I have a 1972 Cutlass S.
Fan operated on all three low speeds, I was only missing the high speed so I replaced the 30 amp fuse in the high-speed relay line behind the distributor.
Worked fine in my driveway after I did the repair on all speeds..... Went for a drive and I lost all four blower speeds.
All the fuses in the junction block are working, did a quick test light on them, could it be just that 30 amp fuse again?
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If the 30A fuse blew, only the high speed would be disabled and the 3 low speeds would still work. There likely was a reason the 30A fuse was blown, such as the motor may have been drawing too much current, and now the motor itself may have failed.

 

Any chance it could be the resistor?

 

The resistor assembly has circuits for each speed. If one resistor should fail, the other speeds would still work.

 

Test for power to the motor and confirm that it has a good ground connection. As previously stated the 30A fuse would only stop the HI speed.

Good luck!!!

 

Jumped the blower motor to the battery, it ran fine.
At this point I figure I'll check all the fuses, make sure they're right and then I have to move on to the resistor

 

If nothing works, I’d look at the relay.

 

Check the connector at the back of the fan speed switch for burned/melted connections, the heat often burns the switch and the connection. My recall is that the relay is for high speed only.

Good luck!!!

 

Thanks all, appreciate the input!

 

The connector at the high speed relay itself carries a lot of current and often melts the insulating plastic.

 

the high speed may be a connection. It is on a FBody. The red wire connection is always the problem.

 

Thanks all for the input...big help.
​Ended up disconnecting harness from relay, plugged it right back in and the blower started working, so I figured I would clean up the relay and the plugs.....anther thing I did was clean up where the relay attaches to the firewall..... I guess I should have known that that is where the relay gets its ground.... Working fine now and I do appreciate the help!

 

I'm having the same high blower blown fuse problem on my 72 Cutlass Supreme with:
New blower relay
New Blower motor
New 12 ga wire with aftermarket 25 amp fuse holder from distribution block to blower relay ( I tried a 35 amp fuse and it melted the fuse holder)
All wire terminals are clean and tight
Lower speeds work fine via the resister at the evaporator housing.

I'm aware that the blower relay gets its ground through the mount to the firewall but that is only to allow the relay to trigger or toggle from the high blower circuit to the lower speed circuit. Once the relay is triggered to the high blower circuit then the grounding is complete and should not impede the high blower circuit once its connected.
The blower motor is grounded to the firewall at the motor housing...right?????

So...what to do ????

 

The lower blower speeds work--this means the blower and its ground are fine.

Double check the cleanliness and tightness of the relay connections.

Take out your test light or meter.

Check at the blower relay to see that it has 12 V supply (through the fuse).
Switch the blower to high and check at the relay to see if 12 V signal is coming in from the switch.
Check at the relay to be sure the signal ground is OK.
Check the output of the relay that goes to the fan.

That should point to the problem.

 

Replaced the fuse holder with a new one .. Why? Because the #12 wires on the fuse holder were getting much hotter than the black/orange OEM wire to the blower relay. The purple wire from the relay to the blower remained cool

I concluded that there was too much resistance in the aftermarket fuse holder and #12 wires. Upgraded to a fuse holder with #10 wire and it's not heating up as much as the previous one.

We'll see.

 

A blower motor that is going bad may still operate but will draw too much current (amperage). The correct way to diagnose this is to use an ammeter to check the actual current draw. Or you can roll the dice and replace the blower motor. Increasing wire size may mask the problem and cause the wiring to over heat in a different area. This can lead to a damaged wire harness or even a fire.

 

On mine that wasn't working at high speed, I wasn't able to find the high-speed fuse at the relay for a while, but what the help I got here I was able to find a tucked behind the distributor, hidden really well ... Pulled the 30 amp fuse out in pieces, cleaned up the connections as well as I could, replaced the fuse and it's been working now for a few weeks.
AC season is here in PA so I'm grateful for that and I appreciate the help!

 

You are correct. That is why I have undersized the fuse to 20 amps following the upgraded wiring. And I am testing the wire temps along their route to the relay and blower for sequentially longer time intervals.

 

You can use a digital voltmeter to test various points of the circuit to determine exactly where abs how much voltage you are losing in the circuit.

Positive probe on the terminal at the blower motor. With the blower on high, bsckprobe blower motor power wire at the relay (both sides) the fuse holder on both side of the fuse, etc. Whatever the meter is reading is the loss of voltage at that point. I bet most of it will be in the fuse holder. If it’s a glass fuse type holder I just don’t think the springs provide nearly enough tension to support the current.

Id suggest a ATO type fuse. You can either cut the connector and wire from a mid 90s GM FWD cooling fan motor, or buy the terminals and make your own. I have repaired lots of blower motor wiring this way, it works much better.

 

If you don’t want to go the junkyard route, here is the ATO fuse and weather proof cover on Amazon for about 10 bucks.