Experience with aftermarket heater blower components?

[dyeager535]

I have the heavy duty heater option (RPO C42, four wire plug resistor, four position fan switch, vented motor) in my truck, and for the second time the fan switch is acting up.

The first switch melted, this one is intermittently turning on/off when in a fan position (worst in high, bad in med), so I expect it's about toast as well. Almost forgot, the fuse panel heater fuse position also melted in the past. To be fair all of it is old, used stuff, so that it lasted this long is probably good.

But...the high heater/AC setting is a consistent problem on GM products this vintage, I'm curious if anyone has had success with the aftermarket (or even GM) switches and resistors available today? I'd consider upgrading to a totally different/modern design, but I expect pretty much nothing will work with the squarebody heater head. I've seen it mentioned that the resistors should be replaced with the switch, although I'm not really sure why, unless they are broken. They don't generally just change resistance with age do they?

Interestingly, to me, it looks like A) GM completely botched the wiring diagram for the heater in 1989 based on the below image (the 1985 diagram appears correct in comparison), and B) it looks to me like you could "adjust" your low and/or medium speeds as there is one extra resistor available, just looking at the resistor diagram?



I'm now a bit baffled why GM even had a HD heater option. The only NECESSARY change was the fan motor and housing, yet they used a different resistor pack and connector, and had to have a separate wiring diagram. In 1989 they don't even list the HD heater option, so I'm guessing they just eliminated the standard heater.

 

[Blue85]

What's different about this setup is that there's no relay. The HI (no resistor in the circuit) connection is made in the dash switch. Maybe that's why they wear out?

Yeah, there's no way that diagram is right. They have another wrong version for 1990, but this one may have all of the circuit numbers correct. You would just have to imagine the connector is really a 3 position. Do you have a picture of the resistor connector to verify?



For the resistance to change, you'd have to get the wire really hot or physically damage it (so it's cut or some sections are bypassed with a short). Probably unlikely. I would be more worried about corrosion on the terminals or relaxed connections.

 

[dyeager535]

Yep, I think the lack of relay is a problem. Maybe not the sole problem, but a problem. This is the first time I've had issue with the medium position though. Visibly the wiring has always looked ok, but that isn't necessarily an indicator of what the wire looks like under the jacket. I may need to spend some time on the connectors and make sure the contacts are all good, and smear them with dielectric grease to try and keep them as clean as possible. As I have no dash to get in the way, wiggling the back of the switch usually "fixes" the medium speed for awhile, but high almost never works.

The way the switch connects I doubt the problem is the physical plug/switch interface, I expect the terminals have loosened up (and/or moved) on the switch from heat as I saw when the first switch failed. Could be something else, but it started with only high being intermittent, then essentially failing completely, then medium starting to do the same thing, so it seems to be progressing, not just random.

In terms of knowing what the connector looks like, I know it's the four wire plug and resistor setup, but as to which wire is where, no. I'm not home again, so it will be some time before I'll actually be able to work on it, but it will be a priority when I can as I typically use the truck only in winter.

Very un-scientific, but there is a pretty big step in fan speed from Medium to High, which gives me indication my resistor connecter is wired like what your diagram shows.

Since the fuse panel cavity failed I've got it running off the auxiliary fuse panel I added, which is close enough to the motor that a relay will be easy to run.

 

[Blue85]

You could put in a couple of relays to cover medium and high speed. Then the switch would last a really long time. You could also use a double-throw type for one of them and cover all 3 speeds. Using a standard style relay lets you replace them whenever needed. Or you could use SSRs.

Another fun project would be to pick up a PWM controller and wire 3 low-power resistors (or potentiometers) to it from the dash switch. You get to fine tune how fast you want each one to go. How about some electric actuators and digital climate control...

 

[imiceman44]

You could put in a couple of relays to cover medium and high speed. Then the switch would last a really long time. You could also use a double-throw type for one of them and cover all 3 speeds. Using a standard style relay lets you replace them whenever needed. Or you could use SSRs.

Another fun project would be to pick up a PWM controller and wire 3 low-power resistors (or potentiometers) to it from the dash switch. You get to fine tune how fast you want each one to go. How about some electric actuators and digital climate control...

I second the relays.
I will be doing it on my 84 as I am having trouble with mine too and already blew a couple of fuses

 

[dyeager535]

Another fun project would be to pick up a PWM controller and wire 3 low-power resistors (or potentiometers) to it from the dash switch. You get to fine tune how fast you want each one to go. How about some electric actuators and digital climate control...

I'm seriously considering this, but I haven't done the legwork to figure out what a PWM that will handle that load would cost. Heater is fused at 25A, so this one is GTG, and still cheaper than a new switch assembly: https://a.co/d/6mokAxu

I used PWM to control my dash lights, but I replaced the potentiometer with the resistor necessary to get the lighting I wanted, so fixed brightness.

I haven't worked through it in my head fully, but either the idea as you suggest of using three resistors controlled by the heater switch, or maybe replacing the pivot for the fan speed lever with the potentiometer so fan speed adjustment is infinite.

I'm not interested in going crazy with climate control, I'm just replacing things that are broken. Anything additional is just more I need to work to keep running later on. Projects like this don't really seem like creating future work to me...I don't see the switches as a good design, and being smarter about the heavier loaded circuits makes sense. Literally one 12V wire to the motor from the heater control head, instead of the 3/4 now.

