Main.TrailIndexPage | Electrical Systems | Dashboard Instruments | Rev Counter or Tacho

Rev Counter

This component is part of Dashboard Instruments.

Definition

A tachometer or Revolution Counter is an instrument that measures 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 analog dial.

German name: Drehzahlmesser.

Function

Describe, in general terms the function of this component. Meaning what is it there for and what role it plays. Describe how it works, the inside mechanism. Use diagrams to explain.

Maintenance

Describe common maintenance procedures, and common faults that may occur. Describe how these may be diagnosed and resolved. Again, include diagrams, photographs and explanations. Where possible, include measures, tolerances, weights etc.

Symptoms when it faults

  • How to test if it is faulty - what tools to use
  • How to fix / change

Link to related components where appropriate.

Tachometer Cable, Removal and Replacement

Note: In the UK the Tachometer is called the Revolution Counter

Symptoms of Trouble

  • Tachometer cable problems include an unsteady tachmeter reading when the Tach pointer swings or moves around part of the dial even when the engine speed is steady.
  • The Tach pointer might not move and remain pointed to the "5" peg position.
  • The tach might emit an unusual screeching or whining sound
  • A tach problem not related to the cable but requiring removal of the tachometer is when the small grey disk covering the center of the pointer loosens and falls to the bottom of the tach unit.

Solution

For each of the symptoms described, the tachometer unit must be removed from the dash to access the tachometer cable or to access the tach unit itself for repair.

The following procedure is based on removal of a tachometer from a USA model 1971 280SL but the similarities among models should make the same or nearly the same steps correct for all 113 cars.

  • Locate the tachometer cable in the engine compartment and examine the hole where the cable passes through the firewall. The firewall hole contains a pyramid shaped rubber grommet with holes permitting several cables to pass through the fire wall.
  • In an unmodified car, the Tachometer Cable and the Temperature Sensing Cable pass through the same hole in the grommet. Make sure the grommet is clear of tar or other obstruction and that the Tach Cable will be free to pass through the firewall when it is pulled into the interior of the car.
  • Inside the car: Remove the decorative leather/vinyl covered panel under the instrument area.
  • Remove the screws holding the metal box and the bracket with the cable connectors under the panel area and push them to one side
  • While on your back resting your head on the pedals, reach behind the dash and the Tach Unit to locate the single knurled nut holding the Tachometer bracket to the Tach dial unit.

For some a somewhat easier procedure is to remove the steering wheel and the corrugated air tube behind the dash in addition to the panels and the wiring connectors. This provides more space and now, while kneeling, one can reach up with the left arm and locate the knurled nut.

  • Remove the nut and remove the ground wire attached to the center post of the tach unit
  • Remove the bracket (note orientation - see photo below!)
  • Gently push the tachometer unit slightly forward into the car and remove the two light bulb sockets with the wiring.
  • Caution: if the tach cable is obstructed and cannot pass through the firewall as described above, when the tach unit is pushed into the car, the center of the tach cable can pull itself out of the cable covering. It is important to make sure that the tach cable is free to pass through the firewall.
  • Remove the tach cable end from the engine. Remove the cable from any brackets or holders along the valve cover. Now the tach unit can be pulled into the car far enough to use a few fingers to remove the cable from the tach. Change your position as needed for these steps.
  • Note the position of the wires attached to two small posts on the back of the tach unit to power the tach illumination bulb.
  • The tach can now be worked on to remove the tach cable from the tach dial unit. For replacement and repair the cable must be pulled into the car. The cable cannot be pushed into the engine compartment. The cable insulation will not allow the Tach Cable to pass into the engine compartment
  • If the cable is unable to pass through the firewall because of insulation, it may be necessary to adust the position of the other cable sharing the grommet hole.
  • As an example of a way to ease the path for Tach Cable movement: if the Temperature Sensor cable prevents the Tach Cable from passing, remove the Temp cable end from the Temp Sensor port of the engine then push or pull the Temp Cable part way into the car until the narrower uninsulated part of the Temp Cable reaches the grommet hole to allow the Tach Cable to pass. Caution: the temp gauge cable can be damaged if bent, cut, or handled roughly.

Cable Fixes

To diagnose and treat the Tach Dial and cable: a quick turn of the small exposed center post on the back of the Tach Unit should move the needle. A frozen needle will not move. A Tach unit can be rebuilt by shops that repair speedometers.

