I have a 91 Toyota previa LE and my heat only comes on when it wants to. The AC works fine and the temperature gauge always stays in the middle. Can someone please help. It's getting cold.

- Ensure that the temperature gauge stays at the middle at operating temperature.
- Check the heater control valve operation, control cable and thermostat.
- Check the heater pipes routing.
- Check the heater radiator for clogged.

you may have a stuck thermostat

You may need to try to find a used shop manual for this repair.
The DIY books don't show wiring diagrams for the a/c system.

stop and go traffic will make the car heat up,, make sure you have air flow through the radiator fins, also make sure that the ac fan is actually engaging the fans work on a sensor and if they don't work, neither does the fan.

bad sensor

blower motor bad or ac clutch bad,,check blower resister as well,it is in the blower box ,blower motor is the most comon to do this

Is the Fuel gauge reading properly?
If so, the fuse is not blown; they are on the same circuit.

If the fuel gauge works, I would guess that you may have a bad temperature sending unit.

Thanks for the quick reply. What you have is a thremostat that is stuck in the "open" position. This would explain why the temp gauge reads low. Also, this condition will also cause the engine to seem like it is "revving up". Why? Because the OverDrive will not engage until the temperature of the coolant is 172 degrees F. When the coolant reaches that temp, the OD solenoid gets power from the ECU, dependant on the temp mentioned above...NOW, when the temp of the coolant DROPS below 172 (as in having a seized open thermostat), the solenoid is de-energized by the ECU, and the trans shifts OUT of OD. Downshifting to the next lower gear cause the RPMs to incerase. In any case, replace the thermostat, and whilst you are doing this, replace the coolant, also. If you have RED coolant, that is factory Toyota coolant. Is is a GOOD IDEA to continue using the red coolant if you already have it in your cooling system. Buy the stat and the coolant at a Toyota dealer. Do NOT buy an "aftermarket" stat. The cheaper ones do NOT have the air "wiggle" valve built into the stat. Install the stat with the little "wiggle valve" pointing straight up; "12 o'clock". Add 50-50 coolant water mix, Bleed the air from the cooling system & you will be fine.

Bring it to a qualified a/c technition! you need a licence to recharge or handle 134a. there are very large fines for improper procedures and handling