Speedo options after swapping diff in 1981 300TD - PeachParts Mercedes-Benz Forum

240dddd · #1 04-01-2018, 03:24 AM

I just changed the differential in my 1981 300TD. I swapped in a 2.88 diff from an '85 sedan. The original diff had a sensor installed for ABS, although the ABS had never worked for me since the front wheel sensors had been removed by a PO. The newer 2.88 diff does not have a hole for the ABS sensor.

After finishing the diff swap and removing the ABS sensor my speedometer stopped working. It seems the 1981 300TD did indeed have an electronic speedo and I suspect it was actually driven by the sensor in the diff.

Assuming I am correct about the diff driven speedo (please let me know if I am wrong) I need to figure out what the other options are possible for a speedometer.

This car is a bit of a Frankenstein. A PO added an OM617 turbo engine and a newer transmission. The transmission in the car is a 722.315 and has the sensor for a tachometer. I know this because I was able to get a tach working using the wires coming off of the transmission. I think this means it was probably a transmission from a later model w123, i.e. 1984 or 85.

My question is whether my electronic speedo can driven off of a sensor on the 722.315 transmission? Or is my best bet to track down a cable driven speedo?

interestingly the speedo does have the circular connection that screws onto the back of the speedomter which I thought was the speedo cable?

97 SL320 · #2 04-01-2018, 09:27 AM

See my post in your original thread here. Given the new info you posted, the sensor wires on the trans would be for an electronic speedo and won't give an accurate tacho reading as the tach should read engine RPM not drive shaft speed.

http://www.peachparts.com/shopforum/diesel-discussion/392245-differential-rebuild-2-88-gears-into-housing-abs-sensor.html

tdoublenastywitit · #3 04-01-2018, 10:05 AM

Buy the 2.88 speedo from the guy selling his w123 in for sale section. He is parting it out now.

240dddd · #4 04-02-2018, 11:42 AM

I'm using the tach from an '85 donor car so it shows accurate readings (see the thread about that install here http://www.peachparts.com/shopforum/diesel-discussion/390289-adding-late-model-w123-tachometer-3-pin-earlier-model-car.html)

I have the speedometer from the same '85 donor car available, would that speedometer be driven off of the same sensor that is driving the tach, or was the 85 speedometers driven by a cable?

97 SL320 · #5 04-02-2018, 11:54 AM

I just remembered something. Are you using a sensor at the trans bell housing? If so, this reads from the engine flywheel and will give an accurate tacho reading.

The sensor I was thinking of would be at the trans tail shaft. Does your tail shaft have wires or a place for a speedo cable? You may be able to swap trans sensors / speedos to get something that will work.

ROLLGUY · #6 04-02-2018, 12:27 PM

In the 123 chassis, the wagons have electric speedo's, sedans and coupes are mechanical. If you put an '85 sedan diff and speedo in your wagon, you will need the trans tail shaft housing and related parts (gears, cable etc) to make your wagon work with a cable speedo. This is only possible if both yours and the donor have the same trans (722.3XX to 722.3XX). If one has a 722.4XX, the parts are not interchangeable. As others have said, the tach is triggered from the flywheel on the '85, and not from the harmonic balancer ('83 & earlier). The entire tachometer system on the two systems are completely different. An ABS equipped car may have yet another altogether different system for the speedo/tach. You may be only able to get the speedo/odo working with the cable setup, but not the tach.

240dddd · #7 04-02-2018, 03:17 PM

Like I said the wagon has been frankensteined over the years. A PO installed a turbo engine and different transmission than what was original to the car. The transmission must have been from a later model 722.3xx trans because it already had the tach sensor installed on the bell housing. I was able to connect to a tach I took off an 85 donor sedan.

I'll have to check and see what kind of speedo connection is on the trans. I didn't keep the transmission from the donor 85 sedan and never checked to see if it was a 722.3xx or 722.4xx.

resto108 · #8 04-02-2018, 03:25 PM

Everything I've read says that wagons used electric speedo in 81 only. 82-85 wagons are mechanical, like my 82 wagon.

vstech · #9 04-02-2018, 05:47 PM

Iirc, the 81 wagon used the transmission from an SD in it, as the 722.318(I think... whatever the 123’s got in 82) and the diff would have no contribution to the speedo at all. However, I am unaware of how the early abs system worked. Amazing to even have that in 81...

If all you changed was the diff, then the speedo would work, but have a reading indicating a slower speed than traveled. If you also changed the speedo... yeah, you are going to have issues, unless you also change the transmission and installed the mechanical cable to drive the newer speedo.

vstech · #10 04-02-2018, 05:50 PM

If the P.O. changed the engine, but not the transmission, that is why your original speedo was working.

The ONLY transmission that works in an 81 TD is the original 81 trans... the tach pickup would be on the engine intermediate plate, not the transmission... it picked up off the flywheel.

vstech · #11 04-02-2018, 05:51 PM

And lastly... no.
You cannot swap tailshaft parts from one transmission to get an electronic or mechanical trans to work with the opposite. One is not compatible with the other.

240dddd · #12 09-19-2018, 12:02 PM

I still haven't sorted out why my speedometer stopped working after I put in a 2.88 differential.

What drives the speedometer on an 85 that came stock with the 2.88 gearing? Were they mechanical or electronic speedometers. If the latter, where would the signal come from for the speedometer?

resto108 · #13 09-19-2018, 12:58 PM

82-85 300TD wagon speedo is mechanical, driven from the trans tail shaft with a mechanical cable on the left side of the tranny.
81 300TD wagon was the first year for the turbo, so they used electric signal, same as 300SD. Its in the same place as where the speedo cable comes off the tailshaft.
I would say you either need a mechanical speedo drive tranny, or you could try putting a 126 speedo in your cluster and use the electronic tranny.

ROLLGUY · #14 09-19-2018, 03:19 PM

Quote:

Originally Posted by resto108

82-85 300TD wagon speedo is mechanical, driven from the trans tail shaft with a mechanical cable on the left side of the tranny.
81 300TD wagon was the first year for the turbo, so they used electric signal, same as 300SD. Its in the same place as where the speedo cable comes off the tailshaft.
I would say you either need a mechanical speedo drive tranny, or you could try putting a 126 speedo in your cluster and use the electronic tranny.

If electronic, a TD speedo is what is needed. I am not sure if a W126 speedo would interchange.

vstech · #15 09-20-2018, 10:36 AM

Grrr. I typed this huge response on phone... call came in and I lost it all...

1. What is on your transmission tailshaft? Cable or wires?

2. What is your speedo? Wired or cable?

81td should be wires. If trans has cable, but speedo is wired, the abs hookup on diff might have been rigged... weird.

Simple is get a cable speedo and cable from any 85w123 and itll work with a cable trans... or tie the electric speedo to the wires on the tailshaft of the trans...

Let us know what you have.