Blower motor wierdness

[dhcomp]

Ok, so i've searched blower motor threads a bunch, but am still confused.

91 V2500 burb, A/C.

Blower motor works fine on all speeds about 1/2 the time, but after an hour or so, the speed drops dramatically. I think the different settings still vary the speed a bit, but it BARELY blows on high.

If i shut it off for an hour or so, it will come back on full speed if i'm lucky for a while.

So, i'm thinking its resistor related. Where the hell is the resistor on this thing? I've never found it.

Think its the resistor, or the motor? Motor is only a couple years old, but it was a cheap parts store one.

What do you guys think? Might as well fix it now that its getting cooler and i don't need it, haha.

 

[xenomarine]

The resistor is in the box itself. Do you see a plug, plugged into a plastic or metal plate screwed to the box? That be the resistor, attached to the other side of that plate.

 

[Fordum]

Don't know for sure. Some trucks have the resistor outside under the hood attached to the air box.
Remember it must be in the air stream when it is being used. It will melt and burn out quickly if not.

Its a fiberglass plate with come coils of wire on it. Here is RockAuto's pic. Pay attention to the plate part with the two screw holes and plug.
That is the part you will see mounted to the air box.

It sounds like a main power connection is getting hot and increasing resistance. It could be the spade connector on the wire, or where the resistors are crimped or welded on.

But, on most of those fans, the resistor is completely bypassed in high. Not going to say for sure on yours, because I don't have the schematic in front of me.

If so, its the fan motor or the connection to it. If its not the resistor, I would examine the plug to the motor real well for signs of heating before buying a new motor.

It might not hurt to check the plug on the backside of the fan switch either......

As a final check, if it is doing it often enough, you might run a hot lead from a fused source that you can hook to the fan motor directly when it quits to see if that fixes it.

 

[dhcomp]

In what box? The heater box under the dash? Or in the firewall box under the hood?

Fordum, where's that item located, shown in the pic?

I am seriously clueless on this one....

 

[tRustyK5]

Top of the box in the engine compartment.

 

[Fordum]

Top of the box in the engine compartment.

^^^^^^^^^^^^

I have worked on too many different makes and models to be sure about one particular one without looking.

About the main thing they all have in common is the fiberglass plate and the fact that the resistors stick into the air stream.

Just look on the air box for the Molex plug with the wires coming out stuck right on the side of the box.

Should be fairly obvious.

 

[diesel4me]

Most trucks with A/C also have a high speed blower relay ,usually located on the firewall side near the blower motor housing...that might affect the high speed settings ,but shouldn't affect the medium or low speeds,the resistor controls those...when that relay fails you'll lose high speed usually,but I suppose one could get burnt contacts and reduce the amps delivered to the blower and slow it down after awhile when it heats up...

 

[4xcrazy]

I could have swore, high speed was full power to the blower motor, as mentioned, maybe the relay, I don't think the resistors are the problem, pretty sure they only control the lower speeds.

Could possibly be a bad motor, but they usually make noise when they are dying out. Or maybe the motor IS going bad, and overheating the relay.

 

[dhcomp]

Well, now i know where the components are. Relay is on the firewall? Guess i'll have to check some connections, then systematically start replacing resistor, relay and motor.

Truthfully, after 250k, relay and resistor should be replaced anyways. Not mission critical items, but 5+ hr tow's with no air suck when it acts up.

Guess i'll get the components on order.

Thanks guys.

 

[Fordum]

Relays can wear out, but resistors are not normally subject to wear unless overloaded or run outside the air flow.
Corrosion or damage due to vibration might cause one to go bad, but its fairly unusual.

Either way, I would pull the resistor and look at it.

Rockauto shows several different ones, and they are obvious. So look at yours before you order.

If I can find my schematics, I will check to see how the high speed works. If the resistor is bypassed like I think, then its either the switch, the relay, or the motor.

 

[byggyns]

My problem was a little different. My blower would stay running on high with the switch in any position and even when the truck was off. The blower relay was stuck on.

If it's not the resistor that Fordum mentioned, I'd look at the blower switch itself. Old plastic, degraded connectors, etc could cause an intermittent issue.

 

[Fordum]

OK Darren, I dug up my schematic. Its a factory 1990 C/K pickup wiring manual. Yours might be different.
It shows two different systems.
One, is for a heater only. Its a simple switch/resistor setup. No relay, nothing fancy.

The other one is for an a AC system, RPO C60.

It is a little more complicated.
Its got two relays. With misleading names. The fan switch is part of the air conditioner module.
There is a low speed relay, and a high speed relay. Plus the resistor pack.

The two relays have a hot wire and are grounded to turn them on.

