Don’t throw away your leaking oil pressure gauge. They can be resoldered. After reading some threads here about it I decided to resolder mine and document the process.

This was in response to reading the current thread about oil leaking from the dash. I realized my 300d was getting up to the age where this starts happening. In the name of preemptive strikes I decided to deal with the leak before it happened. I know I’m crazy but I had to deal with the aftermath of a bad leak in my 240d. The story was the original owner was on a road trip from Texas to California and the leak started getting on his legs. He kept driving and filling oil over three states. In the meantime he turned the car into the Exxon Valdez disaster. He got a new gauge (this was 2003) and sold the car to me with oil saturated mats.

So here is the process. I noticed that a well marketed MB diesel online business sells “used” bourdon tube gauges salvaged from old clusters and I was wondering if he is just reflowing the solder and reselling the parts for a hefty profit. Well, in this thread I will show you how I reflowed my own pressure gauge bourdon tube with solder and plumbing flux. It was relatively easy. A 2 hour job with cleanup.

1. Remove the instrument cluster.
2. Remove the three gauge pods from the cluster. Speedometer, tach and pressure/water temp/fuel.



3. You will focus on the pressure gauge. Remove the needle. This is tricky as you don’t want to bend the shaft. The needle is attached to the shaft using a long brass sleeve that goes down into the pressure gauge mechanism. The easiest way to get it loose was to carefully pry up on the tube from below the gauge face. Do not pry above the the gauge face or you’ll scratch your gauge face. You need to go in through the side of the gauge and find where the needle’s sleeve slips over the shaft. At that point I used a small screwdriver to pry up on the needle. It’s on there tight. I repeat, do not pry against the face of the gauge. Pry from the edge of the gauge mechanism below the face.

You’ll be bracing your little screwdriver on this platform with the two screws in it. I used one of the little screws as my fulcrum. Be mindful of the hairspring. You’ll be prying in close proximity to it and it is super delicate.



4. Now you’ll want to remove the brown circuit board from the back of the gauge. To do so remove the two screws at the pressure gauge fitting. And then remove the two nuts holding the fuel and temp motors to the board. The board is plugged on to the motors using four electrical pins and sockets. To break it free you must gently pry the motors away from the board with a screwdriver. They have been in there 40 years so they will be a little stuck. Once you get them moving they’ll unplug and you can separate the gauge from the board.

Be mindful of your dial and needles. I like to only handle the gauge face by the edges. Don’t lay your gauge down on the table on the needles while you work on it. You’ll mess it up. Support it properly while unscrewing the board mounting nuts and screws.



5. Unscrew the two chassis screws holding the bourdon tube to the gauge. Remove the bourdon tube and mechanism. This is what you are after.



6. Note the hair spring. You will need to be very careful with this because it is delicate and easily burned while reflowing the solder. This needs to be protected with a heat shield. I made one from the shimstock found on a 3.5 inch floppy drive for those of you who remember what that is. I cut one out with shears. I chose to use steel over aluminum foil because it wouldn’t melt under the torch flame. You must protect the spring.



7. Here is my heat shield and some old zinc chloride plumbing flux. I used electronics solder because I have a lot of it. It has rosin core but I used plumbing flux to really get the solder flowing. Everywhere you flux you will get solder bond.



8. I used this torch. It’s a small butane torch.



9. This is my work setup. I use a vise and a welding wire to hold down the bourdon tube in the assembly. I didn’t want it all falling apart once I heated it.



Here is detail of how the heat shield separates the spring from the part to be soldered.



10. Then I soldered. Just like sweating a pipe. I slathered all the key joints with flux and heated gently and watched the solder get sucked into all the critical gaps. I only soldered the end of the tube that is mounted to the oil feed line. Top and bottom. I left the end with the linkage alone. Here is the result. This is the side that I believe develops leaks. Mine had not leaked yet but I could see a side of the assembly where the solder was thin and there was a big gap in the base parts. I filled it all in with solder.



11. Mount it in the car without the gauge and connect the hose. Turn on the engine and see if it leaks at the seams. If it holds you are good to go. At this point I discovered my gauge was not moving. I put a piece of tape on the shaft and it didn’t rotate. I discovered that I’d baked some oil on to the rack and pinion and jammed it up. A spray of brake clean and some gentle pushing got the mechanism freed up. Don’t push it too far. It’s delicate. When you’re happy with the leak free bourdon tube and its action reassemble your instruments and cluster. Pressing the circuit board back on the pins was easily done once I found out how to use the two nuts to press the board back into the sockets.

