300D v 300SD componet question. - PeachParts Mercedes-Benz Forum

Ian White · #1 11-09-2009, 12:58 AM

I searched for this. What is the sensor that does into the bellhousing area on the engine. It looks like a magnet that is close to the flexplate. I has a wire that had a two prong connector on the end. This is on the 84 300D. Also there is a difference between the oil filter canisters on the 84 300D motor and the 81 SD motor, the SD's filter canister had the oil pressure sensor pointed straight back while the 300D's sensor is pointed forward and up slightly. Are the oil cooler lines different between the two engines? Where is the oil pressure senor pickup wire on the 300D? Is is in the glowplug harness like on the SD or is it attached to the car? Picts will be put on if need be.
Thanks in advance!

pawoSD · #2 11-09-2009, 08:46 AM

W123 300D (1984 and 1985 models) has a different tachometer sensor (at rear of engine bellhousing) vs the '81 W126 300SD which is at the front by the crank balancer.

Also, the W123 has a physical oil pressure gauge with an oil pressure line running into the dash, while the W126 has an electronic sensor for oil pressure.

toomany MBZ · #3 11-09-2009, 11:08 AM

^ There ya go.

Ian White · #4 11-09-2009, 01:50 PM

Quote:

Originally Posted by pawoSD

W123 300D has a different tachometer sensor (at rear of engine bellhousing) vs the '81 W126 300SD which is at the front by the crank balancer.

Also, the W123 has a physical oil pressure gauge with an oil pressure line running into the dash, while the W126 has an electronic sensor for oil pressure.

So looks like its time to swap glowplug harnesses huh? Also, my 300D engine does have a magnetic pickup sensor on the bellcrank, why does it have one against the flexplate? When I removed the engine in the 300D I had to disconnect the white wire running to the rpm sensor at the bellcrank, It was plugged into the bottom of what looked like the tach amp on the drivers fenderwell.

Biodiesel300TD · #5 11-09-2009, 02:20 PM

Quote:

Originally Posted by pawoSD

W123 300D has a different tachometer sensor (at rear of engine bellhousing) vs the '81 W126 300SD which is at the front by the crank balancer.

I thought the 123's had a sensor up by the crank pulley.

Ian White · #6 11-09-2009, 04:26 PM

Quote:

Originally Posted by Biodiesel300TD

I thought the 123's had a sensor up by the crank pulley.

I thought the same....

aaa · #7 11-09-2009, 04:36 PM

Crank sensor position is the same for both models. It was moved to the back in late 84, but the front sensor should still be there for diagnostic purposes, assuming someone didn't remove it. I'm not sure if the new rear setup will work with the 81's amplifier setup.

Best way to switch oil pressure sensors is to swap the oil filter housings.

toomany MBZ · #8 11-09-2009, 09:04 PM

^ Yep, it changed in 84.

pawoSD · #9 11-09-2009, 10:25 PM

Indeed, '84 and '85 still have the front pickup for the diagnostic socket (formerly location of tach amp) and the tach sensor was moved to the rear. Same on the W126's.

charmalu · #10 11-10-2009, 05:13 AM

The sensor is connected to the adaptor plate between the engine and trans. picks up the tach signal and sends it to the EGR amplifier that is behind the kick panel to the right of the passengers feet.

The pick up at the front of the engine really doesn`t do anything. just a diagnostic plug in for the shop.

the oil cooler lines are a different part# between the D and SD.

the 300D has an oil line that connects to the back of the gauge.
DO NOT start the engine if you disconnect it.

Charlie

Ian White · #11 11-10-2009, 03:09 PM

Quote:

Originally Posted by charmalu

The sensor is connected to the adaptor plate between the engine and trans. picks up the tach signal and sends it to the EGR amplifier that is behind the kick panel to the right of the passengers feet.

The pick up at the front of the engine really doesn`t do anything. just a diagnostic plug in for the shop.

the oil cooler lines are a different part# between the D and SD.

the 300D has an oil line that connects to the back of the gauge.
DO NOT start the engine if you disconnect it.

Charlie

So this 300D eng is going into my 300SD, I have swapped the cooler lines, filter housing, harness, but I am still wondering if the sensor at the front of the eng will work with the w126 tach amp setup. Thanks in advance..

toomany MBZ · #12 11-10-2009, 05:14 PM

Quote:

Originally Posted by Ian White

So this 300D eng is going into my 300SD, I have swapped the cooler lines, filter housing, harness, but I am still wondering if the sensor at the front of the eng will work with the w126 tach amp setup. Thanks in advance..

Depends on the year of both components. They, 123 and 126 had the same tach set up and tach instrument. As long as both are pre '84, it'll work.

aaa · #13 11-10-2009, 09:12 PM

The q was about the diagnostic-only front sensor in his late 84.

I think it'll work. If they wanted to use the same diagnostic equipment for all the years (thus keeping the port where the amp used to be), then it's probably the same sensor.

Ian White · #14 11-10-2009, 09:31 PM

Quote:

Originally Posted by toomany MBZ

Depends on the year of both components. They, 123 and 126 had the same tach set up and tach instrument. As long as both are pre '84, it'll work.

Well, what to do if the 300d motor is a late 84 one?

toomany MBZ · #15 11-11-2009, 12:38 PM

^ The 84 and newer has a pick up on the transmission and a different plug on the back of the tach itself.
But your swap will be easy. When you pull the engine out of the SD, unplug the wire leading from the crank sensor to the bottom of the amp. After the 84 engine is installed, plug that crank sensor wire into the bottom of the amp, you should have a working unit.
So your current tach in the car will work, no need to re-wire anything, just use the "new" crank sensor wire to plug in where you took the old one out.