Scavenger fan not working

[the1andonly]

I have a 97 4x4
the scavenger fan does not work
story so far
fan test
remove temp sensor connector next to handbrake. Igition on: no fan
fuse 1,2,18 underbonnet OK
Relay B6-06 on Rhs of 1st bank of relays in front of starter battery 7c26
this is same as middle one (whats this one for?) top position live +12v
fan test
put middle relay in rhs for a test Igition on: no fan
so far so good thanks to geofs diagrams
http://www.g8dhe.net/bwm/bwmanualp.htm?1997/Page_46

locations
http://www.g8dhe.net/bwm/bwmanualp.htm?1997/Page_47

Next question where next, have i got anything wrong so far?
what is the relay next to B6-06 can then check relay in that circuit.
Is there an easy way to get to the fan connections for direct 12v application (ignition off relay out totally isolate ECM)

The main fans come on with Air conditioning, so at least fans work

[the1andonly]

I've got at least one thing wrong
The Rhs relay controls the aircon fans on
both relays work in this position
scav fan not on with either relay

[Gasy]

I've put a switch in them cables to engine bay temp sender sited next to me handbrake lever so I can turn scab fan on when hot

So you should just be able to short them wires together and fan will run

[the1andonly]

I measured the rear position in the plug as an unswitched 5V ie ECM

Looking at method to test the engine bay temp sensor
20 deg C 2-2.9kohm
80 deg C 270-370 ohm
so i used a 220 ohm resistor to connect the 2 terminals in the plug (ignition off)
INSTANT scavenger fan blasting.
Remove resistor: fan still on
connect to engine bay temp sensor and turn ignition on :fan off
alternative with ignition off pull out resistor fan stops
So fan does work
Thanks Gasy

[the1andonly]

If i add in a switched x ohm resistor to turn on fan, would a 1/4 watt be appropriate. The 12v power is supplied via relay.
the value of resistor yet to be calculated with sensor in parallel.

[Gasy]

Your way over my head
Think that would already be done as the switch is just doing what the engine bay temp sender will do unless that has resistor built in
I'm prepared to risk it as I only use it if towing up major hills and it drops head temp v quick

[Gasy]

If I leave my switch on and turn off ignition fan runs on for a few mins afterwards
Also makes light on speedo flash when on think it's the glo plug one carnt remember at mo

I got info off here I think and you leave the engine bay temp still connected just wire switch in aswell

[the1andonly]

A resistor is precisely what the temperature sensor is, it varies depending on the temperature
about
2.9k ohm cold
300 ohm hot

if i use a switch with a 220ohm in parrallel with 2.9k it is effectively 204ohm
which the ecm thinks is v hot but probably in an acceptable range.
is a short acceptable
There has obviously been contradictions on this forum in the past on how the scav fan and sensor work.
A proper techie that 1/2 understands the ecm /ecu would be appreciated.
Is either approach with or without resistor acceptable.

My intension like Gasy is to use it in real difficult times, like towing uphill no matter where they are.

[Simon Jones]

Your theory appears sound. You're aiming to get 270 to 370 ohms output from the sensor.

[the1andonly]

depending on the cold value 2.2k to 2,9k
a parallel resistor of 360 ohm will give us 309 to 320 to achieve the limit of 270 to 379
is a 1/4w enough
a suitable candidate
http://www.maplin.co.uk/p/metal-film-06 ... stor-m360r

any idea as to power rating of sensor?

[jimmo62]

The power dissipated in the resistor will depend on the current through it (and hence the voltage). We don't know the voltage at this point but absolute worst case you could assume 13v from a fully charged battery (unlikely but as I said, worst case). Then power = volts squared over resistance, which is 0.75 watts for 220 ohms and 0.5 watts for 330 ohms. Best bet would be to get several smaller power resistors and use them in parallel - say 4 resistors of 1000 ohms at 1/4w in parallel to get 250 ohms or 3 in parallel to get 330 ohms.

Note that the manual says the resistance is in the range 270-370 ohms at 80C but it does not actually say at what resistance the fan turns on. So I would go he route of multiple higher value resistors and keep adding in parallel until the fan comes on with the engine cold. Try getting 5 1000ohm resistors for example allowing you to set 330, 250 and 200 ohms.

[the1andonly]

Thanks jimmo
I measured 5v at cable end, I just happened to have 220 ohm and 1k resistors within arms reach so I think you have given me a way forward.

[jimmo62]

So if it's about 5 volts then a 1/4w resistor will be fine (5*5/220 is well under 1/4 watt)....

[the1andonly]

Having 1k resistors to hand (no 360), ive just tried 3 in parallel with the sensor (not instead of )
equivalent to 286-296
works brill at room temp.
What

What if @ warmer than room temp ie in use, the resistance will be less eg
500 ohm equiv to 200
1k equiv to 248ohm
1.5k equiv to 270ohm
2k equiv to 283ohm
thought of doing a temp calibration but cant get sensor out.
The ECM can obviously take a short (as in Gasys case) am i getting too paranoid?

[the1andonly]

Switch and 3x 1k resistors all in parallel now fitted and working. Not tried it hot yet.