Thanks to Ah-Kay, Diesel911 and Murdock for inspiring me to do this.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Lifting the gauge needle.

Use 2 metal spoons to lift the needle. Tape the dial surface so it wouldn't be scratched. Put the lip of 2 spoons on opposite side of the needle. Press down on the handles on both sides and they needle will pop right out.

Resoldering is one way of fixing the crack solder joint but it is not for the faint of heart.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Love how quickly this escalated.

I haven't stripped the cluster down to this level, interesting to see what that thing is made of (image searched bourdon tubes for reference). Previously had only the vaguest intuition of where the gauge may have seeped oil and it was at the inlet, not where you ended up soldering. Do you have any other photos of the weak points? Still not sure I understand the area you repaired.

Really nice work!

__________________
1981 240D 4sp manual. Ivory White.

This image from Murdock is a better reference on how it all goes together. You can seek the black oil leaking from the seam of his bourdon tube. The tube rests in this u shaped channel that plumbs the oil up from the fitting. I think this is the bad side. Not the crimped over end that is connected to the linkage mechanism.

In other words you have one side of the tube connected to inlet oil (the side I fixed) and the far end of the tube crimped off and linked to the needle mechanism.

W123 Oil Pressure Gauge Leak

Granted mine was not leaking when I opened it up. My logic (possibly flawed) is that the solder ages and cracks by fatigue cycles. Reflowing will fuse all the cracks together. Also it seems like there could have been a bit more solder added to the inlet side of the tube. There are basically four critical seams on the inlet side where the tube connects to the plumbing. By reflowing the top you basically suck solder into the bottom seams at the same time kind of like how solder works its way around a plumbing pipe.

On the photo below there are two seams of solder on each side of the tube. This is the original unit before I soldered to it. These are the two places I added fresh solder to. I don’t have an easy way to markup the photo on this device. The solder is the silver gray material on each edge of the bourdon tube.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Great write-up, thanks for taking the time!

I say temp fix. How long it will last is anyone's guess.

Do the fix and start looking for another pressure gauge. Perhaps a digital gauge that will fit.

__________________
84 300D, 82 Volvo 244Gl Diesel

Totally get it now. Thanks Carl.

Why? Old solder, new solder. Seems like a one to one repair.

__________________
1981 240D 4sp manual. Ivory White.

What!? You sayin’ I can’t sweat copper pipes that don’t leak?

Okay, I’ll start designing a digital retrofit. Best would be a Switec X27 motor mounted in place of the original bourdon tube. Standard electric transducer. Microcontroller and stepper electronics to linearize the sender output and drive the needle according to the OE (backwards) scale. I think I can hack this together.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

There is an underlying reason for the leak. Metal fatigue. Coating it with solder does not fix that. Which is why I personally would not expect it to be a permeant fix.

Or you could just buy a digital oil pressure gauge and sending unit and find a way to fit it to the oil filter housing.

The Engines on the SD's of the same years use a different plate on the rear bottom of the same oil filter housing and use an Eletric sending unit. However, I don't know other gauges the sending unit can be used on.

__________________
84 300D, 82 Volvo 244Gl Diesel

Carl, was there any visible fatigue, cracks or deformations in the brass (body or tube)?

I found more images of these leaks and the failure point was always at the soldered union between the gauge body (hard brass) and the bourdon tube (soft brass).

If the fatigue was localized to the solder, which you replaced, doesn’t track that this wouldn’t be as good as original.

__________________
1981 240D 4sp manual. Ivory White.

No. My gauge was actually in pretty good shape. It wasn’t leaking. This was a preemptive fix.

My thinking was reflowing the solder with fresh flux would fuse together any micro cracks and give me a fresh start. But it’s all a guess. It’s not like we are thin sectioning the joint and checking it under a microscope.

I think the failure is a stress fracture that likely starts around the hole in the bourdon tube and slowly propagates out to the edge over many years. The engineers who designed this thing never imagined us using it this long. They’re likely in the graveyard themselves.

In my mind’s eye the crack is likely filled with oil and contaminants. I don’t think that is optimal for reflowing and fusing it all back together. The heat and flux can only do so much. So in all fairness reheating and fluxing will have limits.

When bad circuit boards are reflowed the hope is the contaminants in the bad joint get lifted up to the surface and washed away with the fluxing agent. The solder is more dense than the oxides so it sinks down while the contaminants rise up in the molten solder. That’s the theory. It’s not quite coating the outside with fresh solder.

