Temp gauge not working
 

My temp gauge stopped working in my 71 250 with M130.
When i ground the wire going to the thermostat housing, i hear clicking in the (not sure if its called this) sending unit mounted on the passenger front wheel well.
The car has been sitting about a year, any one had a similar issue or common faults?

was looking into the control unit, all the circuit board solder connections and solder trails seem in tact and good condition. The main resistor also reads same resistance as the other 2 resisters (not sure what they control) My manual has a wiring diagram but had just about everything but that circuit. Does anyone have a wiring diagram

You are looking at the aux. fan/emission temp sensor.

Look for another sensor in the cylinder head with a green wire. The green wire goes directly to the instrument cluster.

The '71 model year use a "capillary tube" set-up to the best of my recall. It is a manual gauge which runs from the left hand side of the engine.

I hope it is not the manual gauge.
Rick, thank you I will look for the green wire on a sending unit. The ones I was looking at were pink and both going to the control unit. One sending unit on the thermostat housing and the other on the head by firewall.

If it is the manual gauge, is it a copper braided wire with black sleeve of insulation over part of it? Under spark plugs for cylinders 2 and 3?

Yup, that'd be it. I'm pretty sure 1973 was the first year for electronic gauges.

This is from my 1971 M115 engine.

Attachment 140914

Yup, pretty mad at myself now. When I put it on the head, it kinked. Is it no good now? How does it work?

The tube is filled with a fluid. Some say ether, others say alcohol. Whichever it is, the increase of temperature within the sealed tube causes the liquid to expand. This expansion drives a bellows inside the gauge changing the reading. Much as a coiled spring thermometer operates.

There are a couple of write-ups on the forum regarding repairing them but I've never known anyone personally who had any luck fixing them.

I might be wrong but I believe all of the W114's/W115's used this style. Probably all the '60's-'72 Benz. W107, -8, -9, etc.

The only difference I have seen was in the length of the tubes.

The one in my W114 came from a '67 W115.

Drop Ron (Melted Panda) a line. He's probably got a couple taking up space in his shop.

Oh yeah, the senders in the M130 head are for the auxilary cooling fan and the choke/timing/emissions relays. They will NOT work as a temperature sender if you decide to switch over to a 1973-'76 electronic gauge.

It is possible to use a later gauge using the existing cluster by swapping out the gauge, sender and adding wiring. PITA but doable.

I am going to pull the sensor to get the thread size and get a universal mechanical one for now. Just want to drive the car

Pulled the sensor, looks like a m14 x 1.75. Not completely sure, do not have any taps or dies that size to compare it to.

Old Post but possible useful information for those wishing to repair and cosmetically repair mechanical temperature gauges. John Wolf Instruments in Willowghby Ohio,repairs any type of auto, truck, industrial or airplane instrument including capillary tube mechanical temperature gauges. Another one of their specialty ares is the repair of gas tank sending units - foreign and domestic.

Not much to do except a rebuild if the tubing has been breached. Once I had a mechanical temperature gauge that had not moved for years so I carefully removed the sensor, tubing and gauge from the car. Before sending it off for repair, I tried one more thing and immersed the sending unit in boiling water. The needle bounced from its position at 0* to something over 200* Reinstalled everything in reverse order of disassembly and gauge has now been working perfectly for 30+ years.
For those with similar problem, I would suggest simply removing the sensor still attached to the tubing from the engine and applying heat. You may be lucky and find that your problem is simply a stuck needle in the gauge.