In off, both relays are off, and no power goes to the fan.
In low, both relays are on, and the power to the fan goes through both resistors.

In medium, the high speed relay is turned off, and the normally closed contacts short across one of the resistors leaving one one in the circuit.

In high, the low speed relay is turned off, and the high speed relay is turned on. The normally closed contacts of the low speed relay bypass the resistor pack entirely and puts full power to the fan motor.

A little complicated, but it takes the load off the controller and puts it on the fan relays.

BTW, I also have a component location diagram. All the relevant parts are on top of the plenum just to the left of the fan motor. You have three relays mounted on a bracket.
According to my pic, they are, from left to right: Low blower relay, A/C clutch relay, and High blower relay.
The bracket looks like it mounts on the top front of the plenum and the resistor pack is behind it on top sticking straight down into the air stream.

The whole works is behind the instrument panel next to the fan motor.

Of course, if yours does not have all that, then never mind..........

This does throw a new monkey into the mix. Since the relays are controlled by the electronic controller and not the switch, then it could causing the problem.
If your system is like I have said, then there is a way to check.

You could pull the relay pack off and let it hang loose.

Set the fan to high, and when it suddenly slows down, cut the light blue wire going to the low speed blower relay.
Leave enough to splice it back.

If the controller is commanding low speed, then both relays are turned on. Cutting the light blue wire, will kill the low speed relay, leaving the high speed relay on and the fan should go to full speed.

Remember the controller grounds the wires to turn on the relays, so testing with a meter is a little different.

 

[dhcomp]

OK Darren, I dug up my schematic. Its a factory 1990 C/K pickup wiring manual. Yours might be different.
It shows two different systems.
One, is for a heater only. Its a simple switch/resistor setup. No relay, nothing fancy.

The other one is for an a AC system, RPO C60.

It is a little more complicated.
Its got two relays. With misleading names. The fan switch is part of the air conditioner module.
There is a low speed relay, and a high speed relay. Plus the resistor pack.

The two relays have a hot wire and are grounded to turn them on.

In off, both relays are off, and no power goes to the fan.
In low, both relays are on, and the power to the fan goes through both resistors.

In medium, the high speed relay is turned off, and the normally closed contacts short across one of the resistors leaving one one in the circuit.

In high, the low speed relay is turned off, and the high speed relay is turned on. The normally closed contacts of the low speed relay bypass the resistor pack entirely and puts full power to the fan motor.

A little complicated, but it takes the load off the controller and puts it on the fan relays.

BTW, I also have a component location diagram. All the relevant parts are on top of the plenum just to the left of the fan motor. You have three relays mounted on a bracket.
According to my pic, they are, from left to right: Low blower relay, A/C clutch relay, and High blower relay.
The bracket looks like it mounts on the top front of the plenum and the resistor pack is behind it on top sticking straight down into the air stream.

The whole works is behind the instrument panel next to the fan motor.

Of course, if yours does not have all that, then never mind..........

This does throw a new monkey into the mix. Since the relays are controlled by the electronic controller and not the switch, then it could causing the problem.
If your system is like I have said, then there is a way to check.

You could pull the relay pack off and let it hang loose.

Set the fan to high, and when it suddenly slows down, cut the light blue wire going to the low speed blower relay.
Leave enough to splice it back.

If the controller is commanding low speed, then both relays are turned on. Cutting the light blue wire, will kill the low speed relay, leaving the high speed relay on and the fan should go to full speed.

Remember the controller grounds the wires to turn on the relays, so testing with a meter is a little different.

Thanks for the info man! I will let you know what i find out when i can get into it!

 

[Fordum]

Thanks for the info man! I will let you know what i find out when i can get into it!

Not a problem. I can dig out a scanner and post pics of the schematic or the locations if need be.

 

[dyeager535]

One of the problems GM had on the cars was that the high speed (straight 12V) side of the fan drew too much for the wiring connector used, and melted it.

I cannot remember on the AC trucks if there is a connector anywhere in that circuit like the cars had, or if it's only on the back of the heater head switch. Hopefully someone can answer that, because if there is, that would be a potential cause for the problem being mainly on "high" setting. Always a good idea to examine the wiring and any connectors first in any case.

Never had a resistor problem, but my impression of their failure mode, based on their construction (see Fordum's picture) is that the coils would have to physically break/melt in half to cause problems with fan speed. And once they break, the fan speed for that setting would be dead.

If there is no additional connector in the circuit, I would lean heavily towards the fan motor being the problem myself. As mentioned, since 12V does not use a resistor, and you state the problem occurs even on high, then if it's not wiring/connector issues, IMO it most certainly is the motor.