I called out the air con regassing bloke today. He's a nice bloke and has worked on my cars in the past.

Firstly we found that the 10A air con fuse had gone.

simple, just swapped it with the fuse for the horn and we're away.

 

He has got the car hooked up to his monitor and it is working nicely, got the air temp down to 8 degrees.

Then he looks at the sub fan (in front of the normal rad) which should have kicked in by now to make sure the system doesn't get too pressurised and it's not working.

 

After a bit of fiddling about we found that this does work when it is hard-wired, it seems that whatever circuits are supposed to tell it to come on are not working!

So, we leave the fan hard-wired and start the whole process again. Fine for about 3 seconds then blows the original 10A fuse again!

 

I am at my wits end. Does anyone know where I should be looking?

Thanks, Mike...

common misconception: The sub fan is NOT controlled/used by or for the aircon ! ...it's used as an extra/emergency cooler for the rad.

It's controlled by the engine ECU and depends on coolant temp.

However, when the aircon is turned on it will turn on a little earlier (at a coolant temp of 95C, instead of 105C)

 

The reason the fuse blows, could be due to several reasons:

short in the elctr. motor or something is in the fan/axle (plastic/dirt/etc)

Quoted by Lymon "common misconception: The sub fan is NOT controlled/used by or for the air con ! ...it's used as an extra/emergency cooler for the rad. "

 

With the greatest respect for your zed knowledge on this you are wrong.

 

The sub fan is used by the air con to pass air over the condenser when the car is stationary.

When the car is on the move the amount of air pushing its way through the condenser is more than enough for the air con system to work efficiently without the fan operating, however when caught in traffic, the system pressure will rise and the fan will cut in, thus reducing the pressure and the fan cuts back off.

 

Failure of the condenser fan can potentially lead to the system if left on in traffic for too long to over pressurize and at around 22 bar the high pressure relief unit blows, this is a last chance fail safe and all the refrigerant is lost as a result.

 

 

If you want to see this in action stand outside any modern car with air con running and listen to the condense fan cut in and out, on the 300 zx if you have a set of pressure gauges attached you will see the pressure rise to around 16 to 18 bar and the fan then cut in, if it does not cut in the pressure rises very quickly to 20 bar +

 

The sub fan is also used by the ecu if a very high coolant temp occurs.

 

 

Daveman

My fan has burnt out and ive un pluged it and air con runs fine on 10 its frezzeing inside the cabin too cold some times even in traffic .Ive noticed a lot of cars dont go as low as 10 eg fords /bmw /vauxhalls is this becuse the zed was a marketed at the us market eg florida etc.

>> If you want to see this in action stand outside any modern car with air con running and listen to the condense fan cut in and out, on the 300 zx if you have a set of pressure gauges attached you will see the pressure rise to around 16 to 18 bar and the fan then cut in, if it does not cut in the pressure rises very quickly to 20 bar +

 

 

it's controlled by 3 variables: coolant temp, ac switch on/off and vehicle speed

it has nothing to do with the pressure the AC system works at:

 

http://300zx-twinturbo.com/cgi-bin/manual.cgi?list=efec&dir=&config=&refresh=&direction=forward&scale=0&cycle=off&slide=31&design=default&total=190

 

don't forget most (series 1/2) Zeds use an older type ac refrigerant (R12), which operates at a lower pressure and so doesn't get hot as quickly as the newer/safer refrigerants, as used in most modern cars, which are operating at (much) higher pressures

 

Im not sure how it works for zeds that have R134a AC system from factory, but I can imagine the aux. fan is then also used for cooling of the condensor.

The same should be the case if you "upgrade" your AC system to a newer refrigerant

Thanks for the responses

The reason the fuse blows, could be due to several reasons: short in the elctr. motor or something is in the fan/axle (plastic/dirt/etc)

The compressor was a little hard to turn by hand, could this resistance cause the fuse to blow?

 

I am going to undo the hardwiring of the sub-fan and replace the relay. Then I am going to pick up a few more fuses and see what happens.

I'll keep you posted...

Thanks for the responsesThe compressor was a little hard to turn by hand, could this resistance cause the fuse to blow?

 

I am going to undo the hardwiring of the sub-fan and replace the relay. Then I am going to pick up a few more fuses and see what happens.

I'll keep you posted...

 

Get a new temp sensor. The blue one that fits in your top water pipe. This has to be in good nick so the ECU can control when the Fan kicks in.

 

If its knackered it won't kick in.

Thanks for the responsesThe compressor was a little hard to turn by hand, could this resistance cause the fuse to blow?

 

I am going to undo the hardwiring of the sub-fan and replace the relay. Then I am going to pick up a few more fuses and see what happens.

I'll keep you posted...

 

Get a new temp sensor. The blue one that fits in your top water pipe. This has to be in good nick so the ECU can control when the Fan kicks in.

 

If its knackered it won't kick in.

Quoted by Lymon "it has nothing to do with the pressure the AC system works at: "

 

No not directly but you cannot dismiss the law of fluid / gas boiling temperature and its relation with pressure.

 

The sub fan IS primary there for the air con condenser, it maintains air flow across the condenser when required ( i.e. in traffic / stationary ) this in turn reduces the system temperature and conversely the system pressure, this is a fundamental operation of not only the 300zx air con system but all air con systems in general.

 

R22 gas will still operate at pressure of 15 bar and without the fan rise to 22 bar in less than 1 minute if the air over the condenser is insufficient.

 

Mike, as far as your blown fuse I would concider swapping the fan as it most likely is drawing too many amps because the motor is faulty.

 

The compressor is supposed to have resistance when turned by hand as the internals move against the system pressure.

 

The temp sensor for the engine coolant system will not stop the fan running when air con is used so do not be misled by that.

 

Hope that clears it up.

 

Daveman

>> The sub fan IS primary there for the air con condenser, it maintains air flow across the condenser when required ( i.e. in traffic / stationary )

 

sorry...don't want to get into an endless discussion, but I still have to disagree.

the Zed has a engine driven, viscous fan that (when working correctly) is ALWAYS sucking air through both condensor and radiator....to maintain air flow across/through the condenser (and radiator) at all times.

 

The aux. fan is never even mentioned in the AC section of the service manual.

The main viscous fan (among other things) is mentioned in case of pressure problems.

 

>> If its knackered it won't kick in.

 

if the temp sensor is broken or not connected, the aux. fan will turn on permanently and the car will be in safetyboost

 

 

-Eric