I have a 1996 GMC Jimmy ~129,000 miles 4 door 4 wheel drive 4.3 liter engine? My heater blower motor comes on intermittently as seems to really load the new alternator, engine RPM jump up. I have checked the fuse, it is ok. I noticed there are 3 Bosch blower motor relays on the passenger side firewall. I replaced 1 of them, (the one on the right side if you are sitting in the seat) did not work. Any ideas? Do I need to change all 3?
There are infinite possibilities as to why a heater blower motor might not work, blown fuse, blown blower motor relay, blown resistor in wire harness, blower motor is not properly grounded etc. I am not a mechanic, but was lead to a successful resolution. Here is my wisdom. The heater blower in my 1996 GMC Jimmy went from working fine one day to working intermittently overnight. Symptoms: Start up the vehicle and heater blower might engage or not. Hitting a bump in the road seemed to jump start heater blower on occasion. But it was really hit or miss. The search: I checked 20 AMP fuse, in the fuse panel located adjacent to the drivers door first. This was not the issue. I noticed that if I turned the key on with the heater fan on, not turning over the engine I could hear a clicking sound in the engine bay area. I investigated under the hood (passengers side) a bank of 3 Bosch relays ($30.00 and can be crossed to AC Delco PN) located on the firewall. The blower motor relay is located closest to the wheel well on the passenger s side. This relay regulates the fans operation; in my case was not the issue. While under the hood (passengers side) I next looked for a wire harness running to the heater box, a black non descript box with small opening for wire harness connection located directly above the wheel well (passengers side) in some instances the resistor in this harness may need to be replaced. As it turned out I was fortunate in as much as all that was required was to reseat the connection and the blower works like a champ! Best of luck