PeachParts Mercedes-Benz Forum - View Single Post - Another diesel use for the gasser "Economy" gauge

Jeremy5848 · #9 08-09-2008, 01:31 AM

Quote:

Originally Posted by kerry

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.

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