speed.
So is the way my fans run correct, or is there something wrong? I can't figure out the point of only having one fan run on high speed if the engine temp is excessive - would have thought both should run. But since it is the same on all three BXs, I'm wondering if this is correct.
Anyone else tried this and have the same results?

Clogzz · Post by **Clogzz** » Sat Nov 19, 2005 11:34 am

Not sure about the BX, but if it’s the same as the Peugeot 306 diagram, then if the ‘change-over relay’ is not energised or conducting, only one fan will spin at the high speed.
At the low speed, only the ‘left fan relay’ is energised, while at the high speed all 3 have to be energised for both fans to spin.
Shorting the high speed terminal to earth doesn’t activate all 3 relays.

tom · Post by **tom** » Sat Nov 19, 2005 12:25 pm

Dead right Cloggz! The way that it works is that the slow speed switch closes first and provides the feed to the slow/fast relay which only energises when the fast speed switch closes. Naturally in the real world, the slow switch will remain closed once its cut in temperature is exceeded and therefore both switches would be closed to bring in the fast fan setting. Short all three in the fan switch connector and they will both run.
BTW: Research carried out by both myself and Peter Mould shows that as these switches age, they cut in at increasingly higher temperatures, so if you are about to flush and refill coolant at this time of year, buy a new switch just to be on the safe side.

roscoe · Post by **roscoe** » Sun Nov 20, 2005 8:01 am

thanks Cloggz and Tom - I figured I must have something screwed up as all three cars were the same. Tried shorting all three today on the TZD and both fans kicked in on high as you stated. My temperature switch on both TZD and running 16V are fairly new (around 2 years) so will leave them but I do have some new ones from GSF in the parts bin.....