Relays might be easier but I feel like my truck may end up looking like a relay graveyard. There are no less than 6 underhood already (two fans, fuel pump, MAF, auxiliary fuse panel), it pains me to imagine hanging two more, and all that wiring underhood. To be fair if I was smart with my replacement auxiliary fuse panel, it could be pretty clean and minimal runs of additional wire, but not sure when I'll get around to that.

 

[Blue85]

That's why I was thinking of a cheap PWM controller like that. It's less parts overall. You don't know is how reliable it is, but it's also so cheap you could carry a spare. The factory setup or a pair of relays probably gives you the option to hard-wire some reasonable speed in case of a failure (i.e. not max all day). Underhood temps and moisture are questionable, but for the simplified wiring you could go to, placing the controller inside the cab would be as easy as near the blower.

There is such a thing as a linear slide pot with a switch, but I don't think you'd benefit from it if you went that direction. For one thing, part of the goal was to get the high current out of the dash wiring. To replace the controller pot, you'll need a pull-down resistor anyway, to divide the input voltage with your "low" and "medium" resistors (I'm assuming "high" will be wired straight through for max speed). That will drop the control voltage to 0 when you turn the switch off, since "off" is open circuit.

Another question is what the minimum PWM% is on that controller. It's probably pretty low, as a DC PM motor is fairly linear all the way down to zero, but it's something to consider. It's possible you would always get a little airflow, but that's how the older factory setups worked anyway. Actual on/off of the controller is going to be from switched ignition.

 

[Blue85]

I built one of these once for one of my 3800 cars. The resistor blocks failed a lot in some years, so when I lost a couple of speeds, I built a PWM controller out of parts I scrounged up. There was a little pot on the board for each of the speeds. I kept pulling it out to mess with it or repair it and it was in an awkward position under the dash. Then they released a new design resistor block and it was cheap so I abandoned that project.

 

[dyeager535]

That was a question for me, does the potentiometer have an "off" spot? I could go either way (fixed resistor values for each speed or replace the switch with the potentiometer), and a tiny bit of airflow really isn't a concern. Replacing the switch seems a better way to go, reducing complexity, but I'd have to have a switch in front of me to see if it could be modified to work the way it would need to.

Moving the high current wiring does have to be considered, but extending the potentiometer harness wouldn't be tough at all. I suppose if the controller was mounted somewhere behind the glovebox, the wire run to the motor would be very short, and the batt+ at the auxiliary fuse panel is right there on the firewall as well so maybe 3 feet either way or so.

And maybe I've put some numbers in wrong, but it sure looks like the wiring is undersized (stock) or just on the cusp of being too small, for the heaters. If it was working properly I'd check the voltage at the motor and the switch. That's a pretty long run for a theoretical 25A max on I would guess 12AWG wire.

 

[kennyw]

That was a question for me, does the potentiometer have an "off" spot? I could go either way (fixed resistor values for each speed or replace the switch with the potentiometer), and a tiny bit of airflow really isn't a concern. Replacing the switch seems a better way to go, reducing complexity, but I'd have to have a switch in front of me to see if it could be modified to work the way it would need to.

Moving the high current wiring does have to be considered, but extending the potentiometer harness wouldn't be tough at all. I suppose if the controller was mounted somewhere behind the glovebox, the wire run to the motor would be very short, and the batt+ at the auxiliary fuse panel is right there on the firewall as well so maybe 3 feet either way or so.

And maybe I've put some numbers in wrong, but it sure looks like the wiring is undersized (stock) or just on the cusp of being too small, for the heaters. If it was working properly I'd check the voltage at the motor and the switch. That's a pretty long run for a theoretical 25A max on I would guess 12AWG wire.

The motor and resistor in factory form should only draw 80% of the fuse rating if things are in good shape. They would not size the fuse at max draw to start off.

 

[imiceman44]

The motor and resistor in factory form should only draw 80% of the fuse rating if things are in good shape. They would not size the fuse at max draw to start off.

I guess when these motors got older, bearings possibly getting dry adding resistance and raising the amp draw

 

[kennyw]

Also, the wiring diagrams show the factory wire is 2.0 mm which is equal to 12 awg. That is good for 14V source at 15A continuous, max 3% village drop, at 8 feet run according to one online calculator. If you increase the draw to 20A, the wire size is recommended to be 10 awg, if you increase the length to 10 feet, it bumps up to 8 awg.

 

[dyeager535]

The motor and resistor in factory form should only draw 80% of the fuse rating if things are in good shape. They would not size the fuse at max draw to start off.

True, and if that's what the safety margin is, I need to plan for that as well.

GM absolutely cut it too close on the cars, they constantly melted the connector, so they were definitely not trying to oversize the wiring.

I need to check what this one pulls at max. There could certainly be some additional resistance through the connectors due to age. I don't think I've pulled them out of the resistor connector or the switch and cleaned them, but with this many years, it's not going to hurt to ensure they are all clean.

Of course the motor itself has been around for awhile, and is IIRC a parts store cheapy, but I'll find out what it draws with the ammeter.