No fan speeds in my 1980 Chevy K20

The fan in my old pickup turns on with the key but runs very slow (maybe 20%). There is no further response from the fan when I attempt to adjust the speed at the dash. My first guess is that I have a bad blower motor resistor under the dash. Is there something else that I should check? If I am wrong what is the proper procedure for diagnosing my problem? I've got a voltmeter and I'm not afraid of taking the dash apart if need be. Thanks in advance for your help. Jerrad Voth from Lindsay, Texas

Isn't that blower resistor under the hood, screwed into the blower-housing? As I recall, that's where it is in my 79. I know it is in my 82.

Seems to me there is also an inline fuse in the wiring above the distributor.

There's also a relay under the hood on the side of the A/C box near the blower motor that can act up. Sometimes the connectors on that unit get dirty or corroded.

I have an 86 chevy and had the same thing happen. I had power to the blower motor so I ruled that out. Then i took off the blower motor off and hooked it up to my battery charger and when I cranked the amps up it would spin, but would not otherwise. So I bought a new blower motor for around $20 bucks and problem fixed.

JD, you getting snow out your way, or this wintery mix, sleet/snow mix?

Doggone-it,

That sounds just like a draggy blower motor.

I'd pull 'er out of there, yank the cage, drill a tiny lube hole next to both bushings, get some oil to it and see if you can get 'er to spin freely again.

Allan

All my GM trucks 70s to early 90s look like this. Open the hood and yank the blower motor and/or resistor in five minutes.

I've never had a resistor go bad in my Chevys, but have replaced many blower motors. NAPA usually has them in stock.

Now - my Subarus constantly burn up blower resistors.

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Wow that was quick! I should have mentioned that I replaced the blower motor this past week in my original post. It was a step in the right direction because the fan spins now and the old blower motor did not. I've had similar experience with an 02' Focus that used to do the same thing. It had a failing blower motor that cooked resistors every now and then. I appreciate everyone who has replied so far. JV

Next I would replace the switch. It might be "stuck" in slow speed

One more thing you might check. I have fixed many mid 80"s chevy cars with these same symptoms by eliminating a connector located in the engine compartment that runs from the controls to the blower motor. The connector is undersized and will eventually melt open. When this happens you"ll lose the fan speeds, starting with the high speed. Not sure if the trucks have the same connectors or not. The connector is only in there for ease of assembly at GM, and not needed after that, so I typically just solder the wires together and elimnate the connector.

K In Iowa

The blower resistor provides the low and intermediate speeds rather than the higher speeds. The relay provides (only) max speed.

If you have lost all speeds other than the lowest (always on) speed, AND they all disappeared at the same time, you most likely have connection problems and/or switch failure. Note: The max speed (relay) circuit has a separate inline resistor, which you should check, but it is not likely that it is open.

If you lost max speed first, as is most common, followed later by medium speed, both the max (relay) circuit and the blower resistor and/or connections have issues.

The blower motor pulls over 20 Amps on max speed. The spade terminal connections, though adequate when new, become higher resistance connections as the vehicle ages causing the connections to overheat, often melting the connector housing and fatigueing the female terminals, further increasing the resistance and resulting heat.

Once the max speed fails, most owners use second speed, which now overheats at the connection inside the partially melted terminal housing, and subsequently fails. As a result, it is not uncommon to find such vehicles having only the slowest speed functional.

For this reason, I NEVER use MAX blower speed, unless the vehicle is new and well within warranty.

Look for melted or deformed connector housings at the motor (most likely) and/or switch and relay. A reliable repair will REQUIRE NEW female terminals as well as a new connector housing.

An interesting aside: The always on, minimum speed was introduced by GM (at least) in 1968 when vent windows were removed and marketed as a feature of the "Astro Ventillation" system but the real reason was much different.

After alternators were introduced in the early 1960s, manufacturers learned that the energy in the magnetic field of a rapidly spinning alternator could and did damage damage automotive electronics (radio) and even lamp bulbs during a "load dump" condition, which results from a disconnection or intermittant high resistance connection at the battery during a time when the alternator is providing a heavy charge to the battery.

Neither the vibrating contact voltage regulators nor the early (and probably current) electronic voltage regulators could shut down the alternator and absorb the very high energy resulting from the rotating magnetic field in the alternator quickly enough to protect the electronics or even lamp bulbs.

In a former life as a design engineer for GM, I saw laboratory experiments at Delco Electronics in Kokomo, IN, in which load dump conditions destroyed all of the vehicle electronics as well as all of the lamps that were on at the time.

It was decided in the 1960s, to leave the blower connected to the vehicle bus at all times in order to provide an inductive load capable of absorbing PART of the load dump energy making it esier to control the rest.

Dean

In my 78,79,82, 83, 86, 88, and 91 Chevy trucks, the optional heavy-duty blower at high speed draws 9 amps. The standard light-duty blower at high speed draw 6 amps. I have no idea where you're getting 20 amps from.

All my old Chevy trucks run at high speed constantly in winter weather. Also at times in the summer with the AC on. It has never caused any problems other then normal wear over time.

Dean said just what I was about to say. Had the same exact problem in my 92 S-10 last week and all it took to solve the problem was clean up some blackened spade terminals at the switch. Jim

I've worked on many of them, JD, both in the lab and in the garage, and, yes, they were improved over time as the durability issues became well known.

We shall need to agree to disagree and you may have the last word.

Dean

It's not a matter of opinion - or somebody needing the "last word." Electrical devices have certain amp ratings. I'm not saying that there isn't something somewhere that draws the 20 amps you mention . . . but not in Chevy C/K/G or P trucks. They don't draw 1/2 of that.

I attached OEM GM specs for the blowers.

The max amp draw of those motors has been stuck in my head for a long time - because White used to sell a 6 row corn planter that used six of those same blowers (as used in the Chevy trucks). 9 amps per row, and we had a lot of issues with tractors having alternators capable of making enough charge current. 54 amps is too much lot for many tractors running at fairly slow speeds.

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Dont most tractors have a 120-120 amp alt on them? Or are you talking older tractors back in the day?

This was in the late 80s. At that time even brand new 80-100 horse Deere tractors only had 45-60 amp alternators - and that's at full RPM. Less at planting speed. We had the same problem with many other make tractors. Those White planters drew a lot of current at low engine speeds.

We put some big Leece-Neville 120 amp alternators on a few. On some others - we had customers use a deep-cycle battery strapped to the planter and they'd keep charging it.

A while back you were talking about 55 series deeres with MFWD having the wrong ratio in the front axle and burning off tires and such. Was this the 2955-3155 tractors or such ones as 4055,4255,4455? I know that they have nothing in common with the smaller 55s.

I think there's a 30 amp in line fuse holder under the hood on the right side firewall just for the hi speed blower I think. Does'nt take long to pull the resistor out and see if any of the coils are broken or the fan switch wiring plug could be burned.

All 20 series as I recall . . . but might be just because those are the ones we had in stock. Deere seemed to be trying to keep the whole mess quiet. I do know they blamed it on a factory mix-up in Germany.