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Coolant Temp Gauge Rebuild w/parts list & photos
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kirtfitzpatrick
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PostPosted: Wed Dec 16, 2020 2:28 pm    Post subject: Coolant Temp Gauge Rebuild w/parts list & photos Reply with quote

My coolant light has been blinking at me for years and every diagnostic test I tried was inconclusive as to what was causing it. I even unsoldered the capacitor and it tested out okay. Since I was unable to buy a used gauge I was left needing to fix this one. So I unsoldered all the resistors, diodes, chips and cap to test them individually.

Since I had to remove all the components to test them I wasnít about to solder old and possibly bad components back on so I figured out the specs of each part and ordered new ones. It was a grand total of about $2 from Digikey and $10 from McMaster.

All in all it wasnít that hard to swap everything out. I never did figure out which part was the bad one but it must have been one of the diodes or the op amp. The resistors and cap all tested perfect but Iím not very familiar with testing diodes and I didnít test the op-amp.

My gauge is from an í88 and below is the parts list and some photos. I figured this would be helpful to anyone out there contemplating rebuilding their gauge. My gauge finally works again! And it was a simple matter to calibrate it to dead center over the LED once I was finished.



    Hex nut
      3-48 thread

    Button head hex drive bolt
      3-48 thread
      1/4Ē length
      black oxide finish

    Capacitor
      16V, 10ĶF

    OpAmp
      LM741

    Diode 1
      blue, above LED
      BZX55C5v1

    Resistor 1
      4 bands (purple green yellow gold)
      750 kΩ

    Resistor 2
      5 bands (brown black purple red brown)
      10.7 kΩ

    Resistor 3
      5 bands (brown brown green red brown)
      11.5 kΩ

    Diode 2
      orange, at bottom
      1N4148

    Resistor 4
      4 bands, next to cap (brown green yellow gold)
      150 kΩ



The original gauge: The two big solder joints, bottom-left and top-right, are the posts that mount the PCB to the metal cup.
Image may have been reduced in size. Click image to view fullscreen.

The original components.
Image may have been reduced in size. Click image to view fullscreen.

Image may have been reduced in size. Click image to view fullscreen.

I upgraded my soldering setup a little for this. I have plenty of soldering experience but not much with PCBs. This was the most ambitious electronics repair I've yet attempted. Not pictured is the solder wick I used to unsolder everything and clean off the board.
Image may have been reduced in size. Click image to view fullscreen.

The toothed arms are for calibrating the needle position. Set them to the middle of their travel initially. Then adjust the bottom one to set the needle in the correct position when the van is off. And adjust the upper arm when you know the van is at optimal temperature to set the needle to the middle of the LED. I used a temperature gun for this.
Image may have been reduced in size. Click image to view fullscreen.

Everything except the LED here is new. I knew with absolute certainty that the LED worked already. Wink
Image may have been reduced in size. Click image to view fullscreen.

I added some heat shrink over the thin wire because the enamel had worn off in places. From the factory this wire is soldered into a tiny horizontal hole in the post. Pull very gently on the wire and give the post a lot of heat until the solder inside liquifies and lets go.
Image may have been reduced in size. Click image to view fullscreen.

The marks on the face are from a mechanic that tried to work on it years ago. It looked even worse than this before I hammered it flat again. Drill out the faceplate rivets if you want to work on your gauge.
Image may have been reduced in size. Click image to view fullscreen.

Hope this helps!
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PostPosted: Wed Dec 16, 2020 2:56 pm    Post subject: Re: Coolant Temp Gauge Rebuild w/parts list and photos Reply with quote

Killer thanks
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NorthOf50
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PostPosted: Sun Jan 17, 2021 10:34 pm    Post subject: Re: Coolant Temp Gauge Rebuild w/parts list & photos Reply with quote

thanks. I think I need to rebuild mine as I am fairly certain my LED is kaput. the gauge works fine but the LED never blinks. I've got the gauge on my kitchen table, about to drill out the rivets to open 'er up!
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PostPosted: Mon Jan 18, 2021 7:59 am    Post subject: Re: Coolant Temp Gauge Rebuild w/parts list & photos Reply with quote

I just entered this post into the FAQ section.
I know it is a gauge and I debated where to put it. I figured most people would check out the engine cooling system section if they have gauge issues. So that's where I put it.

https://www.thesamba.com/vw/forum/viewtopic.php?p=6236044#6236044

If you feel it should be with instruments, let me know, I'll move it.
Or..... we don't generally do this, but in both sections?

