Do you know if the engine temp goes down or the gauge reads low?
I ran into this problems a few years ago and I found that the alternator would drop below 12 volts when the heater was placed on high. The heater blower draws a lot of current.
Have a tech place a volt meter acroos the battery and observe the voltage with the car idling and the blower motor on high. If it drops below 13 volts you have an alternator problem. The dash has a voltage regulator that needs 13 volts to operate all the gauges.
Let me know and we can explore this problem further. I need to know the country where the vehicle resides.
Monday, January 22nd, 2007 AT 5:58 PM