speedometer gear help - Mercedes-Benz Forum
 
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post #1 of 10 (permalink) Old 02-16-2019, 03:23 PM Thread Starter
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speedometer gear help

I recently put a 79 450 sl engine in a 83 380sl. Because I used the three speed tranny I also used the speedometer which was cable.Because the rear in the 380 is 247 and the 450 was 308 or 307 the mph reading is severely wrong.does any one know what is the best option for correcting this.Are there gears available to remedy this?
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post #2 of 10 (permalink) Old 02-17-2019, 08:06 AM
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Hmm, maybe you need a 79 450SL instrument cluster. Not sure if you want to go back in years from the 83 cluster. I paste a picture below (first pic) of the back of the 79 unit. If the connections are the same (note especially the oil sender, it is not electronic) this should work. The other thing to consider is just replacing the speedo, so examine the mounting points, are they the same?

Here is a unit on flea bay right now for $250
https://www.ebay.com/itm/1972-1981-M...95fm:rk:4:pf:0

Take care and note the 1980 speedo is an 85MPH (third pic). I think you can still buy these from the dealers new, but the 160MPH are unobtainable. If you don't want the one on Fleabay I can sell you mine (pictured) for $150. Send it back if for any reason for a full refund, so both you and I will each pay shipping in one direction. If it works great, if not you can either return it or sell it like the guy on ebay.
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post #3 of 10 (permalink) Old 02-17-2019, 12:50 PM Thread Starter
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speedometer

I used the 79 cluster because it was cable and the 380sl was electronic the problem is the 380 has a different rear end ratio than the 450sl so the gauge is saying 40 but im going 55.
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post #4 of 10 (permalink) Old 02-17-2019, 12:57 PM
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Someone in another thread suggested a Dakota Digital speedometer signal interface. Wouldn't work with cable though.
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post #5 of 10 (permalink) Old 02-18-2019, 10:57 AM
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Quote:
Originally Posted by averageJoe123* View Post
I used the 79 cluster because it was cable and the 380sl was electronic the problem is the 380 has a different rear end ratio than the 450sl so the gauge is saying 40 but im going 55.
I don't know what the mechanical fix will be but think of the mental gymnastics you could be doing every drive!

Performing gozintas every speed zone!




(PS....you could always do a diff swap )

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post #6 of 10 (permalink) Old 02-18-2019, 02:56 PM Thread Starter
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speedometer

I've been using my Iphone with speedometer app. I would prefer not to have to.On a ford all you have to do is pull the cable out of the tranny and stick a different gear on.
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post #7 of 10 (permalink) Old 02-18-2019, 03:04 PM Thread Starter
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speedometer

I do have the other differential but was thinking i could enjoy the highway gears. Maybe even save a sheckle or two on gas.Never even thought this would be an issue.
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post #8 of 10 (permalink) Old 02-18-2019, 04:41 PM
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Not sure about the MB mechanical speedos but most early American mechanical speedos had a replaceable gear on the end of the cable that went into the trans.
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post #9 of 10 (permalink) Old 02-18-2019, 04:51 PM
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Quote:
Originally Posted by averageJoe123* View Post
I've been using my Iphone with speedometer app. I would prefer not to have to.On a ford all you have to do is pull the cable out of the tranny and stick a different gear on.
You know, here's a thought.

Since the speedometer is electronic there may be an easy programming solution.

Why not take a Raspberry Pi Xero and feed the speedometer signal to the Raspberry Pi. Set up a programming interface to basically "translate" the signal coming from the transmission to the speedometer.

Coding wouldn't be that complicated. You'd basically tell the device to apply some sort of step to match what the speedometer is expecting.

You'd just have to power it off of an always on 12 volt power source. The Raspberry Pi runs off of 5v DC so some simple step down source would work with exactly what you need.
https://raspberrypi.stackexchange.co...-a-pi-from-12v

The coding involved would be astoundingly easy. You could even hire someone from Fiverr to do it for you for very little money.
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post #10 of 10 (permalink) Old 02-18-2019, 05:16 PM
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Originally Posted by averageJoe123* View Post
I do have the other differential but was thinking i could enjoy the highway gears. Maybe even save a sheckle or two on gas.Never even thought this would be an issue.
Well look at the bright side, the lower total mileage on the odometer may help in resale value.


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