Location of A/C blower transistor - Found and fixed - DIY

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Jlc
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Location of A/C blower transistor - Found and fixed - DIY

Post by Jlc »

My A/C blower only works on high while the Heater works across all speeds. I plan to replace the AC transistor but am not sure where it is located. I found what I think is the right piece on the right side of the dash but want to be sure. Is it the one with the white plugs? If so, does it unscrew or slide out?

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Last edited by Jlc on Thu Dec 17, 2020 12:43 pm, edited 1 time in total.
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Re: Location of A/C blower transistor

Post by Soperman »

This may help you......

viewtopic.php?f=11&t=2465
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Re: Location of A/C blower transistor

Post by Jlc »

Thanks. That was it. I was a bit confused becasue I was looking for one that looked like the heater blower resistor.

It must have been repalced at some point ass there was one push plug and a loose screw.

Now to try soldering in a new transistor/resistor combo.
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Re: Location of A/C blower transistor

Post by anchored »

if you are replacing it, do two things to greatly increase the life of it.

Thermal compound between the transistor and metal. and use a beefier transistor. I went through hell until I used a beefier one for the heater transistor.
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Re: Location of A/C blower transistor

Post by Jlc »

Well, replaced tansistor and no go. I'd also like to replace the thermal breaker - the little white thing. Anyone know where I can source one or at least its specs?

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Re: Location of A/C blower transistor

Post by hornhospital »

Possibly Mouser Electronics, Jameco, All Electronics, Digikey. Google "electronic supplies".
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Re: Location of A/C blower transistor

Post by Jlc »

Thanks. Any idea what the spec is for the part? I didn't see any numbering on it.
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Re: Location of A/C blower transistor

Post by Jlc »

Some searching suggests it’s a 110 degree C thermal breaker. I can find some related for 10A 250V, any experience using these?
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Re: Location of A/C blower transistor

Post by Jlc »

Anyone know if there is an electrical connection between the base and the pins on the side of the mount? The electrical schematic shows a connection, anyone have a picture of that side of the A/C transistor?

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Re: Update: Location of A/C blower transistor - Got speed control again

Post by Jlc »

Looks like I fixed the speed control resistor on my A/C blower. The steps I took were similar to those in the DIY for the heater with a few additional steps since the two units are different.

1. Removed transistor per the DIY
2. Removed resistor across the two transistor leads
3. Removed the thermal breaker on the heat sink. It is the white part shown in the picture. It is soldered in over pins on the end of the two plug extensions. I wiggled it a while unsoldering it and it came free after some work.

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Here is a picture of the part once it is out. Notice the holes in the end. One of mine looks like it broke from wiggling. As best as I can determine it is a 105degree C thermal breaker; if someone can verify that I'd appreciate it. I bought two from Amazon for aboout $5.

Old:
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New:
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4. Installed new transistor using the upgraded one from the DIY and plenty of thermal paste. I pop riveted it in place since that was easier than screws and would not come loose later. I had to drill out the heat sink holes slightly to get the rivets to fit.

5. Soldered the resistor (1K Ohm 5% - Brown Black Red Gold) across the two transitor leads. I bought a pack at HobbyTown USA since it was cheaper than a box full from Amazon.

6. Soldered the new thermal breaker in place. Be sure to use thermal paste and to make good metal to metal contact with the leads and pin extensions.

Install and enjoy a working a/c blower.

Total cost - about $20, plus a new soldering variable temperature station for about $40 (no job is complete without using it as an excuse to buy a new tool) and a big tube of thermal paste.

I am going to see how hot the transistor gets in use and will update post.
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Re: Location of A/C blower transistor - Found and fixed - DIY

Post by Jubilados »

Amazing work, thanks for the info.
I’ve ordered all those parts and will be giving it a shot.
My blowers work, except for the variable AC blower, so I’m thinking I have the same issue you did.
Is it still working for you?
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Re: Location of A/C blower transistor - Found and fixed - DIY

Post by Jlc »

Yes, of course now the blower motor as started squealing so I'll probably oil I for a short term fix.

One thing I would have done different is wrap the resistor wires around the transistor before installing it and tinning the, That way I could slide the resistor over the pins once the transistor was partially inserted instead of trying to hold and solder it in place.

