I too was having the Heater/AC High Speed Fan issue… Until last night!!
I’ve read through every “heater fan” thread that I could find. For the most part all the issues that I read, naturally revolved around the Switch, the Resistor, and/or the Relay. I did pull the complete truck electrical schematics from various sources within this site and found that everything is right on the money, except for the Fan connection for A/C enabled trucks. The diagram posted by FLEET72 (from FIXIT-P) is close.. but mine didn’t have the extra Relay labeled “Anti-something”? FIXIT-P posted a
different diagram without this extra Relay… and it’s much more appropriate to my setup.. hopefully yours as well.
70GreenBurb – after reading through this thread again.... I really didn’t see that you got your answer. And this is the most recent post on the HIGH SPEED FAN ISSUE, so I’d thought I post a reply on how I fixed mine….
I have a 72 Blazer, with A/C. I rebuilt the switch and purchased a new Resistor and Relay… but was still not getting a High Speed Fan. I only had the low and medium… I did hear the Relay “clicking” during High Speed.. and got the same issue as you when grounded and not grounded.. no speed and slow speed. So something else was amiss… It was the thick 12 gauge orange wire… what is it supposed to do?.. no voltage, nor continuity when with relay was engaged and not engaged. After reviewing the schematic supplied by FIXIT-P it dawned on me… It is another full time power source from the Cigarette lighter circuit. The relay needs two power sources! So to check, I jumped the hot 16 gauge Orange wire (single connector - which is hot when in high speed mode) over to the 12 gauge orange wire… and walla, I had High Speed!! After that, it was just a matter of locating the CIG wire at the fuse box and verify power and continuity back to the relay. I’m good to go now! Hopefully this helps you as well!
