Scavenger Fan Override?

Diplomat · Post by **Diplomat** » Wed Aug 19, 2015 11:47 pm

Does anyone know what the component is that is encased in a long box on the chassis near the bleed hose?

That has a single lead going into it and I wondered if that was a temperature sensor. I noticed it yesterday while bleeding after my radiator change.

Frank

Gasy · Post by **Gasy** » Thu Aug 20, 2015 7:07 am

Fassi
If you have the fan switched on and running
And then switch off the engine leaving the fan switched on
Does your fan keep running ? For 5 or so mins

It does on mine
I use it after a long hard run
Switch fan on switch engine off switch fan off walk away with fan cooling bongo for 5 mins

Post by **Simon Jones** » Thu Aug 20, 2015 12:17 pm

Diplomat wrote:Does anyone know what the component is that is encased in a long box on the chassis near the bleed hose?

That has a single lead going into it and I wondered if that was a temperature sensor. I noticed it yesterday while bleeding after my radiator change.

Frank

I think it's a resistor which is used for something or other.

Fassi · Post by **Fassi** » Thu Aug 20, 2015 1:48 pm

Gasy wrote:Fassi
If you have the fan switched on and running
And then switch off the engine leaving the fan switched on
Does your fan keep running ? For 5 or so mins

It does on mine
I use it after a long hard run
Switch fan on switch engine off switch fan off walk away with fan cooling bongo for 5 mins

Yes it will keep going for 5 mins if I turn off. As long as it has been turned on at 84 degrees or higher which it always is anyway after a run.

If I operate it at under 84 degrees then it just functions on and off in conjunction with the switch and must have ignition power. Above 84 requires no ignition power.

I guess if I was just to leave the switch on and walk away, the fan would run for 5 mins then if the temp was below 84 it would switch off and keep going if it was above. Haven't tested this yet.

It definitely switches off after 5 minutes though as long as I turn the manual switch off.

Gasy · Post by **Gasy** » Thu Aug 20, 2015 5:14 pm

Mine will not switch on with the ignition off

the1andonly · Post by **the1andonly** » Thu Aug 20, 2015 8:50 pm

Gasy wrote:Mine will not switch on with the ignition off

neither does mine

Post by **g8dhe** » Thu Aug 20, 2015 9:50 pm

Its actually controlled by the ECU, if the ECU sees the temperature is above the setting and the ignition is on it turns on the fan, and will then allow it to run until the temperature falls to a safe value, however it won't turn on if the Ignition is not on in the first place!

the1andonly · Post by **the1andonly** » Thu Aug 20, 2015 11:03 pm

g8dhe wrote:Its actually controlled by the ECU, if the ECU sees the temperature is above the setting and the ignition is on it turns on the fan, and will then allow it to run until the temperature falls to a safe value, however it won't turn on if the Ignition is not on in the first place!

Is it known to be temperature or time related? or just another Bongo mystery to be solved by observation.

Fassi · Post by **Fassi** » Fri Aug 21, 2015 7:39 am

It seems to have a couple of modes of operation. Before fitting the switch i was concerned my fan was never coming on. Then a few days ago on a hill climb i parked up to let the engine cool. I removed the key from the ignition. Some time later I heard a fan starting which was the scavenger ( i double checked it was not the front fans). The heat soak had raised the temperature of the head to above 100.

After this there was one more instance of the scavenger fan coming on at a petrol station but this time it happened when I had the ignition key in and turned on so the dash lights were lit but engine not running. The temperature was mid 90's I think but can't remember exactly.

When i decided to do this manual switch I tried with a 400 ohm resistor in series with the new switch, parallel with the temperature sensor.

Started engine.
Switch would turn on fan and off in direct response to the switch. (i had read that it should keep going for 5 mins if i switched off but this never happened)
Stopped engine.
Tried switch with the ignition on and lights on dash lit.
Again switch would turn on and off in response to switch.
Took out ignition key.
Tried switch and nothing happened.

I then tried a 200 ohm resistor.
Turned on engine.
This time when I operated the switch on it would stay on even if I turned it off.

