85 300D. what's this sensor on the block? - PeachParts Mercedes-Benz Forum

funola · #1 06-10-2014, 11:01 AM

What is the first sensor to the right of the dipstick? The second one to the right is for the coolant temp gauge I believe. A quick search through the FSM wiring diagram turned up nothing.

Tanksowner · #2 06-10-2014, 11:27 AM

California or Federal? Could be something unique to the California models, but that's just a guess.

funola · #3 06-10-2014, 11:37 AM

Federal.

Stretch · #4 06-10-2014, 12:55 PM

I'd guess it is also a water sensor - I can't see what else it could be.

Where does the wire go? (Follow the bear)

Is it some sort of electro vacuum when the engine is cold thang?

jt20 · #5 06-10-2014, 01:11 PM

quite certain thats the temp sensor for the emissions system.

It works at a different range of resistance than the gauge sensor and you'll know when they're placed incorrectly.

mach4 · #6 06-10-2014, 01:36 PM

It's a coolant sensor. #83 in the following diagram

compu_85 · #7 06-10-2014, 01:45 PM

Yup, a water temp sensor for either the dash gauge or the EGR computer.

-J

funola · #8 06-10-2014, 03:03 PM

Quote:

Originally Posted by mach4

It's a coolant sensor. #83 in the following diagram

Thanks! That's the diagram I was looking for in the FSM CD I have but couldn't find it in 85 300D turbo. Where in the FSM is it?

RavenTBK · #9 06-10-2014, 03:40 PM

I haven't seen the FSM, but I have been told by several independent sources that one sensor is for the gauge, whereas the other water temp sensor is just a temperature switch directly related to the glow plug duration. I could be wrong, but I do know first hand you cant switch them around and have your gauge work.

DeliveryValve · #10 06-12-2014, 03:39 AM

Quote:

Originally Posted by funola

Thanks! That's the diagram I was looking for in the FSM CD I have but couldn't find it in 85 300D turbo. Where in the FSM is it?

You won't find it. I scanned and posted those '85 diagrams from my 1985 model year specific supplemental manual.

And that coolant sensor is very important for the transmission to shift properly and for the emissions equipment.

mach4 · #11 06-12-2014, 08:39 AM

Quote:

Originally Posted by DeliveryValve

You won't find it. I scanned and posted those '85 diagrams from my 1985 model year specific supplemental manual.

And that coolant sensor is very important for the transmission to shift properly and for the emissions equipment.

Thanks for scanning it! It's been invaluable many times over.

I don't understand how it's connected to the transmission for shifting properly. Can you explain?

Thanks again.

compu_85 · #12 06-12-2014, 09:00 AM

Some cars use a coolant switch to change vacuum solenoids for the shift firmness.

-J

charmalu · #13 06-12-2014, 10:43 AM

Quote:

Originally Posted by RavenTBK

I haven't seen the FSM, but I have been told by several independent sources that one sensor is for the gauge, whereas the other water temp sensor is just a temperature switch directly related to the glow plug duration. I could be wrong, but I do know first hand you cant switch them around and have your gauge work.

When I was having an over heating problem according to the gauge, my Indy mentioned to swap the two wires to see if the gauge changed, then I would know it could be the sensor. from that I thought they were the same Sensor, could be used in either hole.

Swapped the wires and the gauge read the same. was another problem I finally chassed down

Charlie

funola · #14 06-13-2014, 11:46 AM

I took a quick resistance reading of both sensors with a DVM and the sensor on the right (temp gauge) has approx 10x higher resistance than the one on the left (EGR control). So on my 85, I'd say they are not interchangeable.

DeliveryValve · #15 06-14-2014, 12:31 PM

Quote:

Originally Posted by mach4

Thanks for scanning it! It's been invaluable many times over.

I don't understand how it's connected to the transmission for shifting properly. Can you explain?

Thanks again.

I am going to be lazy and quote what I wrote up a few years ago in another forum

Quote:

Originally Posted by DeliveryValve

The ’85 transmission vacuum system does have a positive effect on shifting. When properly working, the shifts should be both precisely smooth and firm at varying acceleration conditions compared to the earlier simpler vacuum system.

The earlier turbo models had basically a direct vacuum source from the main line to the trans vacuum modulator, with a Vacuum Control Valve (VCV) - #65 on the diagram, in the middle to leak down the pressure. A high vacuum pressure around 10-15 Hg leaked towards zero at a different pressed throttle position with a full throttle position at zero Hg.

The vacuum modulator with high vacuum softens the shifts at a low throttle response and a modulator with lower vacuum firms up the shifts with more throttle. This is predicated only on throttle position and no other variables.

On the 1985 models, Mercedes engineered a trans vacuum system that didn’t just rely on throttle position to control the softness or firmness of shifting. It also relies on turbo boost pressure which becomes dependent on engine load and temperature.
The vacuum transducer (amplifier) (#123) works in a system. It works in relation to turbo boost pressure and engine temperature.
Unlike the prior years, the system now starts off at a high vacuum of 12-17 Hg to the modulator. It now leaks down five inches when full throttle travel is given. When the turbo boost pressure goes up, the boost applies an input to the vacuum transducer at connection PRE on the diagram. This produces an upward deflection of the transducer diaphragm, lowering the amount of full vacuum applied to the modulator valve to zero. The hydraulic modulating pressure in the transmission is increased which firms up the shifting.

At a coolant temperature below 50º C, a temperature switch (#20) cuts off vacuum signal from the VCV at a Switchover Valve (#125) on the diagram. The vacuum in the circuit produces a higher vacuum to the transducer from a direct line to the vacuum pump (#76). Then this higher vacuum is sent to the modulator softening the shifts while the engine is cold.
When the coolant temperature rises above 50º C, the switchover valve is powered on and the VCV controls the transducer. The vacuum circuit takes over as describe above which an increase in throttle/power load produces less vacuum to the transmission modulator, thus firming up the shifts.

This is kind of a long winded statement and hopefully it will give some info to make a decision of what direction you want to head towards.
One other note, If you make the vacuum circuit simpler by using the 1981-1984(Fed) model, the modulator will have to be adjusted/tweaked to shift in an acceptable manner.

The '85 300sd and 300D came with 722.416 transmissions for the California version and 722.315 & 722.303 transmission for the Federal version. They then used a version of this vacuum setup on the 603 powered w124 with the 733.4 transmission and the 603 powered w126 with 722.3 transmissions on the later years as well as some other vehicles. So Mercedes thought the improvement was worthwhile to carry it over to the newer models.

With that in mind, theoretically you could use this system on an earlier model and get good results. But only if your able to tweak it to work properly. What you lose is the simplicity of the old system in which more items can now fail over time.