Heater blower transistor failure mystery...

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kxlara
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Location: TN

Re: Heater blower transistor failure mystery...

Post by kxlara »

Did anyone get to resolution on this? I’ve just replaced the transistor for a second time and still only get full speed for heater blower. A/c variable speed is perfect. I put in a new blower motor too as the old one was seized. Would love some more ideas.
Chris Lara

1987 635CSI
2001 E39 ///M5
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anchored
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Re: Heater blower transistor failure mystery...

Post by anchored »

Hi, and welcome to the club.

Does your issue have variable speed at all or was it completely zero and 100 with nothing in between before it burned?


As for an update.
I have been busy but meaning to install the 2n502 transistor to see if that handles the problem. I am currently still rocking the older motor.
SHRKATK - Red 87' 635csi
kxlara
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Re: Heater blower transistor failure mystery...

Post by kxlara »

Thanks. Lots of great resources here! On the heater/vent it all or nothing. If I push in the a/c button 100% variable. Works as it should. Which is why I was thinking it was the resistor. Could be the whole board I guess, but that would seem strange.

Appreciate all of the input!!!
Chris Lara

1987 635CSI
2001 E39 ///M5
northNH
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Re: Heater blower transistor failure mystery...

Post by northNH »

Problem solved, but some mystery remains...

After installing a suggested 2N5302 30A transistor I used a jumper wire from the the supply wire (Bu/Gn) at the AC switch (term5) directly to the control wire at the transistor mount under the hood to eliminate both the AC switch and the downstream wiring as a cause of the problem.
Transistor/blower motor speed control worked fine...

Next I jumpered just across the 2 relevant AC switch terminal connectors, term5 as mentioned and term1 (GN/Vi) to the transistor which allowed the car’s wiring to supply the power to control the blower without involving the AC switch.
Transistor/blower motor speed control worked fine...

Pulled jumpers and reconnected AC switch, held my breath,
Transistor/blower motor worked perfectly.

So the only difference now with my fully working system and previous attempts that failed with 10A and 20A rated pieces is that now I’m using a 30A piece. No other issues were found.

Makes no sense, especially with a new blower motor, as 10-15A transistors were perfectly adequate for years...
Unless, as I wondered earlier, several bad batches of low-rated transistors?
'86 635 5spd 175K Blk/blk orig paint/int
‘88 M6 24K time capsule Black/gray
'92 MBz 300CE Sportlined 175K Blk/wine orig paint/int
‘94 MBz E320 wagon
'62 Volvo Amazon 65K mile time capsule

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emac
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Re: Heater blower transistor failure mystery...

Post by emac »

glad you got it fixed and thanks for posting the solution. I am always holding my breathe as I turn on the A/C!
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hornhospital
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Re: Heater blower transistor failure mystery...

Post by hornhospital »

Air conditioning? What is this sorcery of which you speak?

(Mine has never worked in either E24....that's why they get little use during the summer)
.
Ken Kanne
'84 633CSi "Sylvia"; '85 635CSi "Katja";'85 325e "Hazel Ann"; '95 M3 "Ashlyn"
Mr._Graybeard
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Re: Heater blower transistor failure mystery...

Post by Mr._Graybeard »

As it happens, I'm in the midst of some blower fan troubleshooting. It started out with a dash replacement, and suddenly I find my car without blower fan control except for high speed. This is probably unrelated to the ac/heater transistor issue, but if I can offer a suggestion for overall HVAC troubleshooting, I would check terminal 5 in the AC switch socket with a multimeter early on to see if variable voltage is present with the ignition on as the blower control is adjusted. If you can find juice there, delivered in graduated form as you turn the blower control dial, you can rule out the blower control as the source of your problem.

At this point I suspect I somehow killed both the AC and heater transistor while replacing the dash. The fuse for cabin lighting was blown by the time I had done my work, so I may have committed some other offenses as well. The AC switch may also be the source of my trouble, but I've had it apart and cleaned it, and a continuity check seems to give it a clean bill.
1983 633Csi
1987 325ic
1995 540ia
et al.
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anchored
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Re: Heater blower transistor failure mystery...

Post by anchored »

Forgot to post an update to this.

I also used the beefier transistor with success. No issues so far and I am running a factory motor (well lubricated)


For anyone that experiences this problem save yourself 120 dollars and buy this
https://www.ebay.com/itm/BSC-2N5302-TO- ... 1654649185
SHRKATK - Red 87' 635csi
Jlc
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Re: Heater blower transistor failure mystery...

