1 (edited by gump 2019-04-08 03:26 PM)

Topic: Unobtainable BMW Temperature Switch

For a non-lemons project I bought a 1987 BMW 528e (M20, 2.7L engine).  This thing uses Motronic 1.0.  There are no less than 3 separate systems to deal with the air/fuel for idle etc.  But I digress.

I'm going through this thing pretty thoroughly.  I'm on the front of the motor right now.  Water pump, timing belt, cam, crank & intermediate seals, rocker cover and oil sump gaskets, etc.  The thermostat housing is two pieces.  The bigger of the two has 4 (FOUR!) temperature sensors:  1 for the main ecu, 1 for the gauge and 2 more for idle control.

Most replacement parts are readily available, for example the thermostat housing.  Cheap and easy from URO and available through amazon, rockauto, etc.  I start taking apart the thermostat housing that has all four sensors and the temperature switch falls apart as I start to take the leads off of it.  It has two spade connectors on top.  I don't think too much about this as I already bought the a replacement for the Bosch ubiquitous CLT sensor.

I get to the computer and look up the part number on RealOEM and start searching.  Nothing.  What I do find at some of the usual BMW parts sellers is that this part is NLA-no longer available.  They used this part on a fairly wide range of cars in the 80s but no one seems to have any of them around.

Specifically, this is a 45*C, normally open temperature switch.  I guess what makes it unique is the relatively low operating temp.  45*C is only 113*F.  Kind of an oddball temperature for engine coolant.  But like I said, the whole thing is oddball.

In any case, Ive posted on the e28 forums and there is at least one guy looking for a salvage part.  My inquiries at the usual suspect BMW parts shops have turned up zero.  I'm going to take a look at this thing and see if I can fix it.  Solder new terminals?  Seal it with silicon?  Not sure.  First I will test it to see if it even works.

I brought this issue here because of the various automotive forums I frequent, Lemons seems to have the best balance of ingenuity and resourcefulness.  During all my Google-Foo on this subject I did find a link to a generic 45*C, NO temperature switch.  Form factor is off and the thread is wrong but it could work, if it works.  Since its a switch I don't believe there is too much it has to do to work.

Any thoughts?

Here are some pictures (the part in question is #15 in the diagram and carries BMW part # 12631279716):

https://i.imgur.com/F2NWAFum.jpghttps://i.imgur.com/OtSNWp7l.jpg

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Re: Unobtainable BMW Temperature Switch

Sounds like a job for an arduino, a relay, and a one wire temperature sensor.

Re: Unobtainable BMW Temperature Switch

Even easier, Just grab these, and pot them into a gutted temp sensor body and call it a day.

45°C Normally Open Thermal switch.
https://www.amazon.com/Thermal-Normally … -3-catcorr

There's also this style, but I think they'd be harder to work into the stock form factor.
https://www.ebay.com/itm/1pcs-KSD301-Te … 0745217393

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Re: Unobtainable BMW Temperature Switch

Brilliant!

TheEngineer wrote:

Even easier, Just grab these, and pot them into a gutted temp sensor body and call it a day.

45°C Normally Open Thermal switch.
https://www.amazon.com/Thermal-Normally … -3-catcorr

There's also this style, but I think they'd be harder to work into the stock form factor.
https://www.ebay.com/itm/1pcs-KSD301-Te … 0745217393

10x loser (Arse-Freeze '11 - Vodden '15) 1x WINNER! Arse-Freeze '14 in the Watermelon Volvo Wagon
Swedish Knievel Skycycle('90 Volvo 740 Wagon)

Re: Unobtainable BMW Temperature Switch

I like this. Just out of curiosity, what would you pot it with? I assume thermal conductivity doesn't matter too much as long as the business end of the switch is in contact with the metal housing.

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Re: Unobtainable BMW Temperature Switch

SpaceFrank wrote:

I like this. Just out of curiosity, what would you pot it with? I assume thermal conductivity doesn't matter too much as long as the business end of the switch is in contact with the metal housing.

What else......?...JB Weld  !!!!!

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7 (edited by TheEngineer 2019-04-10 04:37 AM)

Re: Unobtainable BMW Temperature Switch

Thermal conductivity does matter a little. You want the sensor getting heat transfer over most of it's casing to make it react quickly. If you're only inputting through a tiny little contact patch, it will react slowly. But that's the engineering answer, and it likely doesn't matter too much here. You can try JB weld, but it's thermal conductivity is roughly half that of a proper epoxy or potting compound. Might not matter much here, since clearly the sensor is just trying to tell some system that your engine is up and over a specific temp that is well below operating temp. You might tell the system you're good 10 degrees higher than normal, but i doubt that matters much.

