marshal135

Hello,
1968 coupe with air.
Lost low and medium fan speeds.
Hi blower works fan comes on.
Switch or ground?
Thank you,
Marshal

TimAT

Possible resistor failure. Oval shaped plate with a 4(?) wire connector on top of the heater box under the hood. UNplug and check for power. 2 small bolts hold the resistor into the case.

7T1vette

Problem could also be a fuse. The LO and MED speeds are fused at the fuse block; the HI speed is fused with an in-line 30 amp fuse in the engine compartment, around/under the brake master cylinder.

If the fuse isn't the problem, then it's probably a resistor block fault. It could also be a wiring fault or a fan switch fault; but those are much less likely.

marshal135

Thank you Gentlemen,
I will take the above advise and check it out and report back as it may help someone else in the future.
Marshal

0Willcox Corvette

Go to the resistor yellow and blue wires, they are input from the blower switch in the dash. If you have voltage coming out on these two terminals in 2 or 3 position, the dash switch is working fine... If you don't then the issue is your dash switch... If you do, then check the resistor to make sure it has an ohms reading from the yellow to black and blue to black. If it fails this, replace the resistor. If it has resistance then check the blower relay... it may be stuck in the high position.

If it turns that the relay is defective, pry the can off of it and file the points... most of the time this will bring it back to life... Then test it and re-install. If you look at the diagram below, the relay flips between high and lower speeds. Sometimes the points in the relay can become burnt and actually stick causing the issue. You might also check the terminals at the high blower relay... they are prone to corrode and cause this same issue... same with the terminals on the resistor.

-Willcox





68 had an oddball resistor too, it was the only one that had only two lower speeds instead of the standard three.

7T1vette

EXCELLENT response, Ernie!!

0Willcox Corvette

Thank you Charlie.

ksbunting

Had the same problem on my 68 with A/C and it was the resistor mounted under the hood at firewall.
Replaced it and no problems since
Kurt

KapsSA

Worked for me on both my 79s.
Couple swipes with sandpaper.

marshal135

Ok a follow up.
Monday was a beautiful day so I went out and started looking around.
I started with the fuse box as suggested by 71T1.
The 10 amp fuse in the heater location looks good.
I went under the hood and started wiggling wires.
The connection on top of the resistor on the evaporator box(AC) car
was pushed on to make sure its secure.
I placed the key on the accessory position so as not to burn my points.

I placed the fan switch on low and with the engine off you can barely hear the fan turning. Near nothing is felt coming out of the vents.
Position #2 same hear it running, little flow felt.
And of course the 3rd position hi I can feel adequate flow to my hand.

Is the fan ground wire in need of replacement with a smaller gauge wire?
The switch makes contact in all 3 positions, fan either spins or not so I feel that it is working. Resistor in evap box would not complete the circuit if it was burnt.

Any suggestions how I can pick up a little more flow out of low/medium speeds?
Flow is great on the AC hi speed position so I believe the vents are clear.
Thanks,
Marshal

0Willcox Corvette

The fan speed is controlled by resistance from the resistor so possibly there is an issue with the resistor. Monday I can ohm one out for you and see what each reading should be so you would have something to compare to... but the blower really didn't really push very good until you hit hi speed.

IHMO
Ernie

marshal135

Ernie,
Thank you for the diagram and detailed explanation.
The first two speeds as you said are very weak to almost non existent.
The hi speed works satisfactory.
I may have to improve the ground wire just to see if it helps.
I remember now, I replaced the resistor a few years ago when
I rebuilt the evap. box.
Maybe its just a weak after market resistor?
I remember smelling a burnt smell when I first installed it.
I thought it was just seasoning itself
Thank you,
Marshal

0Willcox Corvette

On the first use, the resistor will burn off manufacturing stuff... So when the resistor heats up it'll burn off... (kind of like your furnace when you fire it for the first time each year)...

Have a great weekend Marshal

E