Speedo & tach reading incorrectly - PeachParts Mercedes-Benz Forum

Winston Wolfe · #1 02-09-2014, 12:37 AM

I'm working with a:
1981 300CD Chassis
1982 240D Manual Gearbox
1983 300TD Turbo Engine

The speedo and tach read -1mph for every 10 (so 65mph indicated is really 59)--I've verified this via GPS. And the tach reads accordingly too, so that indicated 65mph is turning at right about an indicated 2500rpm. According to several speed/rpm calculators, those numbers jive together.

I'm seeing conflicting information about where the tach & speedo get their pickup; can anyone tell me how to correct this? If there is a way to correct this?

dieseldiehard · #2 02-09-2014, 01:57 AM

Its common to find errors creep in to the speedometers on older VDO instruments in Benz's.
It sounds like you need to carry the instrument cluster to a speedometer shop, they can adjust the coupling to set the MPH to the K-factor for your particular car/gauge set
The speedometer consists of a rotating magnet driven thru the flex cable from a gear in the transmission and unless you changed the trans it should not be of concern. If the exisiting transmission is not of the original type then tell the shop else they calibrate it to what used to be in your car.

The tach is driven electronically from a magnetic pickup on the harmonic dampener on the front of the engine and I've never heard of one with an error but it is possible the tach is in need of lubrication, again a job for a speedo shop.
DDH

charmalu · #3 02-09-2014, 03:51 AM

As dieseldiehard said the needle has a spinning magnet that move it. Also there is a very small spring that could be weak.

Remove the Cluster and then the Speedometer from the Cluster. On the edge of the spedo to the right of the resting pin, there should be a very small white mark. Lift the needle over the Resting Pin. the Needle should line up with this white mark. this is a reference point when putting the Spedo together.

You could pop off the needle, move it towards the resting pin, and push it back on, and lift it back over the resting pin. this should tighten the spring a bit

Then reinstall it, go for a drive and see if that helps. This worked for me on a Needle that was off.

Charlie

Stretch · #4 02-09-2014, 04:53 AM

The speedometer is matched to the differential. If the differential (probably 3.46:1??) is original then the gearbox and the engine choice makes no difference to the speedometer readings. If you have an overdrive / 5th gear in the mix then the engine revs will drop.

4th gear in a 240D 4 speed gearbox is 1:1

If you are getting within a 1mph agreement of indicated speed on your dash instruments and the GPS then you are doing very well - I'd leave the speedo alone. GPS should really be checked on long level roads - I don't think commercial GPS systems use the military 3 satellite specs that enable an up hill speed calculation; I think you are just stuck with a representation of your speed of travel across an idealised sphere...

Skippy · #5 02-09-2014, 08:54 AM

What's the gauge cluster out of? IIRC, the 81 300CD didn't come with a tach. Are you still using the 3.46 rear end that the car came with, or did you swap to something different? As mentioned above, the speedometer is matched to the rear end ratio.

As for the tach, I can't help you. Mine does what it wants, including reading 6000 rpm at idle, 0 rpm when cruising, randomly reading 500 to 1000 rpm high, or sometimes even seeming to read accurately. It's getting slightly annoying, but there are other things higher on the priority list: shift bushings, fan clutch, slave cylinder, driver seat re-cover...

Winston Wolfe · #6 02-09-2014, 09:06 AM

Thanks for the information, Gents.

DieselDieHard, the gearbox is not original to the car. The PO took out the automatic and swapped in a 4-speed with the 1:1 4th gear.

Charmalu, my speedometer is not off by a consistent amount; it's progressive. So while it's only off by 1mph at an indicated 10mph, it's off by 7mph at an indicated 70.

Stretch, it's worse than that--see what I said to Charmalu.

Skippy, the cluster and diff are out of an '83 TD, so the rear is a 3.07 unit. And that's how I have a tach.

Mxfrank · #7 02-09-2014, 10:17 AM

First off, you have two different problems.

The speedometer is cable driven, geared off the output shaft of the transmission. To recalibrate, you have to replace the speedometer gear in the transmission. You're reading consistently 10% low, which means your rear axle ratio isn't a match for the speedometer gear.

