Red Hot AC! or Hot Pics! or Source of Next AC Fire
09-14-2010, 07:50 PM
#1
Rennlist Member
Thread Starter
Red Hot AC! or Hot Pics! or Source of Next AC Fire
Wow - what a surprise I found today! 
The wonders of the Bimetallic switch:
I'm taking the non-functioning AC system out, with plan to leave the evaporator blower for air circulation. Reattach the battery, plugged the blower back in to test and found it wouldn't run with the 'bimetallic switch' from passenger footwell pulled out of the circuit.
- a quick look at the wiring diagram shows why [see below].
It looks like all the current for low and medium settings has to go through this odd contraption.
So I plug the bimetallic switch back. It's laying there on the passenger footwell floor, loose and I think- this thing looks a bit scary so I propped it up so it doesn't touch and short out on anything. t
Turn ignition on and go to run the blower- it runs low, it runs medium, doesn't run high. So I get out to see how much air high is moving. My son who is helping me was the first to notice- "Something smells bad!". I'm thinking -crap somethings burning! A quick check shows it's not the blower that smells so I go back around to turn car off and notice that the 'bimetallic switch is glowing RED HOT! You could toast marshmallows on the thing!
What's scary is that I had just fished out a huge mouse nest from below the evaporator that must have been just an inch away from this. Had that nest gotten on fire it would have been tinder there at the bottom of the smuggler's box. With the fan going the dam thing would have been a freaking blast furnace with no easy way to access the area to put the fire out.
What kind of arcane thing is this? a toaster oven used as a switch for an AC system? Looks like the fresh air blower has the same thing inside it as well- no wonder these things routinely go up in flames!
I guess there's some logic to this, when the toaster oven gets too hot, then the bimetallic switch closes and the fan runs faster cooling the area more- that way you can have a mouse nest a nice medium rare....
I must have a problem in the switch since I don't have the high setting. I can pull the switch and fix that, remove the toaster oven switch and have the blower just be all on or off and skip the fireworks. And I'm considering pulling apart the fresh air blower and doing the same.
Not being an electrician- Can I just use resistors with similar resistance to replace the toaster coils or would they heat up too because of all the current needed for the motor? Is that why people futz with PWM on DC motors?
I guess if you get an AC blower fire you'd want to aim your fire extinguisher into the passenger footwell!
The wonders of the Bimetallic switch:
I'm taking the non-functioning AC system out, with plan to leave the evaporator blower for air circulation. Reattach the battery, plugged the blower back in to test and found it wouldn't run with the 'bimetallic switch' from passenger footwell pulled out of the circuit.
- a quick look at the wiring diagram shows why [see below].
It looks like all the current for low and medium settings has to go through this odd contraption.
So I plug the bimetallic switch back. It's laying there on the passenger footwell floor, loose and I think- this thing looks a bit scary so I propped it up so it doesn't touch and short out on anything. t
Turn ignition on and go to run the blower- it runs low, it runs medium, doesn't run high. So I get out to see how much air high is moving. My son who is helping me was the first to notice- "Something smells bad!". I'm thinking -crap somethings burning! A quick check shows it's not the blower that smells so I go back around to turn car off and notice that the 'bimetallic switch is glowing RED HOT! You could toast marshmallows on the thing!
What's scary is that I had just fished out a huge mouse nest from below the evaporator that must have been just an inch away from this. Had that nest gotten on fire it would have been tinder there at the bottom of the smuggler's box. With the fan going the dam thing would have been a freaking blast furnace with no easy way to access the area to put the fire out.
What kind of arcane thing is this? a toaster oven used as a switch for an AC system? Looks like the fresh air blower has the same thing inside it as well- no wonder these things routinely go up in flames!
I guess there's some logic to this, when the toaster oven gets too hot, then the bimetallic switch closes and the fan runs faster cooling the area more- that way you can have a mouse nest a nice medium rare....
I must have a problem in the switch since I don't have the high setting. I can pull the switch and fix that, remove the toaster oven switch and have the blower just be all on or off and skip the fireworks. And I'm considering pulling apart the fresh air blower and doing the same.
Not being an electrician- Can I just use resistors with similar resistance to replace the toaster coils or would they heat up too because of all the current needed for the motor? Is that why people futz with PWM on DC motors?