Dave
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NorthOf50
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PostPosted: Mon Jan 18, 2021 10:37 pm    Post subject: Re: Coolant Temp Gauge Rebuild w/parts list & photos Reply with quote

I believe the PN for the original gauge is either: 251919511A or 251919511C depending on if the van is 86+ or 86-


*removed image until I can get the parts list corrected
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kirtfitzpatrick
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PostPosted: Tue Jan 19, 2021 10:03 am    Post subject: Re: Coolant Temp Gauge Rebuild w/parts list & photos Reply with quote

NorthOf50 wrote:
I believe I have the correct parts list from Digikey here, please correct me if I'm wrong and I'll update it as I'm new to sourcing individual components.
...

A couple of those resistors look incorrect to me. Sorry I can't drive deeper at the moment as I'm driving cross country.
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PostPosted: Tue Jan 19, 2021 10:40 am    Post subject: Re: Coolant Temp Gauge Rebuild w/parts list & photos Reply with quote

That capacitor doesn't look like the right specs.
Your list shows 1000 mfd 10v
The list given earlier shows 10 mfd 16v

Haven't compared the other parts listed.

Mark


NorthOf50 wrote:
I believe I have the correct parts list from Digikey here, please correct me if I'm wrong and I'll update it as I'm new to sourcing individual components.

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NorthOf50
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PostPosted: Tue Jan 19, 2021 9:55 pm    Post subject: Re: Coolant Temp Gauge Rebuild w/parts list & photos Reply with quote

yeah turns out I wasn't paying close enough attention, too late at night!

Digikey seems to have everything but the 11.5K ohm resistor. trying to build a parts list @ Mouser instead...Someone want to error check this?

https://www.mouser.ca/ProjectManager/ProjectDetail.aspx?AccessID=a865bde165

Mouser # Mfr. # Manufacturer Customer # Description
1 926-LM741CN/NOPB LM741CN/NOPB Texas Instruments Operational Amplifiers - Op Amps Op-Amp
2 74-199D106X9016C6VE3 199D106X9016C6V1E3 Vishay Tantalum Capacitors - Solid Leaded 10uF 16volts 10% C case .20 LS Radial
3 78-BZX55C5V1 BZX55C5V1-TR Vishay Zener Diodes 5.1 Volt 0.5W 5%
4 603-CFR-50JB-52-750K CFR-50JB-52-750K Yageo Carbon Film Resistors - Through Hole 750K ohm 5% 1/2W Carbon Film
5 71-CMF5510K700FHEB CMF5510K700FHEB Vishay Metal Film Resistors - Through Hole 1/2watt 10.7Kohm 1% 50ppm
6 660-MF1/2CC1152F MF1/2CC1152F KOA Speer Metal Film Resistors - Through Hole 11.5Kohms 1% 50PPM
7 512-1N4148 1N4148 ON Semiconductor Diodes - General Purpose, Power, Switching 100V Io/200mA BULK
8 588-OL1545E-R52 OL1545E-R52 Ohmite Carbon Film Resistors - Through Hole 150K Ohm 1/2W 5% 350 Volt
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PostPosted: Wed Jan 20, 2021 10:47 am    Post subject: Re: Coolant Temp Gauge Rebuild w/parts list & photos Reply with quote

if you don't have the resistors, i'll guarantee that the existing ones are good and you can re-use them, or at least the one you don't have. just measure it for ohms to see if it matches. most likely, the cap was bad and you've substituted a tantalum instead of the aluminum electrolytic. that's fine.

edit: i see you tested the cap.. depending upon the test unit, may or may not reveal a problem but congrats on fixing it!
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PostPosted: Wed Jan 27, 2021 2:49 pm    Post subject: Re: Coolant Temp Gauge Rebuild w/parts list & photos Reply with quote

the plot thickens...

I completed my rebuild last night and tested my gauge today. all new components, LED included. the gauge works (still) however my LED still does not blink!! argh!!! I have verified that I installed it in the same polarity as the original LED. I have tested the new LED in place w/ diode test and it works in place. Why is it not getting V to light up? any ideas? my board was in really nice shape and the needle etc functions.

I'm back to square 1 with a non-working LED!!! Sad
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PostPosted: Wed Jan 27, 2021 2:56 pm    Post subject: Re: Coolant Temp Gauge Rebuild w/parts list & photos Reply with quote

Do you have a photo of the board after you swapped all the parts?

I think 4 or 5 of the parts have polarity and must be installed connected the right way.

Mark
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PostPosted: Wed Jan 27, 2021 3:34 pm    Post subject: Re: Coolant Temp Gauge Rebuild w/parts list & photos Reply with quote

Isn't the original R3 1k25 , not 11k5 ? Did you measure them all?
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PostPosted: Wed Jan 27, 2021 4:01 pm    Post subject: Re: Coolant Temp Gauge Rebuild w/parts list & photos Reply with quote

Before and after photos of the boards. I was pretty careful to match polarity, but I am new to this so it is possible I missed something.


