[rideon]

A friend of mine has a 2006 Honda City ZX Vtec that has run 55000 kms till date. The only issue is that the AC cuts out on speeds above 80 kmph specially during daytime. Once below 80, the AC is back on. He has had the belts checked; they are fine. No heat indications signs, or any loss in power either. Coolant, condenser, gas pressure, etc are all good.
What could be the issue.

[Jaggu]

Indeed a funny situation Since its an ECU controlled car, would suggest to go to a dealer and run a scan while driving to find out what is the cause. Also is the cutting off related to vehicle speed or engine speed? Low gear high rpm does it cut?

Is the vehicle eqiupped with a cut off switch at acc pedal? if so try disconnecting the same and check.

[tsk1979]

Is the condenser fan fine? In case of defect in the condenser fan fitment or central axle, when high speed air hits the fan, it can cause the fan to move, leading to slippage. This can cause AC heating up whenever air flow is high.
As Jaggu said, drive at high rpm in second gear. If AC is fine, then its this issue.

A rarer case can be some problem with ECU speed sensor, but that will usually manifest itself in other things also.

[rideon]

Originally Posted by Jaggu

Indeed a funny situation Since its an ECU controlled car, would suggest to go to a dealer and run a scan while driving to find out what is the cause. Also is the cutting off related to vehicle speed or engine speed? Low gear high rpm does it cut?

Is the vehicle eqiupped with a cut off switch at acc pedal? if so try disconnecting the same and check.

No issues with low gear high rpm. The cut off is related to speed rather than rpm. Also, it does not have a cut-off switch.
OT: please help me understand what a cut-off switch would do on the acc?

Originally Posted by tsk1979

Is the condenser fan fine? In case of defect in the condenser fan fitment or central axle, when high speed air hits the fan, it can cause the fan to move, leading to slippage. This can cause AC heating up whenever air flow is high.
As Jaggu said, drive at high rpm in second gear. If AC is fine, then its this issue.

A rarer case can be some problem with ECU speed sensor, but that will usually manifest itself in other things also.

Yup. Heard this from SS-Traveller too about some Safari's in Spain that ran into compressor clutch issues because of high speed.
He's gonna check the fan.
By the way, the ECU scan is all fine. No error detected.

[Jaggu]

Acc switch is an overriding switch/kill switch for AC which will cut off the compressor when acc pedal is completely depressed. This is to assist with additional power for sudden over taking situation etc. Some cars have this.

And error code needs to be checked while trying to replicate the situation at speeds, just in case.

If its compressor clutch it should show up with higher engine speed.

[SPIKE ARRESTOR]

Originally Posted by tsk1979

Is the condenser fan fine? In case of defect in the condenser fan fitment or central axle, when high speed air hits the fan, it can cause the fan to move, leading to slippage.

Can't understand when you say ^^. Can you explain?

Does this occur during sudden acceleration above 80 kmph or anything above 80 kmph? Also does this occur at night times (cooler ambient temperatures) ? Please clarify.

Spike

[tsk1979]

Originally Posted by SPIKE ARRESTOR

Can't understand when you say ^^. Can you explain?

Does this occur during sudden acceleration above 80 kmph or anything above 80 kmph? Also does this occur at night times (cooler ambient temperatures) ? Please clarify.

Spike

If your condersor fan is not rotating, your AC will cut off due to condensor overheating. Many modern AC systems have this feature.

So if you have dust and debris in air flow path, you will see frequent AC cutoff.

Now imaging a condensor fan, which has a mechanical fault. When car is not moving fast, the fan is blowing air towards the condensor, and as per newtons law of motion, the fan is getting pushed outwards.

At high speed, lets say 80-90kmph, the air flow hitting the fan will be significant, so the fan is going to get pushed inward.

Imagine a mechanical defect, where if the fan is pushed, it moves on the spindle slightly, leading to spindle rotating, and fan not rotating with the spindle. This will be a rare case.

Imaging a defect inside the fan motor, so that when fan is pushed due to airflow, the spindle jams.

If there is a sensor inside the AC mechanism which is designed to turn off AC when fan is not rotating, AC will immediately cut off.
However, if such a mechanism is not there, AC will cut off when condensor temperature reaches above a certain level, which will happen very quickly if fan is not rotating.

I have had such an issue. The condensor fan was not rotating , therefore the AC was cutting off regularly. After the condensor fan was replaced things were fine.

In the above case, lets say fan is lose on spindle.
Airflow pushed it towards the condensor, and the blades touch the shroud, and at high speed spindle rotates, but fan is stationary, actually blocking the airflow. This will lead to temperature increase immediately within a few seconds. So after 15-20 seconds the safety circuit will cut off the condensor. In such a case Ac will come on intermittently.

Since OP has not mentioned that AC is going on/of/on etc., I suspect that when airflow speed is high, the spindle itself is getting jammed, causing a speed sensor to cut off the AC.

A rared case would be a problem with some ECU sensor, which is giving some wrong signals leading to AC cutting off.

For example, if some sensor detects engine overheat, it will first cut off AC to slow down overheating.

Usually this leads to a error code in ECU. Since there is no error code in ECU, it looks like some secondary sensor in ECU.

[Mr_Bean]

Please get it checked by a expert AC mechanic, usually service centres offload these jobs & the expenses sorting the issues out will be high if done through them. This was my experience with MASS, not sure how it is with Honda.