Post by Jlc »

Mr._Graybeard wrote: Sun Sep 27, 2020 9:23 pm At this point I suspect I somehow killed both the AC and heater transistor while replacing the dash. The fuse for cabin lighting was blown by the time I had done my work, so I may have committed some other offenses as well. The AC switch may also be the source of my trouble, but I've had it apart and cleaned it, and a continuity check seems to give it a clean bill.
What did you do to take it apart? Mine sticks so I do the BMW tap to get it to work and have been worried I’d break it taking it apart.
85 635CSi Bronzit
Mr._Graybeard
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Re: Heater blower transistor failure mystery...

Post by Mr._Graybeard »

Jlc wrote: Thu Nov 05, 2020 7:07 pm
Mr._Graybeard wrote: Sun Sep 27, 2020 9:23 pm At this point I suspect I somehow killed both the AC and heater transistor while replacing the dash. The fuse for cabin lighting was blown by the time I had done my work, so I may have committed some other offenses as well. The AC switch may also be the source of my trouble, but I've had it apart and cleaned it, and a continuity check seems to give it a clean bill.
What did you do to take it apart? Mine sticks so I do the BMW tap to get it to work and have been worried I’d break it taking it apart.
The AC switch? There are four tabs at the switch base that hold it in the switch housing. You pry the housing out a bit and work the base downward so the tabs come out of their slots.
1983 633Csi
1987 325ic
1995 540ia
et al.
Jlc
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Re: Heater blower transistor failure mystery...

Post by Jlc »

Mr._Graybeard wrote: Thu Nov 05, 2020 8:34 pm
Jlc wrote: Thu Nov 05, 2020 7:07 pm
Mr._Graybeard wrote: Sun Sep 27, 2020 9:23 pm At this point I suspect I somehow killed both the AC and heater transistor while replacing the dash. The fuse for cabin lighting was blown by the time I had done my work, so I may have committed some other offenses as well. The AC switch may also be the source of my trouble, but I've had it apart and cleaned it, and a continuity check seems to give it a clean bill.
What did you do to take it apart? Mine sticks so I do the BMW tap to get it to work and have been worried I’d break it taking it apart.
The AC switch? There are four tabs at the switch base that hold it in the switch housing. You pry the housing out a bit and work the base downward so the tabs come out of their slots.
Thanks. I’ve pulled the radio for a swap as well as the cluster to fix broken gears, next is repairing the AC transistor assembly so I’ll do the switch at the same time.
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Re: Heater blower transistor failure mystery...

Post by Cogeniac »

Sorry. Been away for 18 years...

Did you ever get this sorted out?

Seems to me, reading this thread, that the issue is overcurrent onthe base-emitter junction of the power transistor.Presumably this damages the transistor quickly enough that the collector-emitter current is not high enough to heat up the transistor a lot (hence the "warm but not hot" description.

That points to an upstream issue, where the transistor in the fan control device is generating away oo much current for the base of the power transistor.
Scott Andrews
Petaluma, CA
1980 Euro 635, "Rocinante", the Truth.
1972 3.0 CS
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northNH
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Re: Heater blower transistor failure mystery...

Post by northNH »

My fan and its control has been working perfectly since I installed the 30A transistor.
I believe in my case the lower rated transistors which failed were crap and not capable of handling their rated amperage.
Tho’ it’s been several years, I do still hold my breath a bit when I turn it on.
'86 635 5spd 175K Blk/blk orig paint/int
‘88 M6 24K time capsule Black/gray
'92 MBz 300CE Sportlined 175K Blk/wine orig paint/int
‘94 MBz E320 wagon
'62 Volvo Amazon 65K mile time capsule

One lucky guy...
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nick88highline
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Re: Heater blower transistor failure mystery...

Post by nick88highline »

Just for information the picture below shows what you get when you buy a new unit from BMW:

Image

Image

The one in my car was missing everything on the rear side and this maybe the same with others as the cars may have passed through many hands.
BMW 635CSiA (1988)
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sideshowbob
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Re: Heater blower transistor failure mystery...

Post by sideshowbob »

Was having the same issue with my blower motor, tried all combinations of transistors, all seemed to fail, my soldering was not brilliant and eventually I 'lost' that little striped resistor somewhere on my bench !
Result , bought a new unit from BMW, lubricated bushes on fan motor and everything works perfectly now !
The new unit from BMW was relatively expensive but the frustration of replacing the transistor and resistor was something else !!
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RossDinan6
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Re: Heater blower transistor failure mystery...

Post by RossDinan6 »

hornhospital wrote: Fri Sep 04, 2020 7:47 am Air conditioning? What is this sorcery of which you speak?

(Mine has never worked in either E24....that's why they get little use during the summer)
.
I got a cool sorcerer's hat and wand from ebay and made mine work. In Florida. In August.

All it takes is time, patience, and of course money. It is not modern cold. Running multiple summer errands makes it struggle a bit. Still worth it to me.
Ross
'84 633 Dinan turbo
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