Heater and A/C Blower Always Blowing On High

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Sbeaud
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Heater and A/C Blower Always Blowing On High

Post by Sbeaud »

Well, my used blower control did not fix my issue. Per my earlier post (http://bigcoupe.com/phpBB2/viewtopic.ph ... &start=950) I replaced the blower control in the dash with another used one. My problem remains. No matter the position of the blower speed control knob the blower is always on high - with the A/C on or not. Looking at the many threads on this subject I'm not sure I found one with this issue.

Chris Wright's diagnostic comments (https://bigcoupe.com/phpBB2/viewtopic.php?t=14791) only detail issues where the blower only works when the knob is turned to max. My blowers are on max regardless of knob position. Not sure if replacing the transistors are the correct fix. Any thoughts/guidance would be appreciated.

Thanks

Scott
1988 M6
TimU
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Re: Heater and A/C Blower Always Blowing On High

Post by TimU »

Scotty, that's a classic "transistor fail" symptom.

Which blower(s) are running on high? Top? Bottom? Both?

The top heater/ambient blower should only run when the A/C button is OFF.
The lower A/C blower should only run when the A/C button is ON.
They should NOT be able to run at the same time.

Edit: I missed a 'NOT' above.
Last edited by TimU on Fri Aug 26, 2022 12:19 pm, edited 2 times in total.
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Re: Heater and A/C Blower Always Blowing On High

Post by 86_6series »

On my 86 I never found both blowers running at the same time.
It's either, or, controlled by the A/C button.

If I'm wrong please explain what settings would make both fans run without some malfunction.

thanks
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Re: Heater and A/C Blower Always Blowing On High

Post by TimU »

86_6series wrote: Fri Aug 26, 2022 12:13 pm If I'm wrong please explain what settings would make both fans run without some malfunction.
Bob, you're correct about only 1 blower being able to run at any time.
If the question was directed at me, I realised that I missed a 'NOT' in my post above.
Corrected now, thanks.
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Re: Heater and A/C Blower Always Blowing On High

Post by Sbeaud »

Thanks Tim for the response. Still not confident we're talking about the same conditions.

A/C button not pushed (heat mode)
Fan runs at max throughout the rotation of the blower speed control knob

A/C button pushed (A/C mode)
Fan runs at max throughout the rotation of the blower speed control knob

Will the failure of the transistors result in the particular fan only operating when the blower speed control knob is in max position (closing the microswitch)? Or does the transistor failure result in the particular fan running at max regardless of the position of the blower speed control knob?

Thanks

Scott
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Re: Heater and A/C Blower Always Blowing On High

Post by TimU »

I understand what you're describing - I believe both failure conditions can occur, depending on how a blower transistor fails internally.

1) Failed, open circuit = Fan only works when knob is cranked to max.
2) Failed, closed circuit = Fan only works at 1 speed AND when knob is cranked to max.

I remember reading about 2) somewhere, but I can't find the evidence now.

Could you identify which fan is running at high speed with the AC button is on and off please?
And also what the other fan is doing at the same time?

Each fan will have it's own blower speed transistor so should operate independently of each other.
I guess it's possible that both transistors have failed at the same time :-k
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Re: Heater and A/C Blower Always Blowing On High

Post by Sbeaud »

Thanks for the info. If it is a failed transistor it seems like it would have failed as a short (little or no resistance)?

I checked and the fans do operate according to the switch. Heater blower when in heat mode, A/C blower when in A/C mode.

So it seems like either both transistors (heater blower, A/C blower) failed (as shorts), or the blower speed control (on the dash) failed as a short. But I already replaced the blower speed control (on the dash) with another used one so...

Either the replacement blower speed control also had a blown transistor, or both the heater blower motor and A/C blower motor have failed transistors?

Thanks

Scott
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Re: Heater and A/C Blower Always Blowing On High

Post by TimU »

Hi mate, I came across a bit a more info that might help:
Both Heat and A/C Fan Only Runs on High
If you have a blower control that is infinitely variable (not the 1, 2, 3, 4 position switches) and you encounter a
problem where the heater AND A/C fan only runs if the blower control is in the Max position, there is a good
chance the problem is the NPN transistor on the back of the blower control (NOT the one that is on the
blower transistor pack next to motor itself.
If you look at the schematic, the motors get their power through this NPN transistor unless the control switch is in the maximum position in which case the transistor is bypassed.

