Help! Does anyone understand the footwell blower circuit?

[Mysterytrain]

Kurt and the rest of you Carrera guys...I have a question about the stock heating system and the Aux cooling of the engine using the rear fan. That speedometer connection on the controller got me thinking. Did the rear blower come on when the car was not moving? If so that would explain the mysterious speedo connection and why the factory didn't use an RPM/tach signal. ok, one more: where is the blower temp switch located?

[aj88cab]

Ron

The blower temp switch is mounted on the oil breather on top of the engine. It is supposed to come on only if the car is stationary (but running). Like if stuck in traffic or left idling for too long. You should be able to test it by pulling the connector and grounding the circuit....I think.

Andrew

[ianc]

The blower temp switch is mounted on the oil breather on top of the engine.

Aha! So that's what that sensor is. I noticed it while changing my t'stat o-ring last week and wondered about it.

So let me see if I've got this straight: if that temp switch grounds the circuit due to excessive heat, and the speedo input shows no forward speed but the car is running, the main blower fan will come on and pump air through the heat exchangers (and out the open flapper valves) to assist in engine cooling?

ianc

[aj88cab]

Originally posted by ianc
Aha! So that's what that sensor is. I noticed it while changing my t'stat o-ring last week and wondered about it.

So let me see if I've got this straight: if that temp switch grounds the circuit due to excessive heat, and the speedo input shows no forward speed but the car is running, the main blower fan will come on and pump air through the heat exchangers (and out the open flapper valves) to assist in engine cooling?

ianc

Yes sir, that's the idea as I understand it.

(edit)

Actually now that I think of it the "engine running" requirement may not be part of the equation. It would make perfect sense that if after a spirited run you turn off the engine there is still alot of residual heat. So it might (either/or) work with the engine off.

My head it starting to hurt! Is it happy hour yet?

Andrew

[Mysterytrain]

Amen...I can sleep now! That damn speedometer connection on the controller had me troubled for weeks. So the engine needs to be running and the car stationary to turn on the blower and the blower does not turn on after the car has been shut off to help with the cooling?

[ianc]

Actually now that I think of it the "engine running" requirement may not be part of the equation. It would make perfect sense that if after a spirited run you turn off the engine there is still alot of residual heat. So it might (either/or) work with the engine off.

You'd have to check to see whether that circuit gets power with no ignition, but I'm inclined to think not. I always hear the main blower fan going on when I turn on the ignit.

ianc

[aj88cab]

According to the Bentley diagram the power circuit for the engine blower is "hot at all times". This leads me to believe it might operate with the engine off if hot enough. Would I bet my next pay check on it....I think not.

Should be easy enough to check though...maybe this weekend.

Andrew

[dfink]

It will run with engine off. Had mine come on one day after a run at the track when it was hot. Heard the fan and thought what the f***. I don't know how hot it has to be but it must be toasty as it has only ever done this once. I may however have a bad temp sensor and should be coming on more. Hmmmmm...... I have been known to turn on the switch and pull the levers but hate to drain battery.. Some one asked about reverse engineering the controller but it actually has an IC "chip" on a small side board. I have also been curious what the potentiometer does. It seems to be quite the little device when you look further at all the functions.

[ianc]

Yes, I believe you guys are right. This goes back to dfink's original comment about devices being hot all the time, and the ground is completed to activate them, so I think when hot enough, the temp switch will complete the circuit to run the fan even if the ignit is off.

ianc

[aj88cab]

I should have placed the bet........

Well guys it is definitely happy hour here on the east coast, and I have a little non-automotive appointment to attend to. So as much as I enjoy our little chats its time to call it a day. I'll probably be back tomorrow once I get my hands dirty with this system though.

A potentiometer you say............hmmmmm........what could it be?

[Mysterytrain]

kinda makes sense. So the factory just figures that the footwell blowers will be in the off position?

[dfink]

OK don't know if this will help any of the engineer types but the IC chip on the board is an HEF 4538BT very hard to find info on. Everything on it is in German. Finally found the following printout hope it loads.

[ianc]

Looks like Dutch to me...

ianc

[rick-l]

Originally posted by dfink
OK don't know if this will help any of the engineer types but the IC chip on the board is an HEF 4538BT

Ah yes, the old one shot.

Data sheet available here.
http://www.semiconductors.philips.com/pip/HEF4538B.html

[dfink]

OK I have de-soldered the side board and will try to take a picture of it tonight so maybe can trace the circuit on the small board.

[efhughes3]

Mysterytrain: You are the FREAKING MAN, my brother!!!

I took mine apart, and looked at the back. No way I wanted to try to solder on the rear as the circuits are close together, not to mention that side sits pretty close to the housing. So, I focused on the front. I didn't have the pigtails that Kurt did, but with a small soldering tip and a little care, a jumper was installed. Whamo-IT WORKS!! This fix took all of 15 minutes. The footwells come on without the motor running, if anyone is interested. For the record, mine don't come on until I get to the red numbers on the dial. Defrost setting kicks that blower on. This has always been the case. At any rate, the top is off for a brisk drive tomorrow morning...Thanks again Mystery!

[dfink]

OK so the footwell are working but for what purpose was the reed switch there to begin with. Either a case of extreem over-complication or there was a reason. Can anyone think why they wouldn't have just done away with the reed switch in the first place if everything works as it should without it. Seems like the rear relay could have just activated the front relay directly. Only reason I can think of is that in some senario the front blower relay will come without the back and there was a possibility of feedback to the rear circuit. In essence the coil / reed assembly is acting like a heavy duty transistor. I know you can activate the front blowers alone by manually engaging the smaller of the two relays. I can't think of any senario with manual heat but with auto heat do the front blowers maybe run when the defrost is on or maybe with the A/C.......... Devils advocate you know would hate to see something happen cause everyone jumped the reed switch..... I know just shut up it's working...

[efhughes3]

Originally posted by dfink
I know just shut up it's working...

You said it.

[Rot 911]

YES, YES, YES, WE HAVE SOLVED THE PROBLEM. Ed beat me by a few minutes. I too went with the jumper. I wanted mine completely reversible (plus my soldering skills suck) so I got out some extremley small diameter shrink tubing and used that to affix the jumper. As there is no strain on the wire it will hold just fine. And the blowers are still variable speed. You guys are absolute geniuses. Ron, write your tech article!!!!!!

[Wavey]

Originally posted by dfink Can anyone think why they wouldn't have just done away with the reed switch in the first place if everything works as it should without it.

It just wasn't complicated enough with 6 fans in the system.

OR, the engineers own Bosch and Hella stock?