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.

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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