If only the heater OR A/C blower motor won't run except on high then odds are the transistor pack on the appropriate motor is bad. This FAQ does not apply to these transistors.

My transistor had a TI logo on it and two numbers, 8417 and BD543A. I was unable to find this transistor so I
used an NTE331. I also bought but did not use a Radio Shack TIP3055. They both have 15 amp capability. I
have had mine installed now for a couple of years and it is working perfectly.
My 1982 (late 1981 manufacture) has one blower each for the a/c and the heater. Also one transistor each
for the blowers. Symptom was, one speed only but for both a/c and heat. Traced the problem to both transistor being bad and also the blower speed switch. Once that was solved the draw on both blower motors was checked and replaced the heater blower. Apparently the motors build resistance and then affect the transistors and then the speed switch.
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Re: Heater and A/C Blower Always Blowing On High

Post by Sbeaud »

Thanks Tim. Still not sure what the issues are. The second snippet you have may be my situation. I've replaced the dash mounted blower speed control (with another used) but the problem remains. Now I'm checking the heater blower transistor. Pics below show my situation. Looks like things got pretty hot - the plastic is melted on the transistor mounting plate. Also, it took a bit of torque to rotate the blower motor - assumed bad/worn bearings. I put some silicone lube on to see if it improved (didn't initially, but better the next day), but decided to buy a replacement. I assumed the new motor would essentially "freewheel" when rotating, but there is still some resistance - assume that's correct. I bought a pair of 2N3771G transistors in anticipation of tearing into this, but given the look of my heater blower transistor assembly I bit the bullet and bought a new assembly.

Transistors: 2N3771G ~$7 each/Digikey
Transistor Assy: 64111381784 ~$134/FCP Euro
Motor: Bosch 0130063013 ~$54/FCP Euro

Assembly is today. We'll see! :-k
1988 BMW Heater Blower Transistor.jpg
1988 BMW Heater Blower Transistor.jpg (358.01 KiB) Viewed 2025 times
1988 BMW Heater Blower Motor.jpg
1988 BMW Heater Blower Motor.jpg (296.39 KiB) Viewed 2025 times
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Re: Heater and A/C Blower Always Blowing On High

Post by Sbeaud »

Well, success and a quandary.

Replaced the heater blower transistor assembly and the heater blower motor. Now the heater fan speed control works - variable fan speed as you turn the fan speed knob. But the A/C fan speed control is now also working.

Original condition:
No matter the position of the blower speed control knob the blower is always on high - with the A/C on or not.

After heater blower transistor and heater blower replacement:
Fan speed control works with the A/C on or not.

Not sure how that happened, but for now I'm happy that it all works!

Scott
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Re: Heater and A/C Blower Always Blowing On High

Post by TimU »

Well done!
That original transistor looks pretty toasted.

I'm not surprised that it failed based on your description of the stiff blower motor.
The harder it is to turn, the more resistance (Ohms) is present and the more power (A) is required to try and turn the motor.
More power = more heat = higher likelihood to fail.

Don't throw the old blower motor out yet though, you might be able to free it up with liberal use of Penetrating oil into the bearings.
If you can free it up enough, you might be able to re-lubricate and sell it as a functional 2nd hand unit on ebay.

The magnets in a good/new motor will still normally give some natural resistance, as you've discovered.

Not sure why this replacement fixed both fans though, I'll have to ponder the ETM a bit more on why that might have been the case.

But we'll done again, another job down. 👍
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Re: Heater and A/C Blower Always Blowing On High

Post by RossDinan6 »

The fan motor quit on my old service van some time ago. Bought a replacement for it, but sprayed the motor bushings with Kroil before replacing the motor. The new one is still in the box. Should have sprayed first. Kroil is the best.
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Re: Heater and A/C Blower Always Blowing On High

Post by Pod »

TimU wrote: Sat Sep 10, 2022 11:33 pm Don't throw the old blower motor out yet though, you might be able to free it up with liberal use of Penetrating oil into the bearings.
If you can free it up enough, you might be able to re-lubricate and sell it as a functional 2nd hand unit on ebay.
These always freeze up if the car is not run for a while, especially when stored outside. You can usually get them moving with some penetrating oil. Best to remove the motor and connect it up directly to a battery. Be careful to grip it tightly, as they can literally jump out of your hand when you apply power!
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