My fan blower. Inst working on high. I checked the relay box underhood and it clicks on and off when i connected it making the blower work on high briefly. Already replace it with a neighboor relay and still no high fan.

Any suggestions?

 

if a known good relay does not fix the problem and all other blower speeds work ok then it’s likely the 12v call for high speed at the relay coil or the ground connection on the other side of the coil. Get a digital meter and when your in high speed setting and the blower is NOT working on high speed, pull the relay and test for 12v on relay pin sockets 85 and 86. One of those should have 12v. If neither one does then it’s the hvac control head or the wire from the controller to the relay socket. If 12v is found on 85 or 86 then put the meter in ohms on the low scale and read the resistance from the other socket slot to battery ground. You should get less than a few ohms. If not then it’s the ground.

George

 

I thought the high speed was on a different set up than the lower speeds. The lower speeds were controlled by transistors. The high speed only was controlled by a set of coils. If the high speed went out, it was a problem with those coils you see on the circuit board. You have a choice to live without 'high' speed or you buy a new blower circuit board. Going from memory.......Jim

 

I read the posts in the 'Related Topics' at the end of your initial post. The first one by Allen Gary from 2019 showed the card sitting on a dash and it has those coils I mentioned. In the posts it says the relay on the firewall under the hood has the control of the high speed. Try checking the ground on that relay. It has a history of causing problems with a bad ground because of its location. It frequently gets wet when you open the hood.

 

Yes the circuit is different for high speed vs all other speeds but the relay is involved in both. There are no transistors or coils involved except for the small internal relay energizing coil for switching the relay position. In high speed the hvac module outputs a low current 12v signal to the relay to energize it and connect the blower directly to a high power fused source on a thicker gauge wire (lower gauge number). In all other speeds the hvac module powers the blower directly and changes the circuit through a resistor bridge to vary the power to the motor. The output of that bridge goes once again to the relay and passes through the normally closed contact of that 5 pin relay. The contacts are a double pole single throw switch that is normal closed to the high power circuit. I gave the OP the easiest point to access to start diagnosis since he only has to pull a relay in the UHFB as apposed to start going into the dash or to start chasing down wires, connectors and splices. Attached is the circuit diagram for his model year, some other model years and/or man/auto use different circuits around the same basic principle.

Let me know if this needs further clarification and I will be glad to help.


George

 

I also own a 96 jimmy. Remove your fan wheel and take a very close look at the switch and terminals. If its lime mine it will be burnt at the contacts. My switch only works on 2 speeds while in 3 different locations on the dial. Hope this helps