I Have a 1969 honda cb 175 the tachometer quit working. It is not an electric tach but instead has a cable.?

Question

I checked the cable. It isn't broken. It pegs the tach at any rpm then tach won't come down.

Answer 1

Your Tach is BENT,,usually happens from crash damage.

Best Solution,,probably the ONLY solution is to replace it.

Those instruments are magnetic -resistive type meters.

The Cable drives a spinning wheel.
The wheel is mounted in Extremely Precise relationship to the NEEDLE.

The needle's suspension is Buffered/Dampened.

Each time the wheel swings by the needle,,it gives a Slight Magnetic "Bump" to the needle.
It twitches "Up" the rpm scale.

As it slowly begins to Fall,,the wheel swings by again giving it another impulse ,,"bump"

It's all calibrated to hold the needle in the correct ranges/rpm speeds.

The SPEED of the spinning wheel establishes an Impulse Frequency.
It does not bump the needle Harder to raise it,,,it bumps it SOONER on the needle's Fall.

Easiest Example to Illustrate the principle is a BALLOON which is Just Barely NOT Floating in mid air.
It falls Very Very Slowly.

If you give it a Gentle,Tiny Tap,,,it rises a little.
If You WAIT for it to Fall back to same place/Height that You originally tapped it,,then tap it Same Way again....

..You will have set a "Band" or Range where the Balloon will oscillate a little,,,but it will also stay WITHIN that Height Range.

Imagine it being at Eye-Level,,tap it Up there just enough to raise it a few inches.
Then when it Stops Rising and Begins Falling,,when it gets Back Down to Eye-Level tap it again.
The Effect is that You can keep it Floating at Eye-Level as long as You keep Doing that consistently,,, It will HOVER,,,Right?

Ok,,,so How do You RAISE it to Higher Level?
#1 Hit it Harder
#2 Hit it SOONER as it falls.

Harder will "knock it to the Ceiling"

SOONER,,will gradually Raise it.

As the Tach's Wheel changes Speed according to engine RPM,,,
It is Tapping the Needle Sooner/Later.
The Impulse Frequency is Changing,,,NOT the Force it applies

I'm sure You've fooled with MAGNETS enough to know that the Closer they get,,the Stronger the Pull.
The Strength Grows/Fades in leaps&bounds according to Distance.

Example:
12" Distance=No Effect
6" distance=No Effect
Ya just HALVED the distance,,and see No Change

4" = some obvious effect
Ya went from None to Some with a 1/3 Decrease in Distance

3"= TWICE the effect as 4"
2"=3X the effect as 3"
1" =5X the effect as 2"
(Imaginary numbers,,but You should get the idea)

Point is that TINY changes have Big effects.
And from any given point--any MORE Change begins a Compounded Effect.
It Snowballs.

Recall the Floating Ballon example--when we hit it HARDER how it just flew up to ceiling?
Instead of tapping it gently,,but Sooner to raise it?

That illustrates the Effect of MORE FORCE.

Changing the Magnetic interaction within the TACH by way of
Mis-Aligning the components creates the effect of MORE MAGNETIC FORCE being applied to Needle.

And you SEE that effect on YOUR Tach as being a tach Needle which PEGS at very low RPMS and appears to just "Stick" up at redline area.
The TACH is BENT>moving magnet Closer>applying Greater Force to Needle>"nailing Needle to the Ceiling"

Imagine "dragging a sewing needle" across a tabletop by using a Magnet spaced far enough away to just Barely Move the needle.
What happens if ya let the 2 get too Close to each other?
RIGHT--the needle slams into the Magnet and is stuck hard to it.

Honda Tachs of that era are constructed in a peculiar way,

The rotating wheel assembly is mounted directly to the Bottom of the actual instrument case.

Any Force applied to the Cable/Cable Connection OR to the CASE actually BENDS the bottom of the Case.

The Needle assy is mounted ABOVE all that on a flimsy frame.

Any FAINT bend in the bottom of the case cause a mis-alignment,,and results in Probs in both Function AND Calibration.

The Tach Case is NOT simply a cover for the mechanism,,
it is actually the Structural Foundation--the mounting base for the mechanism.

Obvious question arises,,,"Can you simply Bend the Case Back to Correct?"

Theoretically,,YES

Reality,,,,Odds of being Successful are Worse than chance of finding Winning Lottery Ticket laying on the ground at your feet.

But just because something is Impossible should not keep you from Trying It:)

The Tach is Trashed,,,with virtually NO Hope for Repair.
So You're not really risking anything to TRY.

*Remove Tach and Cable
*Tighten Cable Onto Tach
*Clamp that Cable Mount Snout in a solidly mounted Vise---Use a block of WOOD on each side/vise jaw to prevent Damaging the connection
*Leave Tach Body Facing straight UP in Vise,,about an inch or so from Vise
*Connect a Power Drill to engine end of Cable and start the Drill
* Firmly Grip Tach Body with HANDS and PUSH in direction of each Jaw,in turn, And WATCH the Tach Needle's Response
* Dont Try to actually "permanently" BEND it at First
* Loosen the Vise and rotate the Tach about 1/8 to 1/4 Turn,,re-clamp and Push Again in each direction toward the vise jaws.
* WATCH the NEEDLE's Response as You apply pressure
* When You see the needle DROP or even Quiver---that's a Suggestion that You are NEAR the Right Direction to TRY to "permanently Bend"

* Be Methodical and Patient in Finding "The Spot" where Needle responds.
*When,,IF,,ya do find the spot---continue PAST it till the Needle Quits responding to your hand Pressure on the case.