If you want to do it right, you get something like 3M TC2810 which is a 2-part thermal epoxy. But it's pricey, especially if you don't own one of the applicator guns.

https://www.digikey.com/product-detail/ … ND/9856929



There are cheaper options, some epoxies, some silicone based, all would work just fine.

Cheaper epoxy example.
https://www.digikey.com/product-detail/ … ND/7317821

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Re: Unobtainable BMW Temperature Switch

If that's the one that turns on the electric fan, then I replaced the one in my BMW track beater with a relay that just leaves it on all the time if the car is on. I also replaced the fan with a 16" SPAL fan.

Mine is on an E30 with a M20 2.5, and it's been like that for a couple of years with no ill effects.

Everybody grab your brooms, it's shenanigans!

Re: Unobtainable BMW Temperature Switch

Oh, if it's just a fan trigger, abandon it with no worries. My solution was given assuming it was required by one of the computers.

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Re: Unobtainable BMW Temperature Switch

No, its not a fan trigger.  It is part of the overly complicated Motronic 1.0 idle control system.  It closes at only 45*C.

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Re: Unobtainable BMW Temperature Switch

Will it run cold if you permanently close the circuit?

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Re: Unobtainable BMW Temperature Switch

Yeah, I'd say any switch that closes at 45c.... Can be permanently closed.

Early Motronic BMWs don't idle worth a damn regardless:)

If you're south of MN, how long is the engine temp EVER going to be below 45c?

But then I'm in the desert. In a Phx Summer, when I park my 300TD overnight... The temp gauge isn't on the peg when I start it in the morning smile

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Re: Unobtainable BMW Temperature Switch

I'm hopeing additive manufacturing costs keep coming down so extinct parts can actually be recreated.

Re: Unobtainable BMW Temperature Switch

I run the same car in Lemons (super slowly). My dumb solution was to do away with the idle control valve and the idle computer. I use the set screw that limits the travel of the throttle to set the idle. I also did away with the cold start injector. Car starts fine if you jazz the throttle a bit.  You can call me a hack, and you’d be right.  Also, the internet says m20b27s react well to boost. Just saying.

TriLamb Motorsports ‘87 528e

Re: Unobtainable BMW Temperature Switch

We have success!  Here is the finished product:

https://i.imgur.com/bGhH0G8m.jpg https://i.imgur.com/GHsUU3Gm.jpg

and this is the adhesive I used:

https://i.imgur.com/6Tl6OPrm.jpg

I am terrible at taking photos of this kind of stuff as I am doing it.  Essentially, I cleared out the guts of the old sensor, used a rotary tool to remove a pocket in the brass to accommodate the new sensor then held it in place with the thermally conductive adhesive.  I was concerned about holding the sensor while the adhesive set but it only took a minute or so for it to become stiff enough to leave alone.  Wired it up and it works great.

These are the sensors:

https://i.imgur.com/4oCq5eOm.jpg

When I received the sensors I noticed the outside case was metal.  This concerned me because I thought that the case might be part of the circuit at some point.  My multi meter and a cup of hot water proved that to be the case.  These thermal switches are SPDT.  When below 45*C the case is connected to one of the leads.  When above 45*C the two leads are connected.  I checked the voltage of the leads coming to the switch in the car and connected the positive lead to the lead on the sensor that is not grounded.  Otherwise, it was a pretty straight forward thing to put together.

Thanks to TheEngineer and everyone else for the help.

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Swedish Knievel Skycycle('90 Volvo 740 Wagon)

Re: Unobtainable BMW Temperature Switch

This is exactly what I did when I converted our Volvo B230FT to microsquirt.  Because racecar.  With the BMW I am really trying hard to keep it as 'factory' as possible.

And yes, Now that I have some turbo experience, I pretty much want to turbo everything.

zakp73 wrote:

I run the same car in Lemons (super slowly). My dumb solution was to do away with the idle control valve and the idle computer. I use the set screw that limits the travel of the throttle to set the idle. I also did away with the cold start injector. Car starts fine if you jazz the throttle a bit.  You can call me a hack, and you’d be right.  Also, the internet says m20b27s react well to boost. Just saying.

10x loser (Arse-Freeze '11 - Vodden '15) 1x WINNER! Arse-Freeze '14 in the Watermelon Volvo Wagon
Swedish Knievel Skycycle('90 Volvo 740 Wagon)

Re: Unobtainable BMW Temperature Switch

Nice, glad it worked.

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