One word of caution - the device gets hot so if you decide to test it before reinstalling it only run it quickly through the sped rang. I thought mine was busted because the motor would cutout after a few seconds, turns out it needs the cooling air not to trip the breaker.
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Re: Location of A/C blower transistor - Found and fixed - DIY

Post by Jubilados »

I wonder if we could safely use a higher temp thermal breaker?
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Re: Location of A/C blower transistor - Found and fixed - DIY

Post by Jlc »

I wouldn't. It doesn't seem to cutout under normal operation and a higher temp would risk transistor damage or worse.
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Re: Location of A/C blower transistor - Found and fixed - DIY

Post by Jubilados »

I replaced the transistor and the resistor, and the variable fan now works.
I used your tip of first wrapping the resistor leads around the transistor legs to get a nice fit, then installing and soldering.
I bought a new heat sink (same part as you got), but ended up not using it, my original one still works fine.
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Re: Location of A/C blower transistor - Found and fixed - DIY

Post by Jlc »

Glad it worked for you.
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Re: Location of A/C blower transistor - Found and fixed - DIY

Post by TimU »

Sorry for resurrecting a bit of an old thread.

Can anyone comment on what the gold stuff might be, that's applied to components of the Transistor and board surface please?
It's a hard, gold varnish of some sort.

Could it be a waterproof coating of some sort to keep moisture away from the components?

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Re: Location of A/C blower transistor - Found and fixed - DIY

Post by Jlc »

TimU wrote: Sun Apr 10, 2022 7:39 am Sorry for resurrecting a bit of an old thread.

Can anyone comment on what the gold stuff might be, that's applied to components of the Transistor and board surface please?
It's a hard, gold varnish of some sort.

Could it be a waterproof coating of some sort to keep moisture away from the components?
No idea; but possibly discoloration from heat - the unit gets real hot and seems to rely on AC air too cool it.
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Re: Location of A/C blower transistor - Found and fixed - DIY

Post by Jubilados »

I had the same on mine. It definitely seem like a coating that was sprayed on to prevent corrosion.
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Re: Location of A/C blower transistor - Found and fixed - DIY

Post by RossDinan6 »

These days circuit boards get whats called conformal coating. There to seal the board and protect from moisture and corrosion. I suspect it is an 80s version.

Ripped my dash apart today for the long awaited AC upgrade. My blower resistor is coated the same. I'm may go ahead and replace the transistor since the evaporator box is out for a new expansion valve and evap clean. It would suck to have a failure right after assembling everything.
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Re: Location of A/C blower transistor - Found and fixed - DIY

Post by Jubilados »

It’s not something you really *have* to do “while you’re in there.”
It’s pretty easy to get to at any time. It’s designed to be accessible for replacement.
If yours is working, I’d leave it alone, but that’s just IMO.
(Of course, while the evaporator box is out, it is sitting right in front of you, so it is a bit easier, and there’s nothing wrong with replacing it.)
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Re: Location of A/C blower transistor - Found and fixed - DIY

Post by Jubilados »

Also, from what I can see, the entire transistor assembly is available from BMW, when I looked last summer it was around $125. That is a complete “plug and play” unit that includes everything (the transistor, diode, heat sink etc).
Of course it’s less expensive to buy the components from Mouser etc and DIY solder it ourselves. But for those that don’t want to DIY, take a look at the replacement part.
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Re: Location of A/C blower transistor - Found and fixed - DIY

Post by hornhospital »

If you are DIYing it, you might consider adding a heat sink like this. I believe the factory heat sink/mounting plate is borderline inadequate.
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Re: Location of A/C blower transistor - Found and fixed - DIY

Post by RossDinan6 »

Since my car looks like this right now it looks like a "While I'm in here" moment
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Re: Location of A/C blower transistor - Found and fixed - DIY

Post by bpoliakoff »

Jubilados wrote: Sun Apr 10, 2022 11:18 pm It’s not something you really *have* to do “while you’re in there.”
It’s pretty easy to get to at any time. It’s designed to be accessible for replacement.
If yours is working, I’d leave it alone, but that’s just IMO.
(Of course, while the evaporator box is out, it is sitting right in front of you, so it is a bit easier, and there’s nothing wrong with replacing it.)
It's preventitive maintenance. It pays to do it while apart. My transistor cost less than $4 and it takes no more than 20 minutes to replace the old one
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