Chevrolet Repair: heater blower only runs on high, chevy s10 pickup, heater blower

Question
"hi i have a 88 chevy s10 pickup. 2.8 w/air . my poblem is the heater blower only blows on high. i already replaced the resistor and switch assembly.im not sure how to trouble shoot the wiring to see if there is power at the switch for the lower speeds. can u help me in how to test the switch and the resistor harness's to see if the problem is in the wiring..i assume that the blower motor is ok, being it runs on high.also i notice when i switch it to high the headlights dim abit momentarily  any help will be appreciated. thanks, john miller


Answer
Hi John,

The blower motor could be the problem.  In a long shot, the motor could be needing 12V to run, and not getting it on lower speeds.  

You are on the right track checking the voltage at the motor.  You need a DVOM meter.  You can get a decent one from ACO hardware for 10 dollars.  

Unplug the motor harness.  Plug the probes into the switch side of the harness.  Black to black, and red to what ever the other color is on the harness.  Turn the switch on low and take a reading.  Then the next speed and do the same.  You should see the numbers going up.  If not, you are not getting voltage.  

Where did the switch come from?  This is usually the resistor.  You changed that, so the switch is usually next.  You changed that, and the next step is readings.  Based on that, if you get progressively increasing numbers, you do the motor next.  

Hope this helps.

C J S