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Heater blower transistor failure mystery...

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northNH
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Joined: Tue Dec 12, 2017 11:29 pm
Location: New Hampshire USA

Heater blower transistor failure mystery...

Post by northNH » Sat Jun 20, 2020 2:06 am

Greetings from northern New Hampshire,
I’m stumped and hoping for some insights...

Over the 20 years I’ve enjoyed my ‘86 CSI, I have replaced the heater blower motor transistor every ~5-7 years with the regular old 3055s.
Always been straightforward, always successful without drama or unexpected surprises.

I recently took the car out of ~3 year storage, and after several weeks of normal blower operation (i.e. fan motor smooth, quiet and powerful for both variable and high speed operation), I lost variable speed control.

As I said, no big deal, BTDT. Everything else that should work does i.e. variable AC fan control, no blown fuses, etc.

So I bought two 3055s, installed one, straightforward as always.
After ~30 seconds it “blew” and fan stopped, back to pre-installation non-variable operation. Transistor did get hot, no fuses blown...

Huh? Removed/replaced the transistor, same result.

Got two higher-powered 3771s, and same results, both blew within 30 seconds.
And dammit, everything checks out correctly with the transistor once soldered to it’s heat sink prior to installation.
And not possible to connect the 3 wires to the transistor board incorrectly, even at my advanced age.

Although I haven’t measured the current draw of the motor (on high), I find it hard to imagine it’s the problem, as well-functioning as it’s always been.
It’s acting like the main power supply to the transistor is somehow grounding through the plastic blower motor case, but that ain’t possible.

Suggestions most appreciated.
9D42F778-7D0A-4101-A198-816026B40619.jpeg
9D42F778-7D0A-4101-A198-816026B40619.jpeg (801.57 KiB) Viewed 365 times
'86 635 5spd 175K Blk/blk orig paint/int
'92 MBz 300CE Sportlined 175K Blk/wine orig paint/int
'94 MBz E420 V8
‘94 MBz E320 wagon
'62 Volvo Amazon 65K mile time capsule

One lucky guy...

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hornhospital
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Re: Heater blower transistor failure mystery...

Post by hornhospital » Sat Jun 20, 2020 4:53 am

Pretty much has to be excessive current draw by the blower motor.
Ken Kanne
'84 633CSi "Sylvia"; '85 635CSi "Katja";'85 325e "Hazel Ann";'87 325is "Odette"; '93 325is "Elvira"; '95 M3 "Ashlyn"; '95 318is "Bebe"

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86_6series
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Re: Heater blower transistor failure mystery...

Post by 86_6series » Sat Jun 20, 2020 9:47 am

hornhospital wrote:
Sat Jun 20, 2020 4:53 am
Pretty much has to be excessive current draw by the blower motor.
+1

Pod
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Re: Heater blower transistor failure mystery...

Post by Pod » Sat Jun 27, 2020 12:20 am

A few drops of penetrating fluid, followed by some light oil on the blower motor bearings could work wonders.

emac
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Re: Heater blower transistor failure mystery...

Post by emac » Wed Jul 01, 2020 2:45 am

I was having a fuse blow every time I turned the car on. Swapped out resistors, cleaned and lubed the fans and everything was fine....until the fuse blew again. It turned out to be the connector to the resistor under the dash. I tightened the female connectors on the plug and it has worked perfect since then. Dont ask me why a loose connection would cause the fuse to blow. No clue. I found it when I had the fan running and jiggled the connector and it popped and blew the fuse.

Just for grins, you may want to check the electrical connector.

northNH
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Joined: Tue Dec 12, 2017 11:29 pm
Location: New Hampshire USA

Re: Heater blower transistor failure mystery...

Post by northNH » Thu Jul 09, 2020 7:19 pm

Thanks for all the reply’s; however they have not turned out to be useful...