  • For the cable: a few drops of oil or graphite forced down the interior of the cable have been suggested as possible lubricants for a screaming cable. For further diagnosis: attach a slow drill to the engine end of the tach cable to see if the tach needle moves.
  • Tach needle flutter and screeching might require cable replacement. Tach cables are available from Mercedes Benz (in 2008) or from third party suppliers. Cables come in varying quality. Best to inquire or examine before purchase. Mercedes Benz cables seem to be of the highest quality and might be worth the higher price for a part that is inconvenient to service in case of future breakdown.

Installation

  • Install is reverse of these steps: push the tachometer into the opening, check the rubber seal for a good fit, insert the two light bulb sockets and then position the bracket with the protrusion over the shorter bulb socket as shown in this photo:

The photo shows a tachometer from a LHD 230SL. RHD W113 cars need a different tachometer model!

Be sure the Tach Cable and the electric wires are fitted correctly and firmly to the Tach Unit. It is a good idea to replace the light bulb while access is convenient.


Tachometer electric wiring

The bulb is a 2 Watt / 12 Volt / Ba9s / MB# N072601 012800 / Bayonet type / Osram Equivalent 3796

  • A new gray rubber instrument seal can be installed if the older seal is dry, cracked, or was damaged in removal. Also a new firewall grommet is suggested if the old one is dry or cracked. A new grommet must be cut from its edge to its center hole to install a grommet on existing cables.
  • For final positioning: make sure the Tach Dial unit is correctly oriented on the dash (that "VDO" at the bottom of the dial is precisely at the 6 o'clock position. Make sure that the grey seal is hidden behind the edges of the instrument.

Tachometer Bushing, Removal and Replacement

Note: Information and pics from Joe A. ...

The Tachometer bushing, also called a "thrust" bushing...

The Vertical timing gear is also referred to as the "driveshaft for the oil pump and tachometer" in the BBB. The specified clearance for endplay is very small (0.1mm - 0.25mm) or (0.004" to 0.010").

The gear drives the oil pump and the distributor and is driven from the horizontal timing gear which drives the fuel injected pump.

The thrust piece or "aluminum/bronze bushing" wears and allows this vertical timing gear to move upward during engine operation. In severe cases, the gear moves up enough to allow the oil pump to dis-engage and stop turning. The engine continues to run fine with no warning until the oil starvation causes mechanical failure!

Checking this endplay or wear is fairly simple. Remove the tachometer cable and insert a screw gently finger tight and move the vertical timing gear up and down to measure "endplay". As specified 0.1mm - 0.25mm (0.004" - 0.010") is ideal. Excessive wear will eventually cause premature gear wear or failure.

If you have excessive endplay the "thrust piece" (alum/bronze) bushing should be replaced and/or shimmed.

The factory recommends shimming between the bronze and aluminum part of the thrust piece. However copper injector seals can be used as shims between the gear and the thrust piece and seem to work fine. Often times the bronze portion of the thrust piece is worn uneven and must be smoothed out before shimming. Replacement of the thrust piece is best.

Removal of the thrust piece is done by removing the 22mm fitting and the 10mm hex head set screw. The thrust piece can be coaxed upward and will narrowly clear the cylinder head. The timing gear can now be removed and inspected for wear at the helicentric cut gear.

Some owners have experienced problems installing the new thrust piece. Light wet sanding of the outer circumference and freezing the thrust piece prior to installation will help if problems arise. Normally they will install without these measures.

Endplay should be checked again after installation of the 10mm hex head set screw. Shim if needed.

The 22mm fitting has a rubber O ring beneath it along with a seal ring inside it. There is also the infamous "felt washer"!!! which is new to my experience also. The seal in the fitting is the most difficult part of the job. Removal from the fitting can be a challenge. Some listers have removed them by drilling them out! If you do not have a oil leak at the tach drive you may elect to leave the original seal in place. You can always replace it later.

Old Yahoo content

The following is the content from the old Yahoo documents on the site. It needs to be structured and edited in the correct sections of the entire document. After moving particular content to its correct place in the manual, please delete it here.


It seems that my tachometer squeals at higher rpm's in cold weather.

Frank says: I have a theory why this happens: the tach and speedo cables contract and shorten slightly. When they shorten, if they are snug in the instrument head, they pull the rotating drum inside the instrument, against the inside back of the instrument case. There is normally no lubricant there, so this makes it squeel & causes the needle to jump around. The solution is not to lubricate the instrument, which would be hard to do; rather, fix the end of the cable so that it doesn't bind & pull. Dress it down with a file, then lubricate it with CLP or some other kind of low-temperature oil (e.g. gun oil).

