I have a 1997 Chevy k1500 Silverado with a 5.7 L 350.

My heater would only come in in high or occasionally wouldn’t at all.

I did some research. First replaced the resistor. Then I had nothing at all. Swapped back to the old resistor and still nothing. Next I replaced the relay, nothing with either resistor. Then the blower motor and nothing with any combination of the three parts. What could it be? If the heater core had a problem then I would still have a blower motor right? The wiring looks fine for a 1997. All three were still factory.

Maybe heater control switch?

 

check your fuses

 

You can put 12v to the blower to see if it works

 

There is a 50 amp for the blower under the hood. It looks fine I’ll double check with another fuse.

 

So there is also a 25 amp fuse on the drive side panel.

Didn’t look blown but I changed it and it behave like previously, only came on in high and shut down when I turned it it lower (because I had put back the original resistor, that is probably bad and my original guess)

put in the new resistor and nothing. Maybe the new resistor is bad? Will try that, send back the replacement for another replacement. I’d rather have full blast than nothing.

 

perfect suggestion thanks!

 

So old resistor still only blows on hi, sometimes nothing. Ruled out fuses

Put in a brand new duralast resistor and nothing. It looks nicer than the el cheapo from Amazon, which looks nicer than the original.

connector to resistor?

 

Did you measure 12V on the fuse?

 

Thanks! I did measure 12v on both fuses with a multimeter.

However I could not get a measurement at all from the resistor harness.

I tried every combination, wasn’t sure what wires to test, one video I watched tested the voltage with the 2nd and 3rd wire and went through fan speeds, that one had a four wire setup but wasn’t a 90’s Chevy.

Wiring harness


I did try a new relay, not sure if I can test there?

I’m leaning wiring harness…?

 

I did find a reading on the resistor harness: Yellow (power) to blue read 9v if the fan was in speed 1 - Lo, read 0v on all other settings.

The fan motor wires read: 9v on speed 1, 2, 3, but 0v on speed 0 - off and 4-Hi.

The relay between red & purple (in and out I believe) reads 9V in every fan setting.

Tested the the resistors I have and they seem to have continuity between all the connections.

still leaning resistor harness.