Help! Does anyone understand the footwell blower circuit?

[aj88cab]

All the speeds work, with or without the jumper. I have switched out the front aux relay and no difference. When I switched out the rear control unit and the problem still occured I came to the conclusion it was something outside the controller..but what? The car only had one PO and he didn't do anything to the wiring.

The only thing I can think of at this point is the diode on the FW blower resistor pack (it shows up on the Bentley diagram) if that were bad and it was allowing power to flow back in the wrong direction, who knows where the current would go. But enough of it could end up in the FW blower relay coil. The resistors work and the front aux relay is good. The diode is pretty much the only device left that is outside the controller.

I know where the resistor pack is. Does anybody know if the diode part of the assembly or a separate item?

[Mysterytrain]

Andrew..I don't have a bentley for the carrera. But the engine controller even with the IC is a pretty dumb device. The only source of power to trigger the second relay in the contoller is first relay/reed switch.
So lets see..all speeds work, the aux relay has been replaced, the controller has been replaced { with a known good? } ...How about some crap on the controller socket..top or bottom? Also, This isn't auto heat is it? Because then you have the extra circuit board and sensors in the picture??

[imcarthur]

Amazing . . .

I go to Vegas for a week (earlier Blackberrys don't browse well) & you guys figure it out & come up with a fix.

Thanks to all for their investigative efforts!

Ian

[Mysterytrain]

Andrew..I'm curious to know more about your heating system. Maybe someone else can chime in also. My question is are there a few versions of manual heat. One with a 3 position switch on the console between the seats that only controls the fan speeds and DOES NOT control the red levers and the flappers. And then a second version with a 10 position switch that controls the blower speeds and the opening of the flappers (meaning the red levers also move).
The reason being..if you look a the SC system the red levers control the engine fan relay by providing a ground for the solenoid thru a micro switch under the levers. Those switches can get funky and be intermittent or be always on. If the levers are still in your car it is possible that they control the first relay.

[dfink]

Are you sure that the rear blower is not running when the footwells are blowing. As mystrytrain stated the only way to get the footwells to run is power from the rear blower engages the smaller of the two relays and that relay sends power to the footwells. If they are running when jumped the rear blower should be running or you put the jumper on the wrong terminals and are directly engaging the small relay. For the other senario of the blowers running then stopping. My guess [and that is all it is] and I am sure it is wrong. Would be that one of the circuits to the controller "maybe the temp sensor" is grounded "pinched" telling the fan to start and when you get some speed the speedometer input is telling it to stop.

[ianc]

Ron,

I think (but am not sure) that there are only two types of heat; auto and manual. The manual heat uses the levers and 4 position switch (0-3), and the auto has no levers and the 10 position switch.

ianc

[aj88cab]

Ok guys, I think I figured it out. I stand before you with hat in hand a little embarrased since the problem has been staring me in the face since I bought the car 2 years ago. It just never registered.

I decided to trace the source of the current that was closing the FW relay in the control unit when I jumpered the reed switch. I removed the engine blower fuse (engine fuse panel) and jumped the reed switch, the relay closed. I removed the FW blower fuse (both fuses out now), and the relay closed. Ahhh, the power was coming from the front fuse/relay. I took the cover off the fuse/relay panel and there they were-- 6 RED relays. No black ones. If I recall there's a diode in the black ones, now I know why. So it looks like the wrong type relay was allowing power to feed back into the control unit and energizing the relay.

I don't know how many times I looked at those relays and it never sent up a red flag (no pun intended). Tomorrow I buy three black relays and keep my fingers crossed that it works. I'll let you know.

Ron, you had mentioned the location of the problem several posts back, but I was just sure the relay was functioning, which it sort of was, it just was the wrong type. Live and learn.

Thanks All
Andrew

[Wavey]

Given the percieved problem that started this thread, would it now be reasonable to say that the Grainger motor is safe to use?

[dfink]

It was safe before it was just the footwells may or may not work. With the jumper you can force them to work.

[aj88cab]

Wavey - I never doubted you

That being said, I humbly retract any blame I placed on the Grainger motor for my footwell blower woes.

