I was replacing my battery on my 1984 300cd and the plug that comes from a wire behind the battery and has a three pin plug that plugs in along the firewall in the middle came apart and the plastic sleeve that holds it to the other connector crumbled. The car still runs but what is that for?
Thanks for your help in advance.

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1982 300TD
1973 220D ambulance
1974 240D pick-up

Sound like the connector for the "rack position sensor" on the injection pump. I think it began in 1984. It is only used for the EGR computer box (behind passenger kick plate). If like me, you installed an earlier engine or injection pump, it makes sense that connector would be open. Most likely, your EGR components are no longer working and new replacement parts are not available. There are drawings here showing how to remove all that clutter.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Thanks Bill. I was staring at that connector last night. I had no idea where it went or what it was. The tach sensor connector is clipped right next to it (two pin plug) and I wiggled it and the clip snapped. Brittle 37 year old plastic. Another zip tie for my harness.

OT: I was trying to trace the tach sensor (85 bell housing style,) to the harness behind the gauge. A little confusing since originally I thought the EGR computer was a simple pass through for the tach signal…it isn’t as verified by a continuity check. Jumping the connector however works for some quirky reason. Had me researching some of your old posts about it.

The three pin plug is item 64 in the 85 wiring diagram below.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 190k miles
97 C280 150k miles

Thanks for the help. I haven’t driven this car for two years and it seems slow no pep. Could that be the cause? Because the plug is unplugged?
Still love the car……

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1982 300TD
1973 220D ambulance
1974 240D pick-up

My understanding (which could be wrong) is that fuel rack sensor is an input to the EGR “computer”. Unhooking it shouldn’t change the operation of your pump. Perhaps it’ll make your car activate EGR at an in opportune time thus decreasing power. You can test this by plugging your EGR line and going for a drive.

Good luck.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 190k miles
97 C280 150k miles

Yes, the tach signal from the transmission bell-housing pickup in 1985 routes thru the EGR box (behind passenger kick panel). But, you can remove the EGR box and jumper across the two associated connector pins to pass it on to the dash gage and Klima box. I did so, using web info (google) when my EGR box stopped working due to the Over Voltage Relay above it failing (not just its fuse). Rather than fix that special relay, I just ripped out the EGR box since it does nothing in my cars and the OVP relay is just to protect it. If you don't pass on the tach signal, your dash gage won't work and your AC compressor won't actuate (only 1985+).

As I recall, my 1984 300D also had the tach sensor on the bellhousing. The front crankshaft sensor has a cable which ends at the "diagnostic plug" on the L inner fender. That is to connect to a special dealer box, otherwise it is just open. I would remove that pickup since it just collects grub, except the wires are hard-wired so would require snipping. In earlier cars, a little electronic box plugged to that connector to generate the tach signal. There is at least one year which has a different dash tach than 1985, having internal electronics, perhaps to amplify the raw tach signal (google). Something like that.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans