Heater only works on low setting - 1990 Chevrolet K1500

Te ressistor is located next to the blower motor usually hold by 2 little screws another possibility is the switch i

Heck the connections on the back of the heater control assembly, you may find the wires to the fan speed melted the black ground wire in the connector. Its common and creates alot of resistence in the circuit, causing only lo speed. I've also seen the connector at the resistor block (located behind the glove box melted, remove glove box to access. There are also relays there, check them.

First, check all your fuses- if you find one that is blown, replace it with one that is the same rating- if it blows right away, there is a wiring problem that needs troubleshooting.

If all your fuses are OK.....

Find where the blower motor is- on most vehicles it is in the right knee area of the passenger side up front... sometimes it is on the passenger side under the hood. If you cannot find it, set the blower speed to low, turn on your key, but do not start the car (don't do this for more than a few minutes, because it can drain your battery). Follow the motor sound....

If the motor has only two wires going to it, the problem is probably with either the blower speed switch or the blower speed control relay. Sometimes, a 2 wire blower CAN wear out in a way that it will only run on low, but you can eliminate that possibility....

With a 2 wire blower, you can check the motor by running a set of jumper wires from a battery to the blower and see if it comes on full speed. First, connect a voltmeter across the connector so you know which side gets + and which gets -. If you are not comfortable with doing this yourself, by all means get a friend with experience or a mechanic to show you how.

If the motor has a bunch of wires going to it, the problem is probably the blower motor itself. These can be very expensive, and your best bet is to shop around online for one.