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Discussion in 'Troubleshooting & Diagnosis' started by CorkyE, Dec 6, 2018 at 8:09 AM.
On my 79, the blower motor doesn't work on high speed. Relay?
It's possible it's the primary resistor but I've rarely seen those go bad. I'd check the connection of the feed wire from the engine harness (10 gauge red wire in the lower LH of this pic from the archives) - connection #2. It's usually in the vicinity of the distributor along the firewall on the engine side. I've repaired MANY of these connections across several GM makes from the 70's.
I remember a fusible link on older GM's that was responsible for that. Maybe near the intake on passenger side. Does it have air? If so, the coil for the high speed breaks on the board the power wire harness plugs into.
If it works on the other speeds... It's most likely a relay problem because the relay bypasses the resistor pack to control the lower speeds.
Or (as stated above) it could just be a bad connection.
Check for power at the 10 ga red wire and the condition of the connection at both ends before you replace the relay.
I replaced one a few years ago and I'm pretty sure it was actually a poor connection at the relay plug.
Beyond that... There's the switch itself and the connections for the smaller dark blue wire at the relay. (968 on the diagram.)
Thanks Terry and xten, time to get the multi meter out and start checking. I installed two, four post power strips on the fire wall, one for battery power, one for ignition. I know I have power headed to it.
The plug in the 10 gauge wire isn't quite up to handling the current of the fan. I've seen that plug burned up and cause this problem.
I just ordered a new AC Delco relay. Wouldn't surprise me if the box is stamped Hecho en Chinesio.
It was the relay. Popped cover off old one, relay is pulling in but contacts are bad, power in but not out, and it was getting warm. It's lived it's life, so into the round file it goes.