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Old 02-04-2003, 07:16 AM
Registered User
Join Date: Jun 2002
Location: Monroe County, GA
Posts: 8
Odometer Recording Half Mileage

1984 300SD

I was checking mileage on a trip the other day when I noticed that my odometer was not accurate. I knew I had been further than it indicated.

Upon further checking, the odometer seems to be reading exactly half of the actual mileage driven. Two mile markers go by and 1 mile rolls over on both the standard and trip odometers.

Does anyone have any ideas why this might be happening and how I can fix it? It's good for keeping the mileage low on the car, but I want an accurate record of miles driven.

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Old 02-04-2003, 10:36 AM
aldedmon's Avatar
Senior Member
Join Date: Mar 2001
Location: Houston
Posts: 809
Talking No But....

I sure would like to know how to fix mine to operate the same way

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Old 02-04-2003, 03:27 PM
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Join Date: Sep 2000
Location: Volcano, Hawaii
Posts: 191
Aloha Wayne...

Had that happen to my SLC. Did a search, got this info, used it, and it worked beautifully. "What typically happens with the odometer is that the trip odometer starts dragging and halts both of them. I don't know if you want to take this approach, but if you don't need the trip odometer to work, you can remove the plastic spur gear that connects the two, and the main odometer will begin working again."

If you want to fix it, here's what's been posted previously:
"Remove the instrument cluster from the car and dis- assemble it till you get to the odometer assby. The problem is the gray metal gear on the number stack. It is supposed to be press fitted to the shaft and spin with the shaft, but over years and age and many miles it loosens up from the shaft and fails to function. Fix it by pushing out the shaft with the correct size drill bit then support the number wheels with the shaft removed. Set the shaft next to the stack and note where the metal gear was positioned. Next take a punch and hammer and make a dimple on the shaft where the gear goes. Press the shaft back through the stack, re-assemble everything and the odometer will now work."

I know you'll be able to fix it,

1980 300SD (Anthrcite Gray) 253,108 miles
1982 300SD (Silver Blue) 242,398 miles
1982 300SD (Black) 263,185 miles
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Old 02-04-2003, 09:55 PM
hill's Avatar
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Join Date: Feb 2001
Location: Northern Calif Sacramento
Posts: 736
My speedo on a 450 sel did that to. I removed the cluster and took it to a VDO authorized repair shop, $90 out the door. I had one fail on my sl when it started screaming and got real hot. It had a couple of new gears put in and ran $150.
Happy Benzing
Darryl, Hill
2005 SL55 AMG Kleemanized
1984 500 SEC
1967 W113 California Coupe
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Old 02-05-2003, 06:17 AM
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Join Date: Jun 2002
Location: Monroe County, GA
Posts: 8
Thanks for the help; looks like I'll be pulling the cluster this weekend.

When you have the cluster out, does this provide access to any of the HVAC vent actuators? I have a vacuum leak, and I believe it is behind the dash. All air flow from the heater comes out the defrost opening. So does a lot of the AC air; enough to cause aggravating moisture condensation build up on the windshield.

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Old 02-05-2003, 02:43 PM
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smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,327

First order of business would be to find the leak.

Check if the system holds vacuum at the check valve on the firewall ahead of the brake booster. One line goes into the cabin, the other line goes to a reservoir. Check both. I suppose you should check the third line which leads to the vacuum pump to make sure you're getting vacuum in the first place.

Next check the vaccum switch bank. Unclip the right kickpanel, remove the right lower dash panel, remove the right transmission tunnel cover. There'll be a slender box with 7 vacuum lines attached to it. The vacuum source, if you will, is the top connector. No need to check this line if the system holds a vacuum. Check each line to find which actuator/s are bad. Note that one of the lines splits downstream of the box to the center vent actuator and one chamber of the defrost actuator. See if you can follow the line to the tee to check each actuator independently.

To get to the actuators:

Recirculation - two dual chamber actuators behind the glove box.

Deforst - one dual chamber actuator at the top of the ACC stack. Helps to have the glove box out.

Center - one single chamber actuator behind the ACC control panel.

Footwell - one single chamber actuator below the center actuator.

Diverter - one single chamber actuator covered by the left transmission tunnel cover.

I can go into detail of how to get to the actuators if necessary.

Replacement diaphragms are available for something like $5 each so there's no need to buy whole actuators.

Fixing the odometer will be a piece of cake by comparison

91 300SE
87 300SDL
83 300SD
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Old 02-05-2003, 06:03 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The problem of the metal gear spinning on the shaft is only for mechanical speedos. The 126 cars all have electronic speedos. They all have plastic gears and while there are a number of senarios for reduced indication, its not likely a gear problem if the numbers work smoothly and the reduced mileage is consistant.

Its more likely that one of the calculating circuit capacitors is bad. its possible the unit could have been built with the wrong gear ratios at some point but the calibration is to run a frequency derived by the "K" number on the back of the unit into the input and this will run a mile a minute and 60mph.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 02-07-2003, 07:18 AM
Registered User
Join Date: Jun 2002
Location: Monroe County, GA
Posts: 8
Thank you sixto for the detailed description of the vacuum system. I agree on the ease of repair. I have done some checks already and have detailed notes on what I found, but I don't have them with me. I will try to start on this project this weekend and I'm sure I will be back with more questions.

I thank you all for your advice on both problems.
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