Author Topic: afterrun coolant fan sensor on AAZ (Read 3034 times)

neimis · « **on:** June 11, 2008, 12:13:05 am »

Can someone tell me where the after-run coolant fan sensor is? I have a Bentley for the Passat B3, but it doesn't cover the AAZ.

Could it possibly be integrated in the coolant temperature sensor? On the gassers it is a seperate sensor, mounted near the valve cover. On my engine I can't find anything that looks like it could be it.

Thanks for any help, Neil

jtanguay · « **Reply #1 on:** June 11, 2008, 03:13:04 am »

it uses the head temp to gauge when to turn on/turn off i believe it turns on at 200F and is located on the front of the block? or maybe at the side... it's been a while since i've checked

Vincent Waldon · « **Reply #2 on:** June 11, 2008, 08:17:57 am »

According to my Bentley, the VR6-equipped models were the only conf of the MK3 that came with an after-run temp sensor... it drives a specific pin on the fan control module. The VR6 also has an electric coolant pump as well, controlled in a similar fashion.

The after-run switch on the MK2s was a retrofit only on gassers as I recall because of heat-soak causing vapor issues in the gas fuel rail, leading to hard hart starts.

It should be pretty straightforward to add an fan after-run to an AAZ... the generic sensor from the MK2 series would work although a new mount would be required (since there are no valve cover bolts directly on the head) and could be wired to the correct pin on the fan control module or drive a additional fan relay.

My approach, fwiw, was to add a manual dash-mountedl switch to control the fan. If the engine is really hot (after a run on the highway for example) I activate the fan manually for the 3-4 minute drive home... that way coolant and oil are still circulating and the engine cools off quickly and uniformly.

jtanguay · « **Reply #3 on:** June 11, 2008, 12:53:54 pm »

ah... i thought my diesel had an afterrun switch. sometimes the fan would stay on for up to a minute after shutting down (after a nice hard run) i guess it could be the residual heat at the thermoswitch though...

Vincent Waldon · « **Reply #4 on:** June 11, 2008, 01:22:49 pm »

Quote from: "jtanguay"

ah... i thought my diesel had an afterrun switch. sometimes the fan would stay on for up to a minute after shutting down (after a nice hard run) i guess it could be the residual heat at the thermoswitch though...

Nope, you could be right... many many versions of VW's out there !!!

Typically the after-run sensor would start the fan *after* the car (and fan) have been off for a few minutes... the engine heat-soaks and enough heat builds up to trip the sensor.

But, ya never know with VWs. !!

neimis · « **Reply #5 on:** June 11, 2008, 10:09:11 pm »

Thanks for the replies. I guess I should have been more clear. My car ('92 Passat TD with AAZ) definitely has an "after run fan" (meaning the fan sometimes continues to run for minutes after the engine is turned off). According to the B3 Passat Bentley, the 16V and G60 engines both have after run thermoswitches "on a bracket near the cylinder head", but I can't find anything like that on my car.

So I think there are three possibilities; first, there is an after run thermoswitch but I can't find it, second, the thermoswitch is integrated in the coolant temperature sensor, or third, the fan continues to run simply because the radiator thermoswitch is still hot (as jtanguay mentioned).

The after run function on my car is extremely sporadic, and I want to get it sorted out. I remember how it worked when the car was new, quite different than it is now...

So if anyone else has more ideas, please let me know!

P.S. jtanguay, there is a sensor on the driver's side of the head, I thought that was an oil pressure sensor?
P.P.S. Vince, come to think of it, what you said sounds right, my after run fan only comes on about 5 seconds or so after the engine is turned off. That would indicate it is a seperate function from the radiator thermoswitch, right?