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.