Check this out...I'm going about rebuilding my first Benz..a 1982 300SD I'm working on the 3/2 vacuum "situation" and I see this strange sensor where there should be a vacuum thermo valve


So I bought a thermo vac valve, and the diameter of the threads is significantly smaller than the hole this sensor is mounted into...can anyone shed some light here? I cant find any info about this in the FSM, or elsewhere...maybe I havent looked hard enough?

another view..


any info you vets could share with this n00b would be greatly appreciated!!

In as much as not all Benz models are the same, maybe you could let us know exactly what it is that we should check out.

Whatever the model, it is a sure bet that a vacuum hose doesn't belong on an electric thermal switch.

Year and model..........

it's a 1982 300SD sorry...I edited the post to include that info

On the SD, the vacuum thermo swtich only serves to cutout the EGR. The 3/2 valves do the same thing. You can, and probably should, eliminate all the vacuum connections to the 3/2 valves, the vacuum thermo switch, and the EGR valve.

There are two T's on the driver's side of the vehicle that can be eliminated to effect this.

You are worried over a none issue.
Disconnect and plug the EGR vacuum circuit, it is useless + sludge's the intake ports/manifold.

The device in the thermostat housing is a temperature sensor, look for the wire, but if all the gauges work = someone may have swapped out a corroded thermostat housing for one from another engine.

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@whunter; Thanks for the good advice, I read alot on this forum about people doing the same. @Brian Carlton; what about the vac line from the 3/2 to the transmission modulator? flaring shifts from 1st to 2nd and 2nd to 3rd are what got me on this quest. I watched most of Kent Bergsma's videos and bought his book on Transmission Tuning..he seems to feel that disabling the EGR is not the way to go...but he's the only one I've seen speak against doing so.

I'm not opposed to following your suggestions, I do however want to get the shifts right to preserve the life of the transmission. What do you suggest? I'm going to go read up now on ERG deletions.

Again thank you all for your input and please keep the advice coming!

__________________
-SpecialDelivery
1985 300CD Silver/Blue H&R Suspension (Sold, still cryin over that)
1982 300SD Silver/Blue '85 OM617 (Sold)
1982 300D - Blue/Blue (Sold)

on my 1983 300CD. It cleaned up the engine compartment considerably. The EGR valve and vacuum lines are now sitting on my shelf. The thermo vac valve is plugged and when I find a right-sized pipe plug that will be gone also.

The black box on top of the cam cover now only serves as a pivot point for the throttle linkage.

Transmission vacuum is supplied from a wye connector attached to the green vacuum dashpot. I also eliminated vacuum to the door locks and seats since they are only potential trouble-making leak sources.

Shifts are still a little hinky when cold but when the tranny is warmed, they are smooth as silk. If you are still having flaring issues, check your shift moulator valve. Put your MityVac on the line and see if it accepts and holds vacuum.

All the more reason to disable the EGR system. The only affect of the EGR system on the transmission is a potentially negative one; the 3/2 valves leak and rob the transmission of the vacuum that it needs to shift properly.

OK I'm sold. I pm'd BC after reading exhaustively on this last night. today I'm going to plug the lock hose as I know they're connected but not working right. They did when I first got the car but now they dont, and the passenger door likes to lock itself as soon as you unlock it and put the key back to perpendicular with the ground. I did MityVac the modulator and it does hold vacuume, I have not however done a pressure test to see what it's making. If i can get ahold of some 3/8 line and fittings I would like to do that today.

__________________
-SpecialDelivery
1985 300CD Silver/Blue H&R Suspension (Sold, still cryin over that)
1982 300SD Silver/Blue '85 OM617 (Sold)
1982 300D - Blue/Blue (Sold)

That is not a vacuum problem; it's a mechanical linkage issue. Only the driver door lock changes the vacuum lineup.

Understand that, on your vehicle, there is no line from the 3/2 valves to the transmission modulator. When you first observe the system, you will see the supply vacuum split at the T and you will observe the vent lines split at another T, but these splits take the vacuum to the EGR system, independent of the vacuum control valve. The vacuum control valve is the white plastic unit at the rear of the IP and it has sole control of the modulator. It is the device that will be tuned to achieve the desired vacuum levels at the trans.

With regard to flaring shifts, if the vacuum is a bit high, it can be tuned lower and the flaring might be improved. If it isn't, the modulator pressure can be slightly increased to achieve the same result. All of these changes are tuning to overcome wear internal to the transmission.

Kent is not wrong when he advocates maintaining the EGR system. In theory, the EGR can coexist perfectly with the transmission modulation system, as it was when the vehicle was originally delivered. However, on a old vehicle with many items out of specification, attempting to maintain the EGR complicates the process, especially if any of the components in the EGR system are leaking in any way.

It is not a sensor and it has nothing to do with gauges. It's a thermal switch that prevents the heater from blowing cold air.

If the system is functional without it, there is no issue.
I note there are two switches, top and side of the thermostat housing, and the side unit is connected.


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