air cond recirculation/fresh system question

B-Hive · Post by **B-Hive** » Mon Dec 06, 2010 7:44 am

Hi there

I have just swapped out a couple of those little electric motors that activate the flaps for fresh/recirculated air.

They are are attached to back of the dual cabin blowers. Anyway there is a cabin switch that selects the mode; Trouble is when I'm testing these motors there doesn't seem to be any circuit that stops them after they have moved the flaps into the full open and full closed positions.

In other words they "run" or are on continually even at the stop point of the flaps.

Since the Haynes book of lies does not cover the air cond system/electrical, is anyone aware of a timing circuit that should go between the switch and these little motors. I'm guessing thats why the original motors were burnt out/seized and unplugged in the first instance..

Thanks

electrokid · Post by **electrokid** » Mon Dec 06, 2010 8:12 am

I'm not familiar with these particular controls - but there are 3 different ways of stopping such a motor when it's reached the end of its travel - a timer as you suggest - a microswitch that is operated by the flap mechanism at the end of its travel - or a potentiometer connected to the flap which can 'tell the electronics' where the flap is (that's called a servo system).

To start with I'd suggest looking for microswitches or a pot / variable resistor - see if the flap mechanism actually operates a microswitch or if there's muck or crud in the way of that happening.

Post by **mat_fenwick** » Mon Dec 06, 2010 9:32 am

They definitely shouldn't run all the time but from memory it is only a 2 wire connection to the motors which would tend to rule out any 'feedback' as to motor position. I'll send you a electrical schematic shortly which might help - I can also check what is happening (electrically) on mine later today. Hopefully the temperature will rise above 0°C soon...

B-Hive · Post by **B-Hive** » Mon Dec 06, 2010 12:23 pm

Thanks for the diagram Matt

As I suspected the switch is just a reverse polarity arrangement. However it feeds straight into the little motors without any timer..and there are no little position micro switches on the flaps..very curious...

I'm keen to hear your findings on the power status of the input leads to these devices..Struggling to see how they would switch off in readiness for the reverse polarity to operate them back the other way and so on..

Post by **mat_fenwick** » Mon Dec 06, 2010 12:48 pm

The only way I can think is in conjunction with a pair of diodes - a limit switch cuts power to the motor at one end of its travel, but with a diode across the switch terminals, when the voltage is reversed the motor would be able to move the other way. There would be a similar arrangement at the other end.

Anyway, I'll brave the cold(!) when I get back home tonight and check whether there is a voltage present at all times, or just whilst the motor is moving. Can you leave the switch in a mid position as a temporary measure? Or does it switch straight over to reverse polarity?

B-Hive · Post by **B-Hive** » Mon Dec 06, 2010 1:17 pm

mat_fenwick wrote:The only way I can think is in conjunction with a pair of diodes - a limit switch cuts power to the motor at one end of its travel, but with a diode across the switch terminals, when the voltage is reversed the motor would be able to move the other way. There would be a similar arrangement at the other end.

Anyway, I'll brave the cold(!) when I get back home tonight and check whether there is a voltage present at all times, or just whilst the motor is moving. Can you leave the switch in a mid position as a temporary measure? Or does it switch straight over to reverse polarity?

The car is laid up at the moment anyway whilst I'm doing other surgery, so battery is disconnected, and I have unplugged the motors till I sort out the design/issue..

The switch is either recirc or fresh ..no middle option..

Hhhmm yeah.. no diodes..and by what i saw when replacing the brushes in one of them, the power goes straight onto to motor spindle.. Before reinstalling the motors, I even tried powering them up and then grabbed them to stop them turning to see if they would power down..some how,, but no!! I let go and they'd keep spinning..

Vanny · Post by **Vanny** » Mon Dec 06, 2010 6:24 pm

i wonder if they work a little more like a stepper and there is a circuit that can detect a 'stall' event and cut the power? Though i don't remember there being any circuitry involved with them other than the console and the switch?

Post by **mat_fenwick** » Mon Dec 06, 2010 8:07 pm

Just been and checked - there is battery voltage present at the two terminals even when the motor has stopped. So there must be something within the flap assembly to stop the motor turning, I can hear it slow then stop as it reaches the end of its travel.
-5 degrees C at the moment and dark so I didn't investigate futher, I must be getting old...

B-Hive · Post by **B-Hive** » Mon Dec 06, 2010 11:37 pm

mat_fenwick wrote:Just been and checked - there is battery voltage present at the two terminals even when the motor has stopped. So there must be something within the flap assembly to stop the motor turning, I can hear it slow then stop as it reaches the end of its travel.
-5 degrees C at the moment and dark so I didn't investigate futher, I must be getting old...

Thanks for the effort Mat...I actually had the whole blower and flap assemblies out and the only thing to stop the motors are the stops on the flap control and of course the flaps themselves when they reach max open/closed. So I am kinda certain that these (quite highly geared motors) are in a perpetual state being on.

as I said even when i stalled them for several seconds by hand they just continue spinning again after u release them...

electrokid · Post by **electrokid** » Tue Dec 07, 2010 7:57 am

i wonder if they work a little more like a stepper and there is a circuit that can detect a 'stall' event and cut the power?

Your right Vanny - I forgot that method - motor current increases when a motor is stalled (due to a lack of back EMF) so that can be detected and the motor switched off when it happens - twas used on some window winder mechanisms to double as an emergency stop in the event of an arm getting in the way of the rising glass.

Doesn't appear to be the system used here though - unless that bit is faulty.

Post by **mat_fenwick** » Tue Dec 07, 2010 8:08 am

electrokid wrote:Doesn't appear to be the system used here though - unless that bit is faulty.

Agreed, as if that was the case mine would appear to have the same 'fault'...

2BXORNOT2BX wrote:Thanks for the effort Mat

No worries at all mate, my other task for the evening was (unsuccessfully) trying to remove a suspension bolt from the spare Discovery to replace a snapped bolt on the other one. Made a nice change to poke around with a multimeter whilst standing up, rather than lying in the snow!

Vanny · Post by **Vanny** » Tue Dec 07, 2010 10:34 pm

electrokid wrote: twas used on some window winder mechanisms to double as an emergency stop in the event of an arm getting in the way of the rising glass.

Its how BX magic one up/one down windows disengage the relay i thought?

I Spend a lot of time with steppers, its fun if you get the coms wrong and they just sit there spinning for days on end!

B-Hive · Post by **B-Hive** » Fri Dec 10, 2010 12:39 am

So I guess the consensus is that these motors are designed to be continuously active even if they are not spinning.. Strange design indeed,

I feel compelled to add an in line kill switch. In fact I think I'll add a little control panel with various switches that 1) kills the power to these motors 2) manually turns on the cooling fans 3) cuts the power to the cabin fan