I guess if you get an AC blower fire you'd want to aim your fire extinguisher into the passenger footwell!
09-14-2010, 08:13 PM
#2
Rennlist Member
You've certainly a problem there,..very not normal,...that contact(s) can be engaged with sufficient current through that coil WITHOUT THAT much current flowing. Maybe someone can correct me if wrong....
Fused footwell blowers should be in order (what year? Probe deeper in the circuits, knowing the year.
Some great threads regarding this circuit operation (search),..some updates require this circuit to be jumpered to function on backdates (I think). Although the AC experts will chime on the AC related things.
Last I recall, as to loosing the high speed blower selection, this was fuse related (maybe rear?) in my case.
Will have to look further.....long time ago.
Best,
Doyle
Fused footwell blowers should be in order (what year? Probe deeper in the circuits, knowing the year.
Some great threads regarding this circuit operation (search),..some updates require this circuit to be jumpered to function on backdates (I think). Although the AC experts will chime on the AC related things.
Last I recall, as to loosing the high speed blower selection, this was fuse related (maybe rear?) in my case.
Will have to look further.....long time ago.
Best,
Doyle
09-14-2010, 09:23 PM
#3
Rennlist Member
Thread Starter
yeah, it definitely seemed shocking to me that this would get that hot. It would be hotter than normal if there was too much current, but- Right now, the only load on that circuit is the AC [evaporator] blower that appears to be running fine - no seized up/not squealing, looks stock... I'll have to run the wires to see if something is shorting out and go back and check fuses everywhere. I'm tearing back into the AC Fan and Temperature controls next.
Switch pulled, and the bayonet for the 'high' setting had come off, with that replaced now have all 3 speeds.
Went back and checked amps in the circuit-
low 2.7
med 5.7
high 10.4
I could not find any official spec's on the motor, but found one posting where their blower was drawing 11 amps....
Have also noticed that if I put the toaster switch in any airflow- then it does not glow at all. What surprises me still though, is with all that heat, the bimetallic switches never closed [motor speed should increase and resistor glow disappear.
Switch pulled, and the bayonet for the 'high' setting had come off, with that replaced now have all 3 speeds.
Went back and checked amps in the circuit-
low 2.7
med 5.7
high 10.4
I could not find any official spec's on the motor, but found one posting where their blower was drawing 11 amps....
Have also noticed that if I put the toaster switch in any airflow- then it does not glow at all. What surprises me still though, is with all that heat, the bimetallic switches never closed [motor speed should increase and resistor glow disappear.
Last edited by RShepHorse; 09-14-2010 at 11:21 PM.
09-15-2010, 10:12 AM
#4
Team Owner
That to me looks like a load resistor that is designed to absorb that much current when the fan settings are on low and medium . I have seen these before and are very common . these should NOT be run outside of the blower air stream as the blower it is designed to control is suppose to be cooling the power resistor. The function osf the bimetalic strip i belive is a safety device in that if it gets too hot the bimetallic strip will bend and open the circuit kiling the power to the blower and rsistor of course .
What i don't know is how much power it is suppos to absorb . I have no idea if that is normal , but if it wasn't I would be looking at the blower as the culprit
What i don't know is how much power it is suppos to absorb . I have no idea if that is normal , but if it wasn't I would be looking at the blower as the culprit
10-01-2010, 01:59 AM
#5
Rennlist Member
Thread Starter
Follow up- reassembled it all, and with the load resistors back down there in the airflow there's not red glow. Checked amperages again to make the motor was not drawing too much and things seem in spec. I added a 'hardware cloth' screen over the opening to keep mice out of there [Bottom of smuggler's box] from now on.
10-02-2010, 06:01 PM
#7
Rennlist Member
Thread Starter
I just don't understand how these things go up in flames so easily when they are fused! You'd think Porsche would come up with a [over-engineered] way to prevent it!
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10-02-2010, 06:17 PM
#8
Rennlist Member
10-04-2010, 10:06 AM
#11
Team Owner
WOW that should allmost be a sticky considering the "noisy blower " questions we get. IA warning of what could happen very quickly ..
Looks like Porsche overdesigned the circuit as usual ....... to protect the fuse ...
Looks like Porsche overdesigned the circuit as usual ....... to protect the fuse ...
