My 71 Chevelle, 2 Dr HT 350/350 has a quirk. Whenever I turn off the ignition, the blower motor runs for a few seconds then shuts off. What is my problem?

Air conditioning car or non-air conditioning car? If an A/C car, does it only happen in high blower speed or in all speeds?

A/C car. Blower comes on in all speeds, even when heater and A.C are shut off.

A few more questions to help diagnose the problem:

How long has this behavior occurred? Is it a recent change or a long-term problem? Have you messed with any wiring or fuses lately?

Does the fan speed switch properly control the fan when the ignition is on?

Been a problem for a long time.
Have not messed with any wires.
The fan only works on Hi.

You say the fan works only on HI setting. The speed control switch or the wiring feeding it might be faulty. Have you tried to disconnect the switch? When on HI setting, current bypasses the blower resistors and feeds straight to the fan on a 14g orange wire.

Weird electrical behaviors like this are sometimes caused by bad grounds. Check the obvious grounding locations, such as the fender ground screw by the battery, and the headlight switch ground.

Please post any results. Good luck!

Thanks for the infor. Too cold today to troubleshoot...will print and file your info.

On an A/C car the orange wire isn't used. Fan is run by a purple wire that comes off the hi-speed relay under the hood. Without looking it up I think it's a blue wire that operates the relay.
Next time, after the fan shuts off, leave the switch in one of the lower settings (not HI).
Next shut the car off.
Does the fan continue to run?
Do you have to put it in HI to make the fan come on?
Guessing you might have 2 problem. One problem seems to be the contacts of the hi-speed relay are sticking causing the fan to continue running after you shut off the car.
As mentioned above, you also may have another problem with the resistors or wiring causing the fan not to run or start in the lower speeds.

John, you got me thinking. If the relay is "sticking", I bet there is some residual voltage at the relay's (blue?) 18 gauge wire terminal. It's the wire from the fan switch that powers the relay coil. ebmcmurray, you might want to test for voltage at that terminal when you turn off the ignition switch. Let us know, this is interesting.

Larry

I wasn't clear enough in my first post. The heater/ac controls are in the OFF position. The blower s/w is in the HI position.The fan comes ON and runs for a few seconds after turning off the key. It was not on/running prior to that. When the blower is in use, it works only on the HI position. Any other position and the blower motor doesn't operate.

I will still take to heart what you have posted and run a diagnosis. You both have given enough ammo to start a search for the problem. Knowing a little about electronics I figured that something had t.b. getting power to the motor in order for the blower to run. After reading the responses, I tend to believe that this may be a combination of problems.

First I have to find out why the blower works only in the Hi position. I wish I had a schematic to run some tests with. You don't perhaps know the voltage/resistence of each s/w position ??

If I remember right the Hi speed is controlled by a separate circuit than that for the lower speeds. Perhaps a fuse is blown that feeds the lower speeds. (I didn't think of that till just now.) I guess the best I can do is a voltage and resistance check on each s/w position.

This is not a problem that has caused any other problems (at least none that I can point to)..it is just a bit un-nerving to have the blower work when the ingnition s/w is turned off.

Doesn't sound like the lower speed fuse is blown. That fuse also provides power to run the hi-speed relay coil. The 71 can be a little difficult to work on because it uses 2 other relays below the glove box.

I'm wondering if the A/C harness has been modified sometime in it's life and now the high speed relay is wired to the compressor clutch circuit, If so the anti dieseling relay, that John mentioned, might be what runs the fan when the ignition is switched off. The two relays, anti dieseling and blocking, allow normal compressor operation when the ignition is on and applies power to the compressor clutch, for a second or two, after the ignition switch is turned off.

Your post caused me to think of another point. While the car is A/C, the previous owner removed the compressor and evaporator. Could this have something to do with the circuitry?

I have not checked to see if all he did was remove stuff w/o cutting wires.

If this thread stays active long enough, I may remember all the things about this car and maybe we can come up with a solution.

Thanks for all of you guy's input.

I used a multimeter to check the blower resistors on my '70 Chevelle with AC and found these results:
Terminal 1 to 2: 0.7 Ohms (one resistor)
Terminal 1 to 3: 1.2 Ohms (two resistors 1-2, 2-3 in series)
Terminal 1 to 4: 1.7 Ohms (all three resistors 1-2, 2-3, 3-4 in series)

The resistance at the blower switch in the dash should be a bit higher than these values.