Andrew

[ianc]

Well, I don't think it's a question of being 'safe'. I think the 'Wavey mod' is always safe (especially for SC's ), it's just that you might have unpredictable behavior of the footwell blowers after doing it. Some people have no trouble. In the event there is trouble however, there is now a bona fide fix courtesy of Mysterytrain. It's all good!

ianc

[ianc]

If I recall there's a diode in the black ones, now I know why.

Andrew, I hate to rain, but I remember it that the red one has the diode, and the black ones not?

In my SC, the red relay was used specifically for the fuel pump because it HAD a diode?

Hmmm... Not sure...

ianc

[aj88cab]

Ianc

You might have made the assumption that my ramblings are based on sound logic and the scientific method. That's always a dangerous assumption.

You very well might be correct about which relay has the diode, and of course that would make my conclusion totally backwards. The only certainty I have to work with is that there is power coming from the front relay/fuse in a manner that it shouldn't. At this point I am relying on the Bentley manual which shows this relay as a black one and I have a red one installed. I got nothing else if this doesn't do it, so I'll change it and see.

Andrew

[Mysterytrain]

LOL, glad it worked out..don't you love this low tech stuff.

[Mysterytrain]

I had some time to look at the schematic on page one again. I pretty sure that the diode on the red relay is across pins 85 and 86 [the coil] if the diode shorted then the relay wouldn't work. If the diode opened..well then so what, now you have a black relay.
Ok, more looking at the circuit. The way I see it the only way the footwell blowers get turned on is via activating the second relay in the controller. If someone [even the factory] reversed the connections for 5&6 with 7&8 on the controller then it would behave exactly as you state when you attempt the modification.
With pins 5 & 6 as an input, 12 volts would travel thru the winding of the reed switch and sit on the contact of the first relay waiting for it to close. Because there is no current draw the reed switch prevents the 12 volts from going to the footwell blower relay. When the contacts are closed the rear blower would spin and the current draw would create a magnetic field causing the reed switch to close and bingo everything operates as normal even though the wiring is ass backwards. Ok, so now you jumper the reed switch..the 12 volts that is incorrectly on pins 5&6 now also travels around the reed switch and directly to the coil of the footwell blower relay independent of the rear blower relay and the footwells are on. I'm betting this is the problem.

[aj88cab]

Ron

I thought about that possibility but remember, I took both the engine compartment fuses out, unpluged the engine blower connector, then jumpered the reed switch..... and the FW relay contact closed. It stopped when I pulled the front fuse. Now that I think about it I don't recall if the front relay was out at the time too...hmmm.

The more I think about it, the less confidence I have it is the front aux relay. A short or crossed wire up front somewhere or in the console betwen the seats maybe. I need to isolate the components and do some more testing tonight.

Oh well, the sun just came out and I've got some work to do. Stay tuned.

Andrew

[Early_S_Man]

A little late to the chase, here, since my car doesn't know what footwell blowers are, or relay control modules with CMOS ICs in them are, either!

That old chip [and datasheets] is also available from Fairchild and was available from National as the discontinued CD4538B ... better, longer, and more detailed datasheets:

http://www.fairchildsemi.com/ds/CD%2FCD4538BC.pdf

http://www.datasheetarchive.com/semiconductors/download.php?Datasheet=535753

... and from OnSemi and TI in the 14538B series ...

http://www.onsemi.com/pub/Collateral/MC14538B-D.PDF

http://focus.ti.com/lit/ds/symlink/cd14538b.pdf

BTW, the 'old' ['70s vintage] 4538s were Motorola parts in their addition to the original RCA 4000-series CMOS line ...

[spooty]

Guys, what a great thread! In the manual heat cars, the red levers move the flapper boxes to allow heat to the cab, right? I have an autoheat car, so little motors move the flappers? I want to backdate, so now with the modified controller to activate the footwell blowers, will the flapper boxes still work correctly with autoheat? In the manual heat cars, you can control the amount of heat by how far you pull the handles. In the autoheat, do the numbers 1-10 on the rotary switch correspond to how far open the flappers go, and will that still function? I assume that when you get to 7-8-9 the flappers are open all the way, and then you kick in the footwell blowers. Is this correct. If so, then with autoheat, all you would need to do to backdate, is modify the controller, and remove the engine blower and replace with the bypass. Please advise...

[Rot 911]

Spooty I responded to your PM about this topic.

[Mysterytrain]

Spooty...it should all work the same.