I went back to the 400 ohm resistor. (should have did more tests at this stage with 200 but never)
I ran the engine and tried the switch as the temperature rose every few degrees.
Still had the on and off in relation to switch until I reached 84 degrees when the fan 'latched on'
Took the temperature up to about 87 then turned off the engine off. Fan kept running.
Inserted key again, turned ignition on and the fan stopped. Seemed it was reset here.

Temperature was reading 86 so I tried the switch with the ignition key and dash lights on but no engine, fan operated and latched.
I removed key and the fan was still runnning. I put the key in again, turned on ignition and reset.

Temp was 85. I then just tried the switch without the key being in and then fan latched on again. This time I timed it and it switched itself off after 5 minutes.

I then tried again with the switch but it would not operate the fan as the temperature was 82ish.

It does seem mine is operating differently and if I left the switch on all the time when parked, all I have done is effectively allow the temperature sensor to give a false low reading to the ECU.

So on mine it is possible that this is monitored without ignition being on due to my test.
But also it could be that this 'latching on' of the fan and subsequent 5 minutes timer needs manually reset by the ignition key.
I will need to check further at some point.

edit
(my temperature readings were just from the tm2 alarm and thus back of head)
The details above are how I remember them but maybe it is different. I will do a proper analysis when I return to Scotland so don't shoot me yet

Post by **g8dhe** » Fri Aug 21, 2015 10:21 am

the1andonly wrote:
g8dhe wrote:Its actually controlled by the ECU, if the ECU sees the temperature is above the setting and the ignition is on it turns on the fan, and will then allow it to run until the temperature falls to a safe value, however it won't turn on if the Ignition is not on in the first place!
Is it known to be temperature or time related? or just another Bongo mystery to be solved by observation.

We can only ever work from observation as its not practical to get at the code running the ECU, and as Fassi comments above suggest it may have yet another mode in that it will turn on if the temperature goes too high even with the ignition off. However given that the fans can easily draw 10 Amps I would have thought that there would be a time limiter built into the code as well to prevent a fault in the sensor causing the fan to remain running until the battery was flat!

the1andonly · Post by **the1andonly** » Sat Aug 22, 2015 11:43 pm

Are we thinking the scav fan ECU system may also be connected to the standard temp sensor
if so what is normal resistance range of standard temp sensor so we can start conning the ECU to turn scav fan on
with ign off.

geoff
I would have thought that there would be a time limiter built into the code as well to prevent a fault in the sensor causing the fan to remain running until the battery was flat!

I've just come back off holiday to find a dead battery because the A4 alarm was on for a day (mentioned a week after we come home)
perhaps Audi is not as savvy as mazda on what could kill a battery
don't comment on last statement as it may contravene something

Fassi · Post by **Fassi** » Mon Aug 24, 2015 11:43 am

I've been out checking again, and with the engine block temperature reading from my haydn at 84, I can turn on my scav fan with ignition off. It then runs for 5 minutes. I would think the ecu probably has some code where if the engine bay temp = a given range, then turn on scav fan while ignition is on. If temperature reaches a higher range, then turn on scav fan even if ignition is off. But I think it is probably just using the scav fan temperature sensor to do this.

I can't verify what happens if i just leave the switch on as i need some way of heating the engine bay back up to temp without turning on the ignition. Maybe I'll get a heat gun and heat up the sensor in situ.

Gasy · Post by **Gasy** » Mon Aug 24, 2015 6:35 pm

Talk about fanning the flames
if it ever caught fire

Fassi · Post by **Fassi** » Mon Aug 24, 2015 7:40 pm

Gasy wrote:Talk about fanning the flames
if it ever caught fire

Haha. Good point. I'm talking a few seconds with the heat gun though so should be ok?

Fassi · Post by **Fassi** » Mon Aug 24, 2015 7:42 pm

And the reason for knowing the outcome of this I just asked myself. I've convinced myself it is in case I forget to turn the switch off I think.

Scavenger Fan Override?

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