PDA

View Full Version : 1991 passat Heater Control Burning Out



Grahame Crispin
21-04-2008, 02:48 PM
On my 1991 passat B3 estate CL, the blower interior control knob (& the speed 4 connecting wire) gets very hot on speed 4 and the switch & spade connector in the 5-way nylon plug has burnt out; the blower does work on all 4 fan speeds (I connected a temporary switch to check tyhings out) but you can tell everything is going to burn out again when left for a few minutes on speed 4.

Feels like too much current being delivered to control knob on speed 4, but is it resisted down from the battery current like the other 3 speeds, or is it supposed to be direct battery current ?

If its NOT supposed to be resisited down, then why would everything be getting that hot on speed 4 ?

Is this a symptom of the a faulty blower resistor ? or can you suggest something else ?.

thanks,

:confused:

scotty33
22-04-2008, 09:09 PM
I think it's not resisted on speed 4. All I can think of that would make it burn out is that it is restricted in some way which is making it take more current than normal. Maybe the blower motor bearings are seizing/tight? Or the airflow is blocked somehow which means the impellor is struggling to drive the air out?

erico
23-04-2008, 06:19 PM
On my 1991 passat B3 estate CL, the blower interior control knob (& the speed 4 connecting wire) gets very hot on speed 4 and the switch & spade connector in the 5-way nylon plug has burnt out; the blower does work on all 4 fan speeds (I connected a temporary switch to check tyhings out) but you can tell everything is going to burn out again when left for a few minutes on speed 4.

Feels like too much current being delivered to control knob on speed 4, but is it resisted down from the battery current like the other 3 speeds, or is it supposed to be direct battery current ?

If its NOT supposed to be resisited down, then why would everything be getting that hot on speed 4 ?

Is this a symptom of the a faulty blower resistor ? or can you suggest something else ?.

thanks,

:confused:

Hi,
Sounds like a blower motor going bad. Have you noticed a somewhat louder noise from it? Motor bearings are known to wear out eventually. Your ride is even more ancient then mine. :)

You are correct about direct current to the motor on the highest speed. There are three resistors and a fuse for fan speeds one through three. I just looked at the schematic..It is a pretty simple setup.

If I were you, I would stay away from the highest speed until the motor was replaced (much smoke and maybe fire follows when the electrons are given too much freedom) in an electron universe anyway :)

Shoot me a PM and with your email so I can send you the schematic in PDF form from a 1995. 17 pages but only @358 kb. It did not change much at all. Really old technology.

Cheers,
Erico

erico
26-04-2008, 11:56 AM
I did some a bit more reading in the Passat VW Heating/Ventilation manual and found this. here is the blurb from the manual.

The manual air conditioning system uses a
5-position switch and a stepped resistor pack
to give four fan speeds. The blower motor is
continuously grounded and Positive voltage
is supplied to the motor through the resistors
and switch.
The switch directs electrical current through
all three resistors in series for low blower
speed. In the second switch position, two of
the resistors are in series in the circuit, and
for the third speed position, only one of the
resistors is used. The resistor pack is completely
bypassed for high blower speed.
The resistor pack is mounted in the air distribution
housing near the fresh air blower for
cooling. Do not operate the blower with the
resistor pack removed from the air flow, and
in winter conditions, do not operate the
blower with the air intake blocked with ice or
snow.
Electrical System and Controls
If the resistor pack overheats, a thermal fuse
will melt and open the circuit. If this occurs,
the fan will run on high speed only.
A resistor pack with a melted thermal fuse
may be an indication of worn blower motor
bearings, or a restriction in the airflow (such
as a blocked dust and pollen filter). These
conditions can cause the motor current draw
to rise, adding to the heat load on the resistor
pack. Always check for mechanical problems
before replacing the resistor pack.

Your HVAC system use the very same setup and theory.

hope it helps you sort it out.

cheers,
Eric

tatham
07-07-2008, 04:53 PM
I did some a bit more reading in the Passat VW Heating/Ventilation manual and found this.
...
If the resistor pack overheats, a thermal fuse
will melt and open the circuit. If this occurs,
the fan will run on high speed only.Thanks for posting that erico. I have a blown resistor pack. I haven't been able to locate a replacement from a scrapyard, so I currently occasionally use it at full speed and it seems OK.

My question is: I thought my resistor pack had just 'worn out' -- how do I know whether replacing it with a new one will result in the thermal fuse in the new one melting?

Thanks!

Colin

erico
09-07-2008, 11:05 PM
Thanks for posting that erico. I have a blown resistor pack. I haven't been able to locate a replacement from a scrapyard, so I currently occasionally use it at full speed and it seems OK.

My question is: I thought my resistor pack had just 'worn out' -- how do I know whether replacing it with a new one will result in the thermal fuse in the new one melting?

Thanks!

Colin
Hi Colin,
Resistor packs do indeed wear out. It's longevity is determined by the quality of the materials used in manufacture. The company that manufactures them does not want them to last forever. If you replace the resistor pack with a new one, chances are that will last perhaps as long as the old one did if the wiring in the circuit that supplies it has not overheated. Overheated wiring would induce high resistance into the Ohm's law formula which quantifies the current drawn by the fan control circuit. When you pull the old resistor pack out for replacement take a look for discoloration of the wire coverings (usually brown is a telltale sign) at the connector. If you see no discoloration and the wire cover coloring is normal then your new resistor pack should last you a long time.

Cheers,
erico

tatham
15-07-2008, 01:34 PM
When you pull the old resistor pack out for replacement take a look for discoloration of the wire coverings (usually brown is a telltale sign) at the connector. If you see no discoloration and the wire cover coloring is normal then your new resistor pack should last you a long time.

Excellent, I'll have a closer look, thanks erico.