Any way to calibrate a W123 speedo? - PeachParts Mercedes-Benz Forum

JackG · #1 08-10-2008, 10:10 PM

My guess is no, but if anybody knows how to adjust it
I would love too know. I did a road trip this weekend
to the Jersey Shore and took along the GPS. I check
the speedo at 30,40,50,and 60MPH
speedo............................ gps
30................................... 32
40................................... 43
50................................... 53
60................................... 64
This is on an 82 300CD and I don't recall anything in the records
about the rear being swapped.
the tires are 195's as they should be.
Is this a normal tolerance or is there so,e other reason?

On a more positive note, this means my "corrected" MPG
is 32+ mpg

.

Jeremy5848 · #2 08-10-2008, 10:32 PM

That's probably reasonable. The speedo may be reading low because of friction due to wear and tear since 1982; you could have the speedo overhauled and it might help, might not. A speedo can be recalibrated for a change in transmission or rear end gearing or different size tires by introducing a little gear thing in the speedo cable. Auto repair places usually have access to such things.

Also, just because the speedo is low doesn't mean the odometer is also low, so don't go bragging on your mpg yet.

79Mercy · #3 08-10-2008, 10:38 PM

I have a 115 MPH speedo in my spare parts stash, shoot me a PM if you want it.

JackG · #4 08-10-2008, 10:40 PM

Quote:

Originally Posted by 79Mercy

I have a 115 MPH speedo in my spare parts stash, shoot me a PM if you want it.

Thanks, but no. I want to keep the original
as it has documented mileage.

JackG · #5 08-10-2008, 10:42 PM

Quote:

Originally Posted by Jeremy5848

Also, just because the speedo is low doesn't mean the odometer is also low, so don't go bragging on your mpg yet.

Had to pi$$ on my parade.....didn't ya

.
I guess now I have too go back out and try again.

Mark DiSilvestro · #6 08-10-2008, 10:48 PM

Are you sure your odometer is also off? If only the speed indication is off a bit, it may be possible to make a slight adjustment. If both your speed AND mileage are off, you'll need a different gear, or to figure out where the error might originate - rear end or tire-size.

I have a prob;em with my stickshift 240D speedometer. Both the speedometer and odometer read about 10% slow. The speedometer was mistakenly replaced by the PO when it really only needed a new cable. Of course the original speedometer was disposed of beefore I bought the car.
And in the case of my Euro 300TD, The speedometer and odometer reads about 15% too fast. I suspect that my original KMPH speedometer was replaced with a USA-spec MPH turbo-diesel speedometer - compatible with a 3.07 or 2.85 rear end, but not with my non-turbo wagon with it's a 3.47 axle ratio.

Happy Motoring, Mark

Jeremy5848 · #7 08-10-2008, 10:50 PM

Quote:

Originally Posted by JackG

Had to pi$$ on my parade.....didn't ya

.
I guess now I have too go back out and try again.

I'm only jealous of your mileage!

Brian Carlton · #8 08-10-2008, 11:52 PM

Quote:

Originally Posted by JackG

My guess is no, but if anybody knows how to adjust it
I would love too know. I did a road trip this weekend
to the Jersey Shore and took along the GPS. I check
the speedo at 30,40,50,and 60MPH
speedo............................ gps
30................................... 32
40................................... 43
50................................... 53
60................................... 64
This is on an 82 300CD and I don't recall anything in the records
about the rear being swapped.
the tires are 195's as they should be.
Is this a normal tolerance or is there so,e other reason?

On a more positive note, this means my "corrected" MPG
is 32+ mpg

.

The speedometer error is almost precisely the difference between the 2.88 and 3.07 diff. If you find the odometer also shows less than the actual miles traveled by 6.6%, it's time to check which diff is in the vehicle.

charmalu · #9 08-11-2008, 12:15 AM

I have two speedometers sitting here on my desk. one out of an 85, and the other out of an 83 300D.

the 85 with the 2:88 diff, has a spedo with the # 123 542 27 57

the 83 with the 3:07 diff, has a spedo with the # 123 542 18 57

Both spedometers are out of 300D turbodiesels.

Charlie

bgkast · #10 08-11-2008, 01:09 AM

Not to hi-jack but I was on a road trip in my '79 240D this weekend and discovered that my odometer is "fast" by 1/10 of a mile every 4 miles. I am running 195 tires in place of the factory 185s, but I believe they have the same diameter as stock. I need to check the speedometer with my GPS.

rcounts · #11 08-11-2008, 01:15 AM

Ditto on the speedo and odo being off by different amounts.