To fix this the right way, you'd have to get the speedometer gear from a 3.07 car and install it in the transmission. But you can do a "poor man's recalibration" by repositioning the needle. Position the needle so that it reads correctly at 60mph...the error will increase proportionally above and below that speed. At 10 MPH, you'd be off by 6mph, but in the range of 50-70 where you need it most, you'll be close to correct. The trick to positioning the needle is to take the instrument out of the car. Use a small section of speedo cable chucked into an electric drill to spin up the speedometer, and note the reading. Whatever reading you have is the baseline. Remove the needle, and reposition it until your reading drops 10% from the baseline.

The tach is electronic. I'm not familiar with this year, but it's usually driven by a position sensor on the ring gear (I assume you installed the 240 flywheel?). Most of these gauges have a calibration mechanism, but I don't have one to examine.

Stretch · #8 02-09-2014, 10:23 AM

Tachometer for an '83 still runs off the front pulley doesn't it?

As for the 10% speedometer discrepancy I'd be happy with that!

Mxfrank · #9 02-09-2014, 10:54 AM

Quote:

Originally Posted by Stretch

Tachometer for an '83 still runs off the front pulley doesn't it?

Might be right...My 300CD has been gone for a dozen years, so I can't run out and look!

Partscarver · #10 02-09-2014, 11:37 AM

Quote:

Originally Posted by Stretch

Tachometer for an '83 still runs off the front pulley doesn't it?

have an 83 sd so yes

Winston Wolfe · #11 02-09-2014, 06:31 PM

Quite frankly, I'm not all that worried about the tach. I thought that since the two gauges were correct for their readings--ie: the tach is an INDICATED 2500 at an INDICATED 65mph, they were getting signal off the same VSS or something. 200-300RPM at speed doesn't bother me in the slightest.

So if there's actually two different systems driving the tach and speedo, then I would focus on getting the speedo to read correctly.

Skippy · #12 02-09-2014, 08:28 PM

What size tires are you running?

vstech · #13 02-09-2014, 09:02 PM

Quote:

Originally Posted by Mxfrank

First off, you have two different problems.

The speedometer is cable driven, geared off the output shaft of the transmission. To recalibrate, you have to replace the speedometer gear in the transmission. You're reading consistently 10% low, which means your rear axle ratio isn't a match for the speedometer gear.

To fix this the right way, you'd have to get the speedometer gear from a 3.07 car and install it in the transmission. But you can do a "poor man's recalibration" by repositioning the needle. Position the needle so that it reads correctly at 60mph...the error will increase proportionally above and below that speed. At 10 MPH, you'd be off by 6mph, but in the range of 50-70 where you need it most, you'll be close to correct. The trick to positioning the needle is to take the instrument out of the car. Use a small section of speedo cable chucked into an electric drill to spin up the speedometer, and note the reading. Whatever reading you have is the baseline. Remove the needle, and reposition it until your reading drops 10% from the baseline.

The tach is electronic. I'm not familiar with this year, but it's usually driven by a position sensor on the ring gear (I assume you installed the 240 flywheel?). Most of these gauges have a calibration mechanism, but I don't have one to examine.

all gear driven transmissions have the same speedometer ratio in the MB"s the problem with the 10% DISCREPANCY is between the speedometer, and the differential... nothing in the transmission needs to be changed.

My bet is you have the diff from an 83, and the cluster of an 85... that would make your speedometer read high. keep the cluster, and find a diff to match it.
post the part number of the cluster.
I bet we can match what it came out of, and what gear ratio it needs to have to be correct.

Winston Wolfe · #14 02-09-2014, 10:08 PM

Skippy, it's got 205/70/14 (should be reading HIGHER than indicated).

VStech, all I know is that the cluster came out of the same car as the diff and engine. I suppose it's possible that the cluster was changed out at some point, however the PO wasn't aware of anything like that or he would have told me. Out of curiosity, what's so special about a 1985 cluster?

And I'll work on pulling the cluster, but it might not happen until next weekend--I hate doing stuff like that in the dark.

vstech · #15 02-09-2014, 10:14 PM

in 85 the diff ratio was 2.88
previous turbo years the diff was 3.07
NA years the diff is 3.46
240 diffs 3.69

speedometer matches the diff ratio...