Another diesel use for the gasser "Economy" gauge

[Jeremy5848]

Our friend Sixto has posted an interesting thread on using a gasser "Economy" gauge as a diesel turbo boost gauge here.

Since I already have a boost gauge, I wondered what else could be done with the Economy gauge. What in a diesel looks like gasser intake manifold vacuum??? [wait for it . . . ] Yes, boys and girls, it's the transmission vacuum line! Mercedes designs the vacuum control circuit (the Vacuum Control Valve and its friends) to mimic gasser intake manifold vacuum, thus allowing them to use the same tranny for both types of cars.

Since vacuum is critical to proper operation of many things in a diesel Mercedes, it makes sense to monitor it. I've long had a gauge in a vacuum line under the hood but it would be nice to have a gauge the driver can see. Since the Economy gauge is, in fact, just a vacuum gauge, it's the perfect addition and gives you the following features (at no additional charge ):

1. Monitor engine vacuum from the driver's seat -- watch for leaks and failures, help diagnose problems with the a/c system.

2. Check the vacuum to the transmission -- compare readings when you make changes in an attempt to eliminate harsh shifting.

3. Economy gauge -- since it reads "lower" (towards the red zone) as you press the footfeed down farther, you can use it for its original (gasser) purpose.

It looks like something similar could be done in W123 cars, with even more uses, since 123s use engine vacuum for the door lock system as well as all of the 124 applications.

Practical aspects -- for a 124 diesel, you'll need a gauge panel (left 1/3 of 3-part instrument cluster) from a 300E. It should fit, mechanically and electrically, with no modifications. The fuel gauge will say "unleaded only", which can be painted out or the faceplate can be replaced with one from an older W126 gasser, which doesn't have the unleaded gas warning. (The gauge panel from the 126 cars won't work electrically.)

Caveat -- I haven't yet confirmed that the fuel, coolant temp, and oil pressure gauges are calibrated the same between 300D and 300E.

For 123 cars, I don't know what panel you can use. The 123 has a mechanical oil pressure gauge while the 126 is electrical. It will be more of a project than the 124. Did the 280E and similar gasser 123s have an Economy gauge?

Starting point -- 1987 300D Turbo (W124)




Use this gauge panel from a gasser W124 car (this one is from a 1989 300E)




Put a tee in the vacuum line from the blue "flying saucer" [located on the left (driver's side) inner fender] to the transmission. Run the line through the firewall into the passenger compartment so you can connect it to the "Economy" gauge.




If you don't want to paint out the "unleaded" warning, swap the faceplate for one from the gauge panel in an older car (this one is from a 1984 300SE (W126).




And . . . enjoy! I'll post an update when I get mine completed.

Jeremy