My speedo reads about 5% HIGH (GPS says I'm going 60 - speedo says 63)
My odo reads about 4% low (GPS says I drove 81 miles - odo says 77.7)

How do you compensate for that? If I were to change tire diameter OR rear gearing to make one of them more accurate, the other one would then be even farther "off"...

Mark DiSilvestro · #12 08-11-2008, 01:32 AM

Quote:

Originally Posted by rcounts

Ditto on the speedo and odo being off by different amounts.

My speedo reads about 5% HIGH (GPS says I'm going 60 - speedo says 63)
My odo reads about 4% low (GPS says I drove 81 miles - odo says 77.7)

How do you compensate for that? If I were to change tire diameter OR rear gearing to make one of them more accurate, the other one would then be even farther "off"...

If the low odometer reading isn't caused by a slipping gear (very common) first try to check/repair whatever is causing the odometer error. Once the odometer is fixed, the speedometer needle/spring can perhaps be adjusted to bring that closer to accurate.

Happy Motoring, Mark

rcounts · #13 08-11-2008, 06:03 PM

Quote:

Originally Posted by Mark DiSilvestro

If the low odometer reading isn't caused by a slipping gear (very common) first try to check/repair whatever is causing the odometer error. Once the odometer is fixed, the speedometer needle/spring can perhaps be adjusted to bring that closer to accurate.

Happy Motoring, Mark

You must've missed my post about doing the odo repair last Friday. Instead of the screwdriver and locktite action (as shown on the DieselGiant site) I used locking pliers to knurl the shaft where the pot metal gear was slipping. Once the newly-knurled part of the shaft got to where it started to slide back into the pot metal gear, I literally had to drive it back in by tapping on it using the butt of a screwdriver. Took some pretty serious whacks ot get the knurling back into the potmetal gear (I was almost afraid it was going to split it fit so tight).

It is 100% safe to say there is NO slippage going on in this odo since all the plastic & brass gears were in perfect shape, and the potmetal gear is so tight on the shaft now.

I just drove 287 miles (measured by both MS Streets AND by my GPS) and the odo shows 281 miles. So it isn't quite as far off as I thought - only about 2% low. Close enough to not want to mess with it...

Speedo still reads 5% faster than actual speed though.

Mark DiSilvestro · #14 08-11-2008, 08:03 PM

Quote:

Originally Posted by rcounts

You must've missed my post about doing the odo repair last Friday. Instead of the screwdriver and locktite action (as shown on the DieselGiant site) I used locking pliers to knurl the shaft where the pot metal gear was slipping. Once the newly-knurled part of the shaft got to where it started to slide back into the pot metal gear, I literally had to drive it back in by tapping on it using the butt of a screwdriver. Took some pretty serious whacks ot get the knurling back into the potmetal gear (I was almost afraid it was going to split it fit so tight).

It is 100% safe to say there is NO slippage going on in this odo since all the plastic & brass gears were in perfect shape, and the potmetal gear is so tight on the shaft now.

I just drove 287 miles (measured by both MS Streets AND by my GPS) and the odo shows 281 miles. So it isn't quite as far off as I thought - only about 2% low. Close enough to not want to mess with it...

Speedo still reads 5% faster than actual speed though.

I remember the post, just not the author. I use wire-cutters to 'notch' the shaft when I fix one of these.
Anyway, I wish my two diesels only had a 2% odometer error. I have to add about 10% to my 240D and subtract 15% from my 300TD odometer reaings in order to calculate the MPG on the two cars!

Happy Motoring, Mark

rcounts · #15 08-11-2008, 08:10 PM

Quote:

Originally Posted by Mark DiSilvestro

I remember the post, just not the author. I use wire-cutters to 'notch' the shaft when I fix one of these.
Anyway, I wish my two diesels only had a 2% odometer error. I have to add about 10% to my 240D and subtract 15% from my 300TD odometer reaings in order to calculate the MPG on the two cars!

Happy Motoring, Mark

Yeah, 2% is more than close enough. I just think its funny that the odo reads low and the speedo reads high. Of course since the odo is a direct drive and the speedo movement is created by spinning magnets, it really isn't too surprising that they aren't in perfect sync. I haven't checked one against the other in any of my other vehicles, but I'll bet it is more common than most of us realize...