I decided to replace the (original) blower motor with a new Bosch unit, very reasonable at $62 delivered (FCP).
Although the old motor ran well at full speed, swapping around fuses showed it would blow a 7.5Amp fuse, heat up a 10Amp one, and run fine with a 15Amp one (normally supplied by 30Amp, which has never blown).
Oil to shaft bushings without benefit...

New motor installed, new 2N3771(15Amp) transistor installed.
Entirely as before, motor ran a total ~10 seconds, correctly increasing speed as I slowly increased knob rotation until 2/3 full, whereupon motor stopped just like when I started with my first recent transistor replacement.

Repeated once again with my last transistor, same result. Motor correctly responded to increasing speed control knob until ~2/3 full travel, then motor stopped exactly as before. Transistor warm but not hot when it failed.

To repeat of significance, all throughout, when AC selector switch is engaged, AC blower motor works entirely correctly from fan switch, both variably and at full speed, eliminating a bunch of possible faults.

Wondering if the AC selector switch, which runs a GN/VI wire to the heater blower housing for speed control (GN/BK from the switch for the AC motor speed control) could be malfunctioning/shorting and sending full voltage rather than regulated voltage to the transistor...
Or an issue with the (hidden) blower housing connector?

If AC switch is OK, what should the voltage on this wire look like coming to the transistor when the fan speed control is advanced?

Not ready to spring for a new complete transistor unit yet (as I have no good reason to believe it will be OK), but I’m getting real irritated that I can’t get to the bottom of this simple circuit which worked fine for years with a simple transistor replacement.

TIA
'86 635 5spd 175K Blk/blk orig paint/int
'92 MBz 300CE Sportlined 175K Blk/wine orig paint/int
'94 MBz E420 V8
‘94 MBz E320 wagon
'62 Volvo Amazon 65K mile time capsule

One lucky guy...

emac
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Re: Heater blower transistor failure mystery...

Post by emac » Thu Jul 09, 2020 7:25 pm

Did you check the electrical terminals in the connectors? If you have removed them a lot, maybe one of them isnt making good contact and causing resistance in the circuit. I would take a small probe and try to tighten each terminal in the connector. I guess you could put your hand on the wires to see if any of them are getting hot.

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86_6series
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Re: Heater blower transistor failure mystery...

Post by 86_6series » Fri Jul 10, 2020 11:34 am

The Bentley manual has a complete section on trouble shooting HVAC.
Including the voltage specs.

If you don't have one, I could scan that section and e-mail you.

BenM635i
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Re: Heater blower transistor failure mystery...

Post by BenM635i » Fri Jul 10, 2020 1:10 pm

86_6series wrote:
Fri Jul 10, 2020 11:34 am
The Bentley manual has a complete section on trouble shooting HVAC.
Including the voltage specs.

If you don't have one, I could scan that section and e-mail you.
Jumping in on the thread... would be really useful if you could scan the bentley blower fault diagnosis section and send to me... not the same problem but my fan speed switch blew (again) after replacement 2 years ago.

Have previously replaced the big transistor and blower motor spins freely so want to check everything out before i sink another wedge on a new variable switch.
'85 M635 #160

northNH
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Joined: Tue Dec 12, 2017 11:29 pm
Location: New Hampshire USA

Re: Heater blower transistor failure mystery...

Post by northNH » Fri Jul 10, 2020 5:54 pm

@86_6series...
Thanks for your kind offer; I do have the Bentley manual, and an original paper copy of BMW’s “1986 635CSi ETM” (complete detailed schematics).

Nothing in either is helpful; no voltage specs for the speed-controlling wire for the transistor from the AC switch...

I haven’t measured this voltage yet (I believe it’s “pulsed”), or better yet, compared it’s value(s) to the other identical-function wire from the AC switch to the AC blower motor transistor, which works perfectly.

Too freakin’ hot to do more than just think about it, even in northern NH...
'86 635 5spd 175K Blk/blk orig paint/int
'92 MBz 300CE Sportlined 175K Blk/wine orig paint/int
'94 MBz E420 V8
‘94 MBz E320 wagon
'62 Volvo Amazon 65K mile time capsule

One lucky guy...

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