https://blazerforum.com/forum/2nd-generation-s-series-1995-2005-tech-41/1997-blazer-fan-doesnt-work-high-78173/

So, a years ago, I posted that I had a problem, got great advice on solving it, and ...didn't do anything. I've decided to reclaim my truck, and I'm back to trying to solve this issue.

I've got a '97 Blazer with a fan motor that won't blow on high. I replaced the blower motor resistor because I didn't read carefully through everyone's recommendation that it fixed issues with the lower 3 speeds (Did I get that right?)

If you'd rather not click through to the link above, I was told that I need to turn the ignition on and switch the mode selector to Heater, then measure the voltage at the Blower Motor Relay on the Orange cable and the Red cable (separately). They should both show battery voltage.

I also determined that the Blower Motor Relay is the middle cube/relay on my firewall. (https://blazerforum.com/forum/2nd-ge...lay-pic-24523/)

How exactly do I measure that voltage?

 

The resistor and the high blower relay are both used for all 4 speeds. The resistor is bypassed for high speed, and only the relay is used. Generally, if speeds 1, 2, and 3 are inoperative, the resistor is faulty. If all low speeds work and high speed is inoperative, the relay is faulty. Circuits need to be checked to be sure.

Here's how to check the high speed circuits:

1. Unplug the blower relay. Measurements are taken at the high blower relay socket.

2. The red wire must show battery voltage at all times. On your 97, it's a fusible link and it comes from the underhood junction block, near the power brake booster. The sole purpose of this wire is to supply voltage to the relay for high speed only.

3. With the ignition in the RUN position, and the blower speed set to HIGH, the orange wire must show battery voltage with the mode switch in each position except OFF. This wire comes from the blower switch on the control panel and triggers the high blower relay.

4. The black wire must show less than 5 ohms resistance to ground at all times. The sole purpose of this wire is to ground the relay so it will latch when you select high blower speed.

When the relay latches, the red wire and the purple wire are connected inside the relay, and the blower operates on high speed.

 

This was the piece I was missing. Also, thanks for pegging the diagnosis. If I'd had sense enough to realize I'd have to unplug the blower relay to test these voltages, I wouldn't have had to ask this question.

Also, upon unplugging the blower relay (which I determined to be the most-passenger-side relay on my '97 4x4 LT), I opted not to bother troubleshooting.

One of the pins was burnt. A new relay and I've finally got a fan that'll work on all 4 speeds. I just need a can of refrigerant now!

Again, thanks for all the help, including the help from over a year ago!

 

No problem, glad you got her figured out.