Btw I face a totally opposite problem, my AC does not engage below a fixed rpm. This does not happen always but when it happens in peak traffic, the AC stops functioning and works like a fan. During this time period, when the AC fails to engage or rather when it tries to engage there is weird sound heard and the engine vibrates very badly with rpm going below the idle.

This goes on for 4-5 times before the AC gets activated, I need to get this checked ASAP by a local AC mechanic.

I own a Baleno!

[Lalvaz]

Originally Posted by tsk1979

If your condersor fan is not rotating, your AC will cut off due to condensor overheating. Many modern AC systems have this feature.

So if you have dust and debris in air flow path, you will see frequent AC cutoff.

Now imaging a condensor fan, which has a mechanical fault. When car is not moving fast, the fan is blowing air towards the condensor, and as per newtons law of motion, the fan is getting pushed outwards.

At high speed, lets say 80-90kmph, the air flow hitting the fan will be significant, so the fan is going to get pushed inward.

Imagine a mechanical defect, where if the fan is pushed, it moves on the spindle slightly, leading to spindle rotating, and fan not rotating with the spindle. This will be a rare case.

Imaging a defect inside the fan motor, so that when fan is pushed due to airflow, the spindle jams.

If there is a sensor inside the AC mechanism which is designed to turn off AC when fan is not rotating, AC will immediately cut off.
However, if such a mechanism is not there, AC will cut off when condensor temperature reaches above a certain level, which will happen very quickly if fan is not rotating.

I have had such an issue. The condensor fan was not rotating , therefore the AC was cutting off regularly. After the condensor fan was replaced things were fine.

In the above case, lets say fan is lose on spindle.
Airflow pushed it towards the condensor, and the blades touch the shroud, and at high speed spindle rotates, but fan is stationary, actually blocking the airflow. This will lead to temperature increase immediately within a few seconds. So after 15-20 seconds the safety circuit will cut off the condensor. In such a case Ac will come on intermittently.

Since OP has not mentioned that AC is going on/of/on etc., I suspect that when airflow speed is high, the spindle itself is getting jammed, causing a speed sensor to cut off the AC.

A rared case would be a problem with some ECU sensor, which is giving some wrong signals leading to AC cutting off.

For example, if some sensor detects engine overheat, it will first cut off AC to slow down overheating.

Usually this leads to a error code in ECU. Since there is no error code in ECU, it looks like some secondary sensor in ECU.

This is why I love Team BHP and spend hours reading the posts. First time I read the opening post, I was racking my brains trying to figure out what the problem might be, but no luck, I just could'nt imagine why the car A/C might behave in such an illogical manner, and when I read your post above, it suddenly seemed like the most logical thing. Thanks a ton.

[SPIKE ARRESTOR]

Originally Posted by tsk1979

In the above case, lets say fan is lose on spindle.
Airflow pushed it towards the condensor, and the blades touch the shroud, and at high speed spindle rotates, but fan is stationary, actually blocking the airflow.

100% agree with your reasoning and understanding. But, cannot understand when you say ^^. The condenser fan is mounted on the condenser itself and is an electric fan right? Are you referring to similar shroud?



Spike

[Jaggu]

Electric fan might not even kick in at 80 kph. So i dont think that is a problem. One thought, is there any wire in front which is getting shorted at high speeds due to wind speed

As i mentioned early its a funny situation.

[SPIKE ARRESTOR]

^^ Jaggu, fan speed depends on coolant temperature and line pressure -> ECU.

Spike

[dhanushs]

Originally Posted by tsk1979

...I have had such an issue. The condensor fan was not rotating , therefore the AC was cutting off regularly. ...

You mean, at high speeds the AC would stop working, just like in OP's post?. So that would mean in a Safari(?) the safety system is cutting off the AC when the fan stops working and not when the condenser reaches a certain max temperature?. But then, is the fan always ON?.

Originally Posted by SPIKE ARRESTOR

^^ Jaggu, fan speed depends on coolant temperature and line pressure -> ECU.

At above 80kmph and featherlight throttle, ram air is sufficient to cool even the engine. So, coolant temperature at that speeds shouldn't be high. Hence the statement, I guess.

Originally Posted by Jaggu

.. As i mentioned early its a funny situation..

Another thought is that, if the AC recirculation is malfunctioning, and is always in fresh air mode, high vehicle speed means more fresh air into the cabin(?) and less AC effectiveness. Also, the problem should be prominent if he has an already not-so-efficient AC.

[SPIKE ARRESTOR]

Originally Posted by dhanushs

At above 80kmph and featherlight throttle, ram air is sufficient to cool even the engine. So, coolant temperature at that speeds shouldn't be high. Hence the statement, I guess.

Not really, if that was the case fan must cut out as soon as a preset high speed is reached. Fan still rotates but with a preset minimum speed (modulating fan). Moreover, we are talking about AC, so condenser fan here.

Spike

[SS-Traveller]

I think Tanveer said it...

Originally Posted by tsk1979

If your condersor fan is not rotating, your AC will cut off due to condensor overheating. Many modern AC systems have this feature.

Check if both the main and condenser fans start up as soon as the AC is switched on. If either fan fails, the AC will still work, but there won't be enough cooling of the condenser (esp. when the car's radiator needs to get rid of a lot of heat from the engine when driving at high speeds) - and a little switch will then cut the AC compressor out and stop cooling.

Also check for a AC compressor clutch slippage / failure.