Author Topic: tacho question  (Read 124 times)

floyd111

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tacho question
« on: 19 June 2020, 03:27 PM »
He guys, does it matter if a tacho is for M110 or larger engines? I know top speed indication is different, but does it affect the accurate speed indication?
What about miles vs KM? Is that still the same tacho underneath the front bezel?

ptashek

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Re: tacho question
« Reply #1 on: 19 June 2020, 04:04 PM »
He guys, does it matter if a tacho is for M110 or larger engines? I know top speed indication is different, but does it affect the accurate speed indication?
What about miles vs KM? Is that still the same tacho underneath the front bezel?

Speed indication is through the tranny, so I'd like to think that in theory all cars with the same type of tranny should show the correct speed.
However, miles v. kilometer tachos have different internal gearing and are not interchangeable.
1993 "Pearl Blue" W124 280TE
1988 "Arctic White" W124 200T
1979 "Icon Gold" W116 450SE (history, resto)

Max-NL

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Re: tacho question
« Reply #2 on: 20 June 2020, 03:55 AM »
The final drive ratio in the differential also plays a role with the speed indication.
1971 R107 350 SL (Dad's)
1975 W116 280 S (Dad's)
1975 W116 450 SEL 6.9 #140 (Dad's)
1977 W123 230 (Mine)
1992 W124 230 CE (Mine)

floyd111

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Re: tacho question
« Reply #3 on: 20 June 2020, 03:43 PM »
Wait.. it can't be both, not? Either it gets its signal from the diff, or from the transmission, I, as an amateur, would deduct.. I thought it got it's signal from one of the wheel hubs, haha!

The tacho I found has miles as a primary on the bezel, and KM as a secondary, inner circle.
I should think that, if I find an exact reverse bezel..-if that is a thing at all-, the bezels should be interchangable . But that answers my own question.. It would have to be an exact revers-KM/MPH bezel, both of them listing the same top speed, but would they need to come from the exact M110 tacho underneath, and are those tacho's M110 or M116/7 specific?. I have seen quite ba few bezels, with  quite different top speeds, but havenot yet cracked the mystery yet.

Randys01

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Re: tacho question
« Reply #4 on: 20 June 2020, 07:30 PM »
How interesting.?
 In this part of the world a "tacho" is a tachometer.  .. a device for measuring engine rpm. Nearly always these are electronic pulse generated.

"A tachometer (revolution-counter, tach, rev-counter, RPM gauge) is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolutions per minute (RPM) on a calibrated analogue dial…"


A speedometer is the device for measuring road speed. It is usually KPH or miles per hour. The odometer is the bit that records the distance. either in miles or kilometres.

Am I missing something here?

ptashek

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Re: tacho question
« Reply #5 on: 21 June 2020, 02:50 AM »
Indeed. But it's sometimes used to describe the whole instrument cluster too.

You're right. The actual tachometer is driven by a signal from the ignition module. It's branched off of the output stage via the TD pin.

They are likely the same electrically between cars using the same module, just would have different faceplates to mark redline.

1993 "Pearl Blue" W124 280TE
1988 "Arctic White" W124 200T
1979 "Icon Gold" W116 450SE (history, resto)

Max-NL

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Re: tacho question
« Reply #6 on: 21 June 2020, 03:22 AM »
Wait.. it can't be both, not? Either it gets its signal from the diff, or from the transmission, I, as an amateur, would deduct.. I thought it got it's signal from one of the wheel hubs, haha!

It's driven via the transmission. If you connect a wheel directly behind the transmission then indeed you only have to look at the transmission. But the W116 has a diff behind the transmission, with different ratio's based on the engine. The diff works with ratio's, which means that the input speed of the diff is not the same as the output speed of the diff.

So let's say you have a input speed of 2500 rpm from the transmission. A diff with a 1:3,46 ratio will have a different output speed than a 1:2,65 diff at a 2500 rpm input, and thus a different rotational speed of the tire and different vehicle speed.

So as an engineer you have to take into account the ratio of the diff to calculate the gears in the speedometer.
1971 R107 350 SL (Dad's)
1975 W116 280 S (Dad's)
1975 W116 450 SEL 6.9 #140 (Dad's)
1977 W123 230 (Mine)
1992 W124 230 CE (Mine)

floyd111

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Re: tacho question
« Reply #7 on: 21 June 2020, 02:19 PM »
Right, and that is a familiar part to me, after having considered changing diff's myself.
Well,  that rules things out good'n clear. The V8's do not share the same diff with the M110, so I'll need a M110-specific instrument panel.

The actual tachometer is driven by a signal from the ignition module. It's branched off of the output stage via the TD pin.

They are likely the same electrically between cars using the same module, just would have different faceplates to mark redline.

And with that, the story is complete.
Imma gonbna have to find myself another NOS M110 tacho somehow, or NOS parts thereof.

Thanks a lot guys!