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Mon Dec 15, 2008 6:38 pm

My 85 Weeekender has always (since I owned it) had one speed of heater fan - High---

I took out the heater today, figured I'd take a week and fix some electrical issues, including the heater rheostat replacement. Also finish the new 'Fear No Deer' steel bumpers (photos when finished next week!)

The usual suppliers have no source for the VW resistor (rheostat)

VW used the same switch and blower motor in Jetts/Golfs - any reason to not use the Jetta resistor and fid a suitable mount for it?

Thanks
Bates

Mon Dec 15, 2008 7:05 pm

You don't need a rheostat, just a resister. One from another VW product or many other different makes should work is the resistance is similar.

Mon Dec 15, 2008 8:24 pm

A rheostat is like a variable resistor--a knob that produces greater or lesser resistance. The dash knob for the blower motor is not a rheostat: it's just a switch. If I remember correctly, it sends 12v either directly to the motor (this is 'high'), or to one of two routes through the big resistor, one of which lowers voltage more than the other (the resistor functions as two resistors in one part--one has a higher ohm rating than the other).

Mon Dec 15, 2008 8:27 pm

Is the resistor pooched?

My understanding is that it's usually the switch that fails first. And FWIW.......

Not suggesting it's the right way to do it, but when I rebuilt the air exchanger/replaced fan etc., since I was adapting the wires to the new motor, I extended the wires and mounted the fan resistor "outside" up by the rad (you can see the 3 wires going up at the RH end of the fresh air ram) for me, I figured it was a whole lot easier to get it if it fails.

Someone elses idea, but makes sense to me. I assume the resistor gets hot at times, so I was careful where I mounted it.

Neil.

Tue Dec 16, 2008 5:04 am

Next time fan resistor goes out mine will be changed to the variable reostat type ,u get any speed u need and is off an older Plymouth car and flaps may carry simuliar ones.

Tue Dec 16, 2008 5:05 am

Next time fan resistor goes out mine will be changed to the variable reostat type ,u get any speed u need and is off an older Plymouth car and flaps may carry simuliar ones.

Mon May 04, 2009 9:02 am

Any source for new resistor?

None at VW! GoWesty? Bus depot?

Thanks, Ben

Mon May 04, 2009 10:24 am

If resistor not available, could a Jetta/Golf resistor be used?

I understand they use a similar/same fan.

Neil.

Mon May 04, 2009 2:05 pm

You know, a while back this resistor discussion came up and others had pointed out the Jetta resistor to be the same values, it may not have the same form in mounting. I cannot remember (that neuron was sacrificed for some other bit of trivia) what the issue was, but it had been successfully used as a replacement. The switches are cheap and frequently the issue with the fan.

A rheostat would not really be a good idea for a couple of reasons. First it would be really expensive needing to handle 12-18 amps load. This means it needs to be able to shed a lot of heat, the reason the original resistor is on a ceramic substrate. The rheostat would get quite warm itself, kind of an alternate heating source on your dash. It also may not fit the dash, needing to be a large wire wound affair.

Mon May 04, 2009 2:22 pm

Dogpilot wrote: You know, a while back this resistor discussion came up and others had pointed out the Jetta resistor to be the same values, it may not have the same form in mounting. I cannot remember (that neuron was sacrificed for some other bit of trivia) what the issue was, but it had been successfully used as a replacement. The switches are cheap and frequently the issue with the fan.

A rheostat would not really be a good idea for a couple of reasons. First it would be really expensive needing to handle 12-18 amps load. This means it needs to be able to shed a lot of heat, the reason the original resistor is on a ceramic substrate. The rheostat would get quite warm itself, kind of an alternate heating source on your dash. It also may not fit the dash, needing to be a large wire wound affair.

Obviously my suggestion was already covered. :)

FWIW, here's a pic of a FLAPS switch I bench tested as a replacement for fan switch during my heat box/dash area rebuild fun times. It has a coil type arrangement. Pic is fuzzy, but shows how hot the coil got. Sorry. Can't recall if it was a variable pot, or had switch positions. Pretty sure it had switch positions.

Neil.

Mon May 04, 2009 4:24 pm

If you want variable speed, then what you want is some form of high amperage light dimmer that will work on 12VDC. Way less heat being generated than using either the stock resister or a rheostat.

Mon May 04, 2009 4:55 pm

Wildthings is on the correct path. You need a circuit that switches off rapidly varying the duration of the pulse to the motor. These run a lot cooler than the resistor brute force technique. These circuits also use a lot less power. Nickels mean so much to manufacturers so spending even a tiny amount more on parts means big losses for them. So until the cost of a switching circuit is cheaper than a switch and mondo resistor, they will continue to use the Flintstone technique of a switch and resistor.

Mon May 04, 2009 4:55 pm

Right, to do variable speed without the expense, bulk, and heating with consequent early demise of a true rheostat, you would want a Pulse-Width controller instead. They are out there, it's a much more common way of controlling a DC motor these days, but for the expense and complexity, three speeds is fine with me.

Mon May 04, 2009 7:37 pm

High power PWM is a recipe for RF noise. Lots of harmonic content in that square wave. I think modern cars are doing it three-phase with a brushless motor, and varying frequency.

Tue May 05, 2009 7:29 am

Academic at this point, but we use many different solar controllers that are all PWM circuits, all at higher power than would be needed in this app, and far higher than that. None of them makes any RF that can be picked up on radio sets. A quality controller is not a problem in this regard.

Besides, the whole vehicle is a rolling RF generator.