Image may have been reduced in size. Click image to view fullscreen.


and after


Image may have been reduced in size. Click image to view fullscreen.



I measured all the resistors that were removed to confirm that they were in fact what was stated by the OP. He's bang on. The only difference is that I ordered 1% tolerance resistors for my replacements, where previously some are 5%. I tested everything that came out, and it all seemed to test good. I don't know how to test the OpAmp but regardless the LED didn't work before and doesn't now. I know the ground is good as I also replaced my high beam indicator LED which shares the same ground and it now works properly....
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PostPosted: Wed Jan 27, 2021 4:12 pm    Post subject: Re: Coolant Temp Gauge Rebuild w/parts list & photos Reply with quote

unable to determine from your photos the polarity of the tantalum cap. the negative lead should be closest to the IC chip.
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PostPosted: Wed Jan 27, 2021 4:31 pm    Post subject: Re: Coolant Temp Gauge Rebuild w/parts list & photos Reply with quote

Awesome! So glad to see someone else swinging for the fences.

I double checked your work. The polarity of your LED looks correct as are the directions of the diodes and op-amp and the value and position of your resistors. I can't tell from the photo if the cap is in the correct orientation.

NorthOf50 wrote:
the LED didn't work before and doesn't now.

I would guess that your issue may not be in the gauge, or at least not the electronics of the gauge.

If it is in the electronics there are four things I don't know: if the different type of cap you used is ok to use, the orientation the cap was soldered in, the specs of your new LED and if that will affect the circuit, the soldering work on the other side and whether you have a short or a bad connection.

Do you have a photo of the other side?

Is the cap in the right way?

Can you test the old LED to see if it works and if so, solder it back in place to test?

I have a bunch of leftover "audio quality" caps and could mail you one or two if you want.
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PostPosted: Thu Jan 28, 2021 11:57 am    Post subject: Re: Coolant Temp Gauge Rebuild w/parts list & photos Reply with quote

ok. I have verified everything on my board. the cap polarity is correct. have traced out the circuits going on photos on the board empty. verified the gauge needle works as it should by pulling the coolant level probes, it pegs.

led still does not blink under any circumstance!!! argh.

"if" my circuit trace is correct. the LED should blink @ ign on via:

10V in, straight through LED, through the 150k resistor, charge up the 10uF cap then direct path through gauge ground (connector 14/5).

Things I know:
-10V in is good from voltage stabilizer
-gauge ground is good both through foil and gauge (tested and jumpered)
-cap is soldered w/ correct polarity



ideas?
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PostPosted: Thu Jan 28, 2021 12:19 pm    Post subject: Re: Coolant Temp Gauge Rebuild w/parts list & photos Reply with quote

The flashing should done by the 741, configured as an oscillator. You'll need to draw out the circuit diagram to work out how this circuit is supposed to perform.

I've googled for such a diagram, and it seems no-one has published it yet in a discoverable form!

To help, here's the components and the PCB (flipped) to make it easy:


Image may have been reduced in size. Click image to view fullscreen.


It I can find a round tuit (in short supply here ATM) I'll have a stab at it myself...
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PostPosted: Thu Jan 28, 2021 12:20 pm    Post subject: Re: Coolant Temp Gauge Rebuild w/parts list & photos Reply with quote

It looks overly complicated to me. Confused Looking at the manual, the warning light is getting a chassis ground via cluster track "S" which terminates at the T 14 connector. Perhaps, its an external problem to the actual gauge.
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PostPosted: Thu Jan 28, 2021 12:25 pm    Post subject: Re: Coolant Temp Gauge Rebuild w/parts list & photos Reply with quote

I've been down the road of chasing the external ground already, before I rebuilt the gauge going so far as to running a ground wire directly from the rear of the gauge to a ground on the body. The 14pin track is actually "5" which shares the ground for the high beam indicator, which in my case works (now that I replaced the burnt out LED there).

I'm working with a colleague to try and trace out the entire circuit. Hopefully we'll get somewhere or if anyone else out there has further insight or locates a "round tuit" we can add to the collective knowledge here.


For those that are in the "know" are we certain that the LED blinking (at all points or any) is controlled by the gauge? Apart from sending a signal to peg the gauge, does the relay (43/42 - low lvl warning control unit) have anything to do with lighting the LED? I have a 43 in my van. It pegs the gauge.
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PostPosted: Thu Jan 28, 2021 12:34 pm    Post subject: Re: Coolant Temp Gauge Rebuild w/parts list & photos Reply with quote

I think you need some rope, sulfur, charcoal and a section of bamboo. Wink
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