6N Revving when air con is on

[t_mike]

Hi all, I have a 97 polo with the updated electrics. The car does everything right, until the air con is turned on. Every time it kicks in when idling the revs jump to 1800 and surge between this and about 2000. I've tried a couple of throttle bodies and map sensors but no change. Vag-com shows no error codes and with each throttle body a tba has been done. With limited info it's hard to diagnose. Anyone have experience with this? Googling has found no useful answers or clues.

[pontiff]

Would have to think throttle body still as hunting like that is a normal indication . Interesting though only happens when AC is on, but the car will try and raise its rpm a few hundred to compensate

[t_mike]

I've had the throttle body and map sensor on another car and it works perfectly. I'm wondering if there is anything in the air con system that could be doing this. Obviously the ecu gets a message from somewhere to up the throttle and this is not a reactive thing, there is no drop in revs as the compressor kicks in, just an immediate increase.

[pontiff]

I've had the throttle body and map sensor on another car and it works perfectly. I'm wondering if there is anything in the air con system that could be doing this. Obviously the ecu gets a message from somewhere to up the throttle and this is not a reactive thing, there is no drop in revs as the compressor kicks in, just an immediate increase.

Exactly... Possibly comes from the fan resister as it engages the radiator fan at the same time the compressor kicks in...

Do you have a vagcom cable? Have you checked all the inputs in that? Random things like coolant temp sensors and the triggers that show the engangent of the A/C etc? I.e is the ECU acting on all the correct inputs?

[t_mike]

Finally had the opportunity to try a known good thermo fan relay which made no difference. I do know for a fact that this is the switching point for the air con because if you disconnect the pressure switch the thremo fan still switches on without the compressor clutch kicking in. I do have a vagcom can hex but still have my L plates on with it. I wasn't aware that you could find the ecu inputs for the air with it
I don't have a work shop manual for the car and I'm not inclined to get one, when the car is sorted I'll sell it.
Your help is appreciated pontiff.

[pontiff]

Finally had the opportunity to try a known good thermo fan relay which made no difference. I do know for a fact that this is the switching point for the air con because if you disconnect the pressure switch the thremo fan still switches on without the compressor clutch kicking in. I do have a vagcom can hex but still have my L plates on with it. I wasn't aware that you could find the ecu inputs for the air with it
I don't have a work shop manual for the car and I'm not inclined to get one, when the car is sorted I'll sell it.
Your help is appreciated pontiff.

That is really weird... I mean if you did not try another throttle body (and TBA) I would just say that its the throttle body and would be 99.9% certain thats what it is, but if you have tried known good units and its STILL doing it that is most peculiar, especially when it only happens when the AC is on and the car just raises the RPM's ever so slightly.

Not that it should matter but is the car auto or manual? the auto ones stock idle is about 650 and the manual ones is 850 give or take.

there isnt any air leaks in the intake?

[t_mike]

Definitely no air leaks. pontiff, you're based in Melbourne, correct? May need to contact you for parts. What are the build dates of the polos you're wrecking. The 2 I have have different electrics so I'm wondering if the tb may have differences. I don't have access to part numbers to check. May just have to try other components. Not the way I like to do it but diagnostics are telling me nothing.

[pontiff]

Definitely no air leaks. pontiff, you're based in Melbourne, correct? May need to contact you for parts. What are the build dates of the polos you're wrecking. The 2 I have have different electrics so I'm wondering if the tb may have differences. I don't have access to part numbers to check. May just have to try other components. Not the way I like to do it but diagnostics are telling me nothing.

the 6n1's, so 96 through to late 98 (which were complianced and called 99's here)

[t_mike]

About march 97 there was an upgrade to the electrics, new dash, revised fuse/relay panel and the thermo fan was the later type mounted on the chassis under the battery as opposed to next to the battery. I'm wondering if this also equated to a different tb given the fan module was different. I've put an early 97 tb into a later 97 car, could this be the problem?

[pontiff]

About march 97 there was an upgrade to the electrics, new dash, revised fuse/relay panel and the thermo fan was the later type mounted on the chassis under the battery as opposed to next to the battery. I'm wondering if this also equated to a different tb given the fan module was different. I've put an early 97 tb into a later 97 car, could this be the problem?

most of mine are the later type and yes, i think they could be different. put it this way, i would try the correct part for your car before you do too much as you may end up chasing your tail.