I’ve been humbled before. There is a possibility that my solder job will leak in the near future. But I like to distinguish between possibility and probability. We will see. The experiment is the ultimate test.

I’ll circle back if it starts leaking. Before that I’ll likely have an electronic solution to show off. I ordered some pressure sensors and instrument stepper motors (X27 Switec) yesterday and I’ll have everything in place to start prototyping a new measurement system.

ETA - I guess to reiterate it is easy to misconstrue adding new solder to an existing joint as recoating a bad solder connection with a thin layer of new solder. This technique often works for repairs because you are also adding a small amount of fresh flux to the joint if you use flux core solder. This is a paltry amount of flux but it will often get old electronics working again. Next best thing is to strip all the old solder away and resolder. What I’m doing is flooding the thing with gobs of flux paste and reflowing. The theory is the metal in the bad solder is primarily the same eutectic alloy it was from production. It is just recrystalized from heat and stress cycles. So fluxing and remelting kind of cleans it up like straining dross of a cauldron of molten metal after flux (like borax) is tossed in. Ok, I’m nerding out too much on this. I wasted too much of my life breathing solder fumes.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Permanent fix? What a dumb proposition. No fix is permanent, this is what a mechanic is for. How permanent is permanent? The copper pipe ( sweat joints ) in your home also will fail due to whatever reasons. Personally I would not re-sweat all of them as a preventative measure.

I found JB weld very effective. You remove the gauge from the dash, clean it up and reattach to the line, start the engine without the dash and you can pin point the exact leak. Clean it up real good, roughen up the area and apply the glue. Retest when dry, it gives you 100% confidence that the leak is fixed.

JB weld is rated at 2500+ psi. It will fix a pinhole leak with no issues. Personally it is less messy, less chance to go wrong and as good, if not better than resoldering it. Good luck.

BTW: I have used JB weld to repair evaporator of my 190D. The part is no longer available. I cut off the leaky tube, inserted new tube and JB weld it. Evaporator is running at 250psi in liquid form, much higher than oil pressure. I pressure tested the fix with compress air, dipped into a big trash can with water. It was 2 years ago and it is as good as new.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Got it AK. JB weld is tough stuff. I considered doing it that way. Solder can be easily reworked. Glue is awful to remove if things don’t go right. That’s pretty cool that you fixed your evaporator that way.

Progress on the digital pressure sensor project. . My Switec x27 motor arrived. I wired it to an Arduino Nano and I installed the Switec X25 libraries to it. They’re free open source on GitHub and it saves a ton of coding time. It’s all in C++ but it is pretty easy to use. Just two commands, zero gauge and send needle to position. It works very well. Here I have it reading a 0-5V electrical signal off a potentiometer. Turn the knob and the needle rotates. Easy. I do have to code it to flip and scale the direction of the needle since the w123 oil pressure gauge is backwards.

I have an oil pressure sensor on the way from Amazon. $7. It runs on 12v and puts out a linear voltage from 0-5V as pressure goes from 0-100psi. So it matches my simulated output from the potentiometer. I think this will be an easy hookup.

Last details.

I need to install the stepper motor into the gauge. There is the small black plastic frame that the bourdon tube sits on and the circuit board out back. On the circuit board there are two screw mounts that I may be able to use to secure the stepper motor. I’m hoping it’ll all fit in there. I may need to fabricate an adapter mount.

The sensor is 1/8” NPT but I’ve managed to locate an M12x1.5mm to NPT fitting that uses a copper crush washer. I’m hoping I can get that to seal on my filter.

I’ll be hitting the yards in search of an instrument gauge to butcher. I’m not feeling like mangling my newly soldered one.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Got my pressure sensors in the mail. $7 each. Quality looks good. 0-100psi/0-5v. That means I want about 50psi or 2.5V full scale. 3 bar works out to 43 psi so that should cover it. I need about 90 degrees of sweep on the gauge face.

Unfortunately I’ve been thinking this device through and I realized when I kill the engine the power to the microcontroller will go down and it won’t be able to drive the needle down to 0 like the original gauge. Some 90s GM cars would do this. The needle would get stuck at the last known measurement and wouldn’t zero out till you power cycled the electronics.

I guess that’s ok. There are ways to get around this using unswitched power but I think I’ll just keep going with this quirk as a part of the design.

Next step is to rig up some plumbing and air pressure to test the sensor with the stepper motor.

Then mount the motor. Then flip the scale and calibrate it so it reads the correct pressure

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles