big hiccup in gasser econ/boost gauge mod
 

I'm finally ready to attach the gasser economy gauge as a boost gauge and I've hit quite a snag. There are significant funcitonal differences in the left clusters although there are no structural differences. Between an early 80s 380SE left cluster and 87 SDL left cluster the relevant differences are:

- the round connector of the 380SE is missing two pins
- the 380SE doesn't have provisions for oil level, coolant level and washer level warning lights
- the park brake and service brake warning light positions are reversed

I don't know if these are as much problems as inconveniences. The potential problem is that if the pin-outs are different, it might cause problems with dash illumination unless I do some rewiring.

An alternative is to transfer the 380SE gauge unit to the SDL left cluster board. The trouble is that the SDL cluster board has a resistor where the economy gauge body goes and traces where the economy gauge input nipple breaks through the board. Not only that, it requires undoing solder joints that are difficult to reach. It might be possible to undo the solder joints by heating the back side of the board, but I don't know how to get a solder sucker thingy to the front side of the joint so the solder doesn't take a set while I'm undoing another solder joint. Aiya!

I'll look for the left cluster out of an 87 420 or 560 and see if the arrangement is more compatible with the SDL. Maybe the 380 left cluster will work with an SD.

Sixto
95 S420
87 300SDL

Keep us updated as to your progress. I've been trying to locate an early '90's 300 e cluster for this same modification. I wonder if the 124's have the same interference issues?

Economy/boost gauge conversion
 

I made this modification today on my 1987 300D. Should be similar to the SDL. The donor was from a 1987 300E. Only irreversible modification was drilling two holes, one through the circuit board and the other through the instrument face. It would have been easier to use the donor instrument face, but I didn't want the "Unleaded Fuel Only" sign looking at me. Routed the pressure hose through the firewall at the rubber manifold holding the vacume lines, which was easily accessed through the void in the dash left from removal of the instrument cluster.

Did you mark the instrument face to delineate the various boost pressures?

Economy/boost gauge
 

I didn't mark the instrument face because it would have looked like graffitti with my artistic skills. The needle rests at nine o'clock with no pressure and swings counterclockwise to three o'clock at fifteen lbs.. Used the Mityvac's pressure side to see where the needle would be. My boost is .9 bar or 12.9 lbs. (checked with another boost gauge) and the needle ends up at about four o'clock at max boost.

Mike,

Wow - congrats! I bought a donor pod and also planned to use the 3- and 9-o'clock positions, which gives a range of about 5-15psi IIRC. Could you tell us more about how exactly you did the conversion? Did you tear apart the donor gauge and remove the Bourdon tube assembly, and mount that in your stock cluster? I wasn't going to attempt that because it looked like it was too difficult. What are you using for stop pins at each end? Any photos? Enquiring minds want to know... :D :D

economy/boost gauge
 

Dave

Too difficult for you-nah. It took a couple of hours. Toughest part was reinstalling the needles on the delicate wires through the holes in the instrument face. I removed the Bourdon tube assembly from the donor and simply grafted it into the 300D's left instrument pod. There was a "X" on the underlying circuit board indicating where to drill the hole for the tube portion that the pressure hose is attached to. Overlay the 300E instrument face on the 300D's to mark the spot where the needle inserts and drill another hole. The Bourdon assembly mounts on the 300D subframe with two screws, no modification needed. There are also threads at the base of the pressure tube that accept a nut and washer from the rear. No stop ppins are needed and I think they woul dislodge the needle if it banged into them. A photo would only show an extra needle at 6 o'clock on the intstrument face pointing west at rest.

Mike, thanks for the details. It sounds like your donor has less range than mine and Sixto's. We both saw about 270 degrees movement from 0-15psi, i.e. if it starts out at 9 o'clock (west) at rest/zero, it would be in the 1-2 o'clock range at 12psi, and pointing north (noon) at 14-15 psi. My donor was from an early 300E also, I don't know the exact year. I wonder if there is some way to adjust the movement? Yours sounds PERFECT, I was going to have to "settle" for 5-15 range instead of the 0-15 that I wanted. Hmmm. Now I have some incentive to go play with it again! :D