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

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Don't forget to fill up with premium unleaded fuel!

I have thought of using the economy gauge for the same purpose, but the only one I have found was for a W126, and that was before I got my W126.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

When I found an economy gauge from a 1985 280CE at the local pick-a-part, I did the same thing. I tee-d into the transmission vacuum line and it functions like the gauge on a gasser. Gassers produce about twice as much vacuum at idle than what's recorded at the VCV on diesels, so the gauge only reads about halfway (that registers about 12-13 in.Hg vac) at idle. It'll drop as you give it more foot, but it's not good for anything besides eye candy and another moving gismo in the cluster.

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

He is right, its not too accurate and its slow to respond so its really only good for eye candy.

Its still nice to have though. It fills in that dead area very well.

Cool retrofit, but near useless.

When I put a gas fuel/temp/oil econ in my SDL, the fuel level read 2X as high, so at 1/2 tank on the gauge, it was nearly empty....

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91 350SD
14 F150 Eco
19 Fusion Hybrid
11 GT500

Very creative work, thanks for the info...

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'01 Escape 2.0 5 speed- 101k
'96 Cherokee 5 speed- 120k
'92 Mercedes 2.5l turbo diesel- 212k
'90 Aerostar ext Cargo 4.0- 29k
'02 ZX2- 93k
'98 F250 CNG- 54k

I've always had a soft spot in my heart for vacuum gauges since the one I had in my 69 Ford van allowed me to diagnose a bent pushrod in about 10 seconds (the gauge was fluctuating since that cylinder wasn't pulling vacuum anymore). I think I'd want it tied into the main vacuum line to monitor the overall state of the vacuum system. It could give you early warning that your brakes are going to be hard.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

That is certainly possible and one could think of many places in the vacuum system where a gauge could be useful. I have had a gauge in the main line for years. It is very nice to have but it is under the hood and hard to see while driving.

The "Economy" gauge pins high (left side, "in the black") at about 17" Hg, while the main line of a diesel vacuum pump (in good repair, no leaks, etc.) is typically over 22" Hg. The "Economy" gauge would thus spend almost all of its time pinned high, never moving. I would get bored and forget to watch it. In the transmission line it lives in the middle of the range, moving with your right foot. Over time, you'll get used to seeing it and should be quickly aware that something is wrong in the vacuum system if the gauge begins acting up.

A crude calibration of my "Economy" gauge is shown in the following picture. The gauge is shown at 5, 10, 15, and (maximum) 17 inches of "Mityvac vacuum." The range is ideal for monitoring vacuum in the line to the transmission modulator.



The choice of which "donor" gasser to use for the Economy gauge is clearly important and there is no manual or guide. I recommend using the model that is closest to your diesel. A W123 diesel should work with a similar gasser -- a 280E, for example. A 300SD could use a 300SE/SEL of similar year and chassis. Thus, for my 1987 300D (W124), I found a 1989 300E, which is an almost identical car. Here is a photo of the rear of the instrument cluster for these two cars.



The only difference is that the 300E has a fuse and a vacuum connection for the Economy gauge. The gauges are otherwise mechanically and electrically the same. My tests today showed that the two sets of fuel, oil pressure, and coolant temperature gauges responded the same in my car.

Next: Installation proceeds.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

The four-gauge panel is finished, installed in my '87 300D, and working. Changing the three-gauge panel (originally in the '87 300D instrument cluster) for the four-gauge panel of the 1989 300E was easy. The 300E version of the instruments (coolant temp, oil pressure, fuel) appear to work just like the 300D gauges. It is plug and play.





Changing the faceplate for an older one that doesn't say "unleaded only" added a lot more work -- I would say it doubled the time needed to do the job. It also means you must find an older cluster to rob of its faceplate, one that doesn't say "unleaded only." You might prefer to simply paint out the warning with some flat black paint and a small brush. I wouldn't advise leaving it unmodified, some idiot might try to "help" you by filling the tank with gasoline ("but that's what it said . . . ).





The older faceplate must be reamed out to accommodate the plastic support piece of the newer gauge set. You could also swap the plastic pieces and leave the faceplate alone.





Routing the vacuum line to the "Economy" gauge is easy. There's a multi-hole grommet in the firewall behind the brake booster. Anything passing through this grommet comes out behind the instrument cluster. Tee into whatever line you want to monitor and pass the line through the grommet and attach it to the gauge.

Best of all, it's reversible -- if you change your mind, you can always put your old three-gauge panel back.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970