Will: I have had several cars made in the same era as the 113 and all of them suffer from either squealing tach or speedo. All I do is have the instrument serviced. I assume they take it apart and refresh the lube inside.

Bernt Damm: I was really proactive recently and took out the rev counter cable to oil it. It was all gunked up with old lubricant and when I was done, it turned really easily. However, as it goes, if it isn't broken, don't fix it. My rev counter now jitters (like a very fast vibration) around the actual reading. It isn't much and it stops at the higher revs but the smoothness it had is gone. I tried moving the cable etc. I also re-lubed it with some grease but it now remains the same. I obviously destroyed the damping effect of the sticky old lubricant. Any ideas on how to remedy this perhaps?

Cees: the jittery effect, as I understand it, comes about due to normal wear and tear. The cable starts slipping slightly where it connects into the tach. Probably the gunk kept it reasonably tight in there, so in fact by cleaning it up you probably loosened the connection. My tach was jittery for a long time, and then it started to screeeeech (loud) at times, and I replaced the cable with a new one (aftermarket, they're much cheaper than original, anyone know the difference in quality / life expectancy?) It was quite easy to replace and it fixed both the screetch and the jitter (sounds like the old "wow and flutter").

Tom Hanson: A new cable might be in order. 113-542-07-07 for left-hand-drive.

Bernt: just got the price from local MB for that cable... Has to be ordered from germany as well. US$155,- !!! -> Now am jittering too. For that amount I will have to let it jitter along for many years to come or come up with another solution.

Cees: Van Dijk in Holland lists a reproduction cable for around $25, and an original one for around $55.

Bernt: Yes, I know I am getting ripped off here but there isn't much I can do about it. It is the airfreight, customs duty and the greedy local dealers that kill the prices :-(

Cees: Van Dijk (and probably Caliber motors, etc) ships worldwide and if you get lucky, a small item like a cable does not get taxed by customs, at least not in The Netherlands. Anyway, even with customs duties, a $25 cable would still be much cheaper than your $150+ ?

Jittering might come from your cable, but if your tach starts screeeeech it is most likely vibration from inside of the tach. This same effect occurs for speedo's as well. Cause is dry drive, I mean the drive where cable goes into. This drive rotates inside of the housing and can not be lubricated without disassembling entire meter. Adding a small amount of grease on top of the cable might help. In case of loud screeeching I would consider at least disconnecting the cable to save the meter. A kind of a shortcut. As when tach is in it's place the drive points down. In some cases I have got the drives lubricated when pulling out the tach and placing it upside down and applying some light liquid oil (wd-40 kind) on the drive and let it sit and run down (or up). Any others experiencing this phenomenon? I think it is most likely cold climate problem.

Cees: well, I had the screeeeech and the new cable did correct that (at least it did not return during the past ca. 12 months, when before it was beginning to do it almost every time I went driving - a firm tap on the glass would stop the screeereech). Maybe the new cable had some oil to it, but not really enough to lube the inside drive I would think. I don't recall the screech happening more often when it was cold or not. Just for my understanding, I thought WD-40 was not a lubricant, but only a Water Displacement liquid, although I have always thought it lubricated as well. Then, when some posts put me right, I carefully read the can and in fact it does not mention lubrication properties anywhere. So yesterday I bought a spray can of general purpose lubricant on some kind of a teflon basis.

Cees: you are right, WD-40 is not a lubricant, but that is something that everybody recalls when mentioned. That's why I advised to use light liquid oil, and wd-40 only as an example how light. Sorry if I misled someone. For rusty bolts and nuts I use water. I am positive that when you replaced your cable there was just enough new grease/oil on it to lubricate the drive (it doesn't need much).

In cold weather the tachometer cable seems to have trouble working and makes a whining, whirring sound. This fades as the car warms and later disappears. Had the cable greased but the noise though less, still remains.

The easier and cheaper first step is to try and replace the cable. If that does not solve the problem it is time to have your tach rebuilt. The cable is about $30 US and I got my tach rebuilt for about $100. Getting behined the tach is the difficult part of the job.

Normally not the cable but the tach itself. In mine the bearings in tach are worn out. There are guys that rebuild the tachs in hemmings.


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