What sets the speedo ratios? - PeachParts Mercedes-Benz Forum

300SD81 · #1 05-27-2007, 04:15 PM

I pulled another speedo from a different car a few days ago. It reads way too fast. My speedo is intermittant. What part sets the ratio on them? A resistor or capacitor? I'd like to swap the part over so the new one reads correctly.

lietuviai · #2 05-27-2007, 04:17 PM

This is something I'm really interested to know myself. Got a subscription going on this one.

#3 05-27-2007, 04:23 PM

The speedo used in the SD's are electronic and can be reprogrammed.
The speedo used in the W123 (except the 1981 300TD) are mechanical and can't be altered to read differently.

Both are tuned to the rear diff ratio.

300SD81 · #4 05-27-2007, 04:30 PM

I have an SD. I'm trying to figure out which component to change, maybe even put in a variable one to fine tune it...

#5 05-27-2007, 04:43 PM

You should do some searches, I know I've seen a picture here that shows what tuner is used to change it's settings.

300SD81 · #6 05-27-2007, 05:03 PM

The only thing I can find are external things that can modify the signal to the speedo. I'd prefer to modify the speedo itself, with parts from my broken one that reads correctly.

lietuviai · #7 05-27-2007, 06:15 PM

So by the sounds of it, the mechanical ones can't be changed.

miljohnj5, try posting a new thread, you'll get more attention that way.

Mark DiSilvestro · #8 05-27-2007, 11:37 PM

Quote:

Originally Posted by lietuviai

So by the sounds of it, the mechanical ones can't be changed.

miljohnj5, try posting a new thread, you'll get more attention that way.

Yeah, my '81 240D's original 85 mph speedometer was replaced by the PO, with a 100 mph unit that reads 26% slow. I checked it against the highway mile-markers on a recent trip.

Happy Motoring, Mark

Whiskeydan · #9 05-28-2007, 12:06 AM

I believe all SD speedos from 81-84 would be calibrated the same. There was an axle ratio change in 85 which would require a different calibration. Also, SE, SEL (W126) would be different as they have different axle ratios from the diesel 126s.
If you have the two speedometers you could compare the components on the board. I suspect you will find one or more resistor color codes different.

The Porsche 911SC has a pot to calibrate the speed. Too easy.

lietuviai · #10 05-28-2007, 12:40 AM

Quote:

Originally Posted by Mark DiSilvestro

Yeah, my '81 240D's original 85 mph speedometer was replaced by the PO, with a 100 mph unit that reads 26% slow. I checked it against the highway mile-markers on a recent trip.

Happy Motoring, Mark

100 mph speedo in a 240?

Bill Ladd · #11 05-28-2007, 07:39 AM

Quote:

Originally Posted by Whiskeydan

If you have the two speedometers you could compare the components on the board. I suspect you will find one or more resistor color codes different.

Once you've found that, just replace it with an appropriate potentiometer and viola, adjustable ratio.

Mark DiSilvestro · #12 05-28-2007, 08:12 AM

Quote:

Originally Posted by lietuviai

100 mph speedo in a 240?

Yeah. The car in question is an '81 manual-shift 240D. The PO insisted that speedometer was the correct replacement for the original 85 mph unit. There was even a decal stuck to the instrument panel stating "This speedometer is a replacement and reads in excess of 85 mph etc...", but I verified the inaccuracy in the unit using the highway mile markers during a recent trip. The irony of all this is that after the PO replaced the speedometer, it turned out that the cable was bad. Meanwhile the original speedometer got tossed.
The diff. housing in this car is still marked with '3.69', so unless someone replaced the guts inside the diff. housing, it should be the speedometer that's wrong. I'm going to get an original 240D speedometer and see if that's the answer.

My other diesel, an '82 automatic 240D, still has it's original 85 mph speedometer, which seemed accurate when checked against one of those portable roadside radar speed-monitors, but I notice the engine revs a bit higher at highway speeds than my manual-shift car. I assume this is due to inherent slippage in the automatic as I don't believe it has a lock-up converter.

Happy Motoring, Mark

lietuviai · #13 05-28-2007, 01:08 PM

They should rev very close to the same since they have the same final drive ratio. There could be slippage but I wouldn't think that there would be too big a difference.
Worth starting a new thread?

#14 05-28-2007, 01:28 PM

Torque converter slippage should be about 500-ish rpm at highway speed.

My engine runs about the same RPM at 65mph in my 240D as it did in my 300TD because of torque converter slippage in the auto vs no slippage in the manual, despite the 3.69 vs 3.07 diffs.

#15 05-28-2007, 01:38 PM

Quote:

Originally Posted by ForcedInduction

Torque converter slippage should be about 500-ish rpm at highway speed.

Really? So you have 15% slip at 3000 rpm? Sounds like speedometer error between the two cars.

It's probably closer to 50 rpm than 500 rpm. I have no measurable difference in rpm between climbing and descending steep hills with the cruise control on.