Posted: May 10 2009 at 12:23pm | IP Logged

Hi

I am new to thuis forum. This is the 1st form I have joined.

I have some problems with my 1999 Frontier and looking for assistance in solving them.

The fan does not work on high speed(setting 4). The AC button is in and the AC light is on. The compressor is engaged and the system seems to be charged sufficiently( I have not checked the system presseure with guages)

When I bring the fan switch back down to 1-3, the fan works. The AC light goes out and the air blows cold for a little while(Compressor not on anymore).

According to a wiring diagram that I have.

The AC indicator is fed from either the AC relay and then back to the ECM or thru thermal control amplifier(thermister) which is also controled thru the ECM( Engine Control Module).

I am not sure why there are 2 paths, but they all lead back to the ECM.

I have swapped out the AC relay with another that was identical and used for something else, so iI don't think that is the problem.

I am not sure where the Thermal Control Amplifier is  or what it's purpose is.

I have pulled the ECM and did a visual on it. I don't see any visual evidents of damage or overheating. But I suspect this could be my problem. They are not cheap to replace and you can't return electrical items, so I don't just want to replace it!

One other reason I think it may be in the ECM is that occasionaly I have a sevice engine light come on. The code says it is a misfire on the on cylinder 1.

I have already replaced cap , rotor, wires and plugs tring to fix this occasional misfiring issue but it's is still present.

The service engine light also comes and goes, does not stay on all the time.

Any help or direction with my problems would be appreciated.

Thanks

Jim C

Posted: May 11 2009 at 6:56am | IP Logged

The thermal control amplifier monitors the temperature of the evaporator in the AC system.  It shuts off the AC compressor to prevent the refrigerant from freezing.  Usually, there is also a safety cutoff of the compressor if the fan is not running (air needs to be blowing throught the evaporator, or again, the refrigerant can freeze).

It sounds like you have a faulty circuit making the ECM think the fan is off in "1" through "3".  Before you go off spending a lot of money, the first thing you might want to try is take a look at the fan control, clean or repair wire connections, and keep an eye out for any short or open circuits.  A multimeter or continuity tester will come in handy, and be sure to disconnect the battery.  Most likely, whatever is causing the fan not to work at "4" may be what is causing your AC compressor to not come on.

For now, also be careful of running the AC in "4" when the fan is not running, if you freeze the refrigerant, it can burn up your compressor.