My a/c will blow cold, than it will blow hot while going down the road?

AC blows cold, then hot, then cold... - Car Forums and Automotive ...

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A/C now turns on but will blow cold air for 5 seconds then back to hot for the same. ...It seems be be left of the heater box somewhere but it is definately inside. .... on this '98... the things just keeps turning in there, hot, cold, hot and so forth.

If you have a viscous fan hub have it replaced as it is not working. The problem you have is that the engine is not driving the fan to draw air through the radiator when idling so the engine overheats . When moving the air flow from driving cools the radiator and so the temp goes back down. If you have electric fans have the codes read to find the fault or check why the electric fans are not working.

Check if the fan is working, it could be a weak or bad fan or just a bad fan relay. You may also want to check the freon you have in the system.

it is either low on coolant, has air trapped in system, or your blend door is stuck in cold mode. rotate it from hot to cold, back and forth and see if you can see or hear it moveing. Some have vacuum motors that operate door, some have electric motors. report results.

Have your a/c condensor fan check out, sit in back of the radiator, fan should always be on when a/c is on to reduce the freon pressure in the condensor or a/c will not get cold till air goes through the condensor while driving down the road, no air blows through at stop light because fan not working so a/c will not work right.

I would check the following possibilities:
1. Is compressor clutch working? - You need to do this when the ac is blowing hot air only - locate compressor drive pulley on engine - rubber belt wraps around it. Start engine and set to max ac. When the ac is blowing hot air, look at the center of the compressor drive pulley using a good flash light - watch it for a while, it should be spinning with the pulley - this means the compressor is engaged. If not, the compressor is not engaged. If your ac is blowing hot air, then the compressor should be engaged and running (trying to cool). If it is not engaged, then your compressor clutch is probably going out. Could be low voltage to clutch, but more likely the clutch is worn out and only working some of the time. Note: if working normally, the compressor clutch will engage and disengage in cycles- but the air should always be cold. If clutch is bad, compressor would need to be replaced (sold as one unit). This is tricky to diagnose and could take some time - you need to do it when ac is blowing hot air but set to max cooling.

2. Low on refrigerant or refrigerant has moisture in it? If your ac blows cold initially after being off for a while, then heats up, and continues to regularly cycle back and forth between hot and cool air, then the system may be low on refrigerant or the refrigerant may have moisture in it that is freezing and restricting refrigerant flow. An a/c shop can check refrigerant level and even replace refrigerant if moisture is suspected.

Could be other issues, but I would start with these.

Good luck

Heater control valve malfunction will not cause engine to overheat unless coolant is leaking externally...Overheating going down the road can be the result of a few different things...Partly clogged radiator...Insufficient air flow due to possible fan problem...External coolant leak...Make sure fans are operting properly and have cooling system pressure tested for leaks...Gaining and losing heat on the inside of the vehicle usually means system is low on coolant but you must find out where coolant is going

try turning the blower down, if you have the fan on full you will feel cooler air from time to time. as the thermostat only measures the ambient tempreture inside the car when this level is met then it will turn off the heat so it will only blow the air from the car which will feel cooler.
if the car is still cold suspect the tempreture control in the car maybe faulty or the water level in the system maybe low.

It could be a number of things. Check to see if the clutch is turning on the compressor. If it keeps turning on and off (cycling) it is low on freon. Is the condenser clean and free of bugs and road debris? Is the radiator fan coming on? If not that could explain why it is cold while driving, but hot while idling. Most all vehicles will blow colder on the AC when driving down the road because you are getting cooler air through the condenser. Hooking up a set of gauges to the vehicle will tell you best what is going on. Basic rule of thumb is: low side should be in the 30-40 lb range while running and the high side should be around 100 lbs higher than the ambient air temperature. Anything drastically different means you have a problem.

How cold is it there and does your engine cool down any while going slow? If it does it may mean you need a hotter thermostat installed, My first thought would be to check the coolant level in the radiator and the coolant reservoir! Hope this helps and Merry Christmas