Speedometer/trans/differential question.
 

When I inquired about which speedometer to use on the diesel into a gas body I was persuaded that in order to get a proper reading on the speedo I'd have to use the speedo from the diesel car.

Both cars are early w126 and I wanted to use the differential from the gas setup because I like the gear ratio. Now I read on the thread about differentials that in order to use the gas differential I'll need to use the gas speedometer to get an accurate reading.

The diesel engine and transmission came from an 84 300SD and went into a 85 380SE body. The original differential got a yoke change to accommodate the diesel drive shaft.

Which speedometer will work? If neither with this setup what should I do to fix it short of changing the differential?

I don’t think it’s as much gas vs Diesel speedometer as keep the speedometer with the differential. In the W126 series, the speedometer is set to the differential ratio.

Sixto
98 E320s sedan and wagon

I could have sworn someone told me I'd have to change the rear housing on the transmission if I wanted to get the gas speedometer to be accurate.

So to achieve accurate readings I should use the speedometer from the gas setup with the gas differential?

If you’re talking ‘84 SD transmission into ‘85 SE body then all you need is for the speedometer to match whichever differential you’re using (3.07 from SD or 2.47 from SE). The SD transmission should have the same speedometer drive as the SE transmission. It’s a W123 transmission that needs conversion from gear drive to work with a W126 speedometer. Does the SE have traction control?

Sixto
98 E320s sedan and wagon

Yes, the SE has a wire coming from the differential which is absent on the SD. Is that traction control? If so, how is it going to work without the electronics of the gas ignition?

That’s more likely the ABS speed sensor which isn’t associated with speedometer or engine function. If the SD differential doesn’t have an ABS speed sensor, you might have a persistent warning light at best and intolerable brake function at worst. I don’t know if you can disable ABS by just unplugging the module or pulling a fuse.

I’m not familiar with traction control of that vintage but it would likely involve hydraulic components like a tandem pump and pressure lines to the differential.

Sixto
98 E320s sedan and wagon

The brake lines run through a module on the left front fender well just in front of the brake master cylinder. It seems since the wiring is there for the differential and all the rest of the brake system is intact it may work as it should.