1987 TDT Speedometer 10 mph off
 

On to next thing on my new 87 tdt -- W124. The speedo is about 10 mph slow exactly when checking with gps. I searched and didnt find anything.

Thanks for any help. I'm trying to find in fsm how speedo works.

what diameter are your tires in relation to the stock MBZ ones?

Is your mileage counter out as well?

I think they are right. Just got new rear tires put on and Firestone would have said if they were'nt spec i think. They are R15s Not sure exact numbers will check.

thanks

good question. I'll check that with gps too. I think it might be ok as my mpg 24 in city and 28 on highway. But That would be cool if it was reading less too :)

Also, i'm having trouble finding speedo and tachometer troubleshooting in the FSM. Anyone know how to find those? I can find R&R of gauges but not the diagram of system of gauges to the engine. Are these both electrical and thus just in the big wiring diagram pdf -- ETM.pdf?

thanks

The True Test is to find one of those police speed carts and see if it's really off, I've seen some GPS unit's poorly estimate Speeds.. thats the only use i've seen those mobile speed traps good for... not that I speed.

Actually now that this is all mentioned, to the OP:
Now would be a really good time to inspect your odometer gears. For my w123, VDO designed these speedo/odo units such that some of the odometer gears were plastic, some metal. This didn't work too well in the long run. Fortunately, my speedo/odo unit was really easy to replace. That's another story.

What I would do if I were you? I would look at the gears. If they show any decent amount of wear and tear, you now have 2 good reasons to pick up a replacement unit: you can now either eliminate or identify the speedo/odo unit as the problem, and get a better condition odometer that will not fail as soon. The For Sale/Fastlane section on this site can help you there!

Thanks all for help. Well, the tires are 195/65 R15. Are those oem? I'll have to check service manual as i don't have an owners manual.

The odometer works according to gps and mileage signs on road. This gps has been pretty accurate with all our other cars. So its just the speedo thats about exactly 10mph off. It is even like that from start. It reads nothing until i go over 10 mph. At 20mph , it reads 10mph.

Where does the speedo cable connect? Is it back by transmission, in a big black plastic plug? There is some cable back there on drivers side of tranny that has the plastic plug insulation busted a bit. I was pulling good size pieces of plastic from around it. The connector is a couple square inches at least i'd say. Don't know what that is and still kind find speedo and rpm function in fsm.

thanks.

Has the Diff been replaced? Mine has and I am 10mph above actual speed.

I use the GPS to check speed.

Don't know
 

Hmm, don't know. Is your diff a different spec? is it geared higher to get better mph?

Yes, a different spec. I could not find an exact replacement when mine needed to be fixed.

is it relative to speed or just off by 10mph all the time? maybe the needle got bumped?

Off by 10 all the time. I guess its a good way to be , if it is going to be broken.

It's likely IMO that the speedometer needle has been removed and replaced. Whether it was to paint the needle orange (they fade to yellow-orange) or to disassemble and repair the odometer, or to turn back miles, hard to say.

If it's off 10mph all of the time, needle is off. If it is off 10mph at 30, 20mph at 60, etc (linear error) then it is gearing or the wrong speedometer for the car/gearing.

Thanks babymog. So are you saying that i should be able to pull the needle off and rotate it clockwise (by about 10 mph ) , and put it back on? So it has some detents in shaft and needle. Sounds do able. Worth a try when i get in there.

Yeah, hard to tell with these old cars what they did to em except through owning one for a year . Makes a bit hard on buyers . I know the po said it was original miles, he got it form local german -- car fax says indiana car its whole life, a couple owners which is where i bought it. It has original #14 head. Tach doesn't work either and radio wont come on. Weird is driver seat is really comfy and passenger seat is worn. Wonder if they were switched. Ok with me though :) Could use a new passenger seat. (oops went off topic). All in all, i love this car -- the way it drives, the room, awesome -- but i don't like the engine as much as 617. Just hoping it is reasonably trouble free. Don't mind fixing things and tinkering but don't want to be stranded.

thanks

Tach not working?

Replace your ovp relay.

Maybe it will fix the odo too.

I think you have a different rear end or a different cluster causing the speed difference

I have the exact same issue On my 87 300D that I just bought. The speedometer being 10 miles an hour off. Found the thread whilst searching for an answer as your are.

Is the 87 spedo electric or mechanical like the 123? Iam not familiar with the w124.

As mentioned above, someone might have been repairing the spedo at one time. and if the needle was taken off, It might not have been replace in the same spot.

On the bottom edge of the spedo face is a small white mark to the right of the resting pin for the needle. this probably can only be seen with the spedo removed.

Remove the spedo from the cluster.

Gently lift the tip of the needle over the resting pin, and see where it falls down to. It should line up with the white mark on the edge of the face. If it is above or below the mark, it will not read accurate.

If it doesn`t line up, use 2 spoons with some tape on the back side. place under the needle and pry up. the needle will pop off the stem. It is just pushed on. line the needle up with the white mark, and push it down on the stem. lift over the resting pin and you are golden.

W124s have a mechanical speedometer.

-J

Careful! Two spoons method neatly ripped the needle WITH shaft and coil spring right out of my speedo. It actually snapped the shaft in two. No way to repair that mess. If your needle doesn't pop off the shaft with mild pressure, stop trying.

I am 10mph off but I replaced the Carrier with a different ratio. Speedo says 70, Actual 60, same% off over the entire range.

Chesk your Carrier ratio.

Carrier=differential

mine is off about 10% too. the speedo and odometer reads about 10% higher than true...based upon GPS and mile maker signs over a 100 mile run.

wait. you have a lower gear ratio in your TD wagon? I thought 2.65 was the lowest gear ratio you could get in a TD... is yours from a manual equipped 124?

no, it's an auto...i would they would all be the same gear ratio for a given model....but if I'm showing 70 on the speedo it's really only going 65. After 100 mile markers on the interstate my odometer shows 110 miles.

and the tires are the stock size. i figure it's typical german speedo error. older porsche cars are notorious for a 10% higher than true speedo reading. if anything it probably helps prevent a few speeding tickets, lol

If your speedometer AND odometer are off 10%, it is a speedometer/final-drive mismatch.

As VS mentioned, you're reading 10% high it is probably a speedometer from a higher-ratio gas model (pre-'94).

You might want to add a speedo to your shopping list. Read the little white numbers on the speedo face, I think that this is the part number of the speedometer subassembly.

BTW, the speedos on my older Mercedes have all been within 2% of GPS, which is within the tolerance of tire construction and wear, pretty good IMO.

Also, are you looking for a used turbo?

so my 522,000 mile speedo came from a gasser? guess they last a long time too...lol

what's the part number on your speedo? odds are good, it was put in due to originally low indicated miles, and it's had 475K put on it since... meaning your 522K odo reading car actually has 600+K... wow. not bad for a #14 head! if it still has one.