* Target the CENTER point of where Needle Started Responding and where it Stopped Responding,,,
that should be the approx Centerline/Direction which to try the Bend.

Imagine it's a Clock Face---
You get NO Needle Twitch at 12:00
None at 1:00
But at 2:00 You see a Reaction--ok Thats a STARTING Point.
Continue around to 3:00--it Moves some.
then 4:00--still moves.
Then at 5:00,,Your hand pressure gets No needle reaction

So it Started at 2:00 and ended at 4:00----
The Suggested Direction /Line to Apply Force is "Between those 2 points" at approximately 3:00 Position

*Clamp Tach Drive Snout Firmly in Vise,,,with case bottom a bit above the jaws for working clearance----
And with "3:00" Point against Vise Jaw(so Ya have a solid,,perpendicular support)

* GENTLY and GRADUALLY try to Bend Tach Case/Head in that Direction......WHILE watching the Drill-Driven Needle's response

* The Needle SHOULD DROP considerably from PEGGED position,,IF You're Lucky

At That point,,You will have only succeed in Releiving some of the Excess magnetic Force by virtue of withdrawing the magnetic back towards it's Correct Position.

There REMAINS the Issue of CALIBRATION--the Accuracy of the instrument.

* If at That Point You can put Force on the Case and Cause Needle Fluctuation,,,
If in effect you're able to "Rev the Tach Up/Down" by bending it....
There's some small hope of Getting it CLOSE to Correct.

***Realize we're talking about Less Than Hair-Width Measurements here,,especially in terms of accurate calibration.

#1,,You're going to have to Do Some Math and make some assumptions

The ASSUMPTION is that You Know an Accurate Speed for the Electric Drill thats driving the Tach

Most 3/8"s Drills are 'advertised" as 1,000 RPM...who knows how accurate that is???

Next,,the Math,,,and some More mechanics.
Connect Tach Cable to engine,,and on Tach-End fold a piece of Tape onto the Inner Cable to make a little "Flag">Pointer.
Note it's Position.

Remove L/H Engine cover(Round Plate w/ 3 Screws)
Turn engine Clockwise as You Count the Turns on both Engine and Tach-Cable Flag.

That gives actual,physical indication of Ratio between Engine Revs and TachDrive Revs.
(Cam runs at 1/2 Engine speed,,but I have NO Idea what the Tach Drive reduction ratio is)

Use that Ratio multiplied by the DRILL SPEED to determine the "artificial" Engine Speed created by the Drill

Example:
If Engine x Tach Drive =4:1,,,
meaning 4 engine Turns= 1 Tach Drive Turn.....

Then with Tach Cable turning at 1,000 RPM(The Drill Speed??) that should equate to a Engine RPM of 4,000

Following that example and those figures,,,,
Spin the tach with the Drill at 1,000 Drill RPMS,,,
And TRY to Bend the tach to show 4,000 rpms.

Ya GOTTA determine overall ratio between Crankshaft and Tach Drive/Cable revs.
Then figure whatever your Drill RPMS are 'supposed to be"

IF,,Drill is 1,500 RPM and Tach Ratio is 4:1.......
The Drill Should drive the Tach to show 6,000rpms.
1,500 X 4

Whatever those 'Constants" happen to be,,Drill Speed & Tach Ratio multiplied by each other should be the Indicated Engine RPM.

By Tweaking the Tach Case Carefully,,You MIGHT can raise/Lower the Tach Readings to match your Calculated Engine Speed.

If by Some MIRACLE You have any success....
Make Certain the Tach Mounting Cup & all the Rubber Grommets are in good shape.

The CUP itself,,and the bracket it bolts to can cause some Distortion when tightened-up if they are Bent.
The way the actual Mounting Studs are fastened to the TACH Body/case,,,is directly to the "problem Area " that Bends during impact and "breaks" the Tach's function.

The Mounting System is Great for Vibration Isolation---
But it's also the EXACT SAME effect as having the Case Clamped in the vise.
Any force applied in any way to the Case IS gonna distort the bottom of the case,,and knock it outa whack.

Yes,,I know that all sounds Crazy .
This is the INTERNET!!
Stuff is 'Sposed to sound Crazy:)

But whether You Beleive any of That or Not,,or attempt it or not....

I think ONE thing should be More Clear and certain than ever>>>>>>

"Best /Easiest/Simplest Fix" is to simply Replace the Tach.

...........................
If you DO Try it,,I cannot overstress being Patient and Methodical.
You'll be using substantial Force to yield a MICRO-move result.

One thing in your favor is that Metal when bent tends to want to go back where it was,,,,IF it's not bent Too far.
The Bend Damage in Tach Cases is usually only VERY SLIGHT.
And rarely is it a Compound Bend,,bent down then sideways for example.
It's Usually just a simple straight-line bend.

If You can possibly find the "Centerline" of the bend's direction by monitoring Needle Action as the tach is running,,
There's a decent chance( but still Small) That You can simply bend in that direction and have some success.

Dont get your hopes up,,or blow all your "New Tach Money" on Beers & Women just yet.

But ya Never Know till ya Try,,and I doubt You have much to lose


Good Luck!
........................

Answer 2

More than likely, the inner cable has broken off at one end or the other and needs to be replaced. Guages rarely malfunction internally. Due to the age of your equipment, I must assume that the inner cable has not been oiled in a while causing it to bind and break inside the outer sleeve.

Answer 3

tach needs replacing. not much you can do there are no serviceable parts inside it, you may be able to take it apart and see what is wrong and fix it you may want to start looking at motorcycle salvage yards to try to find a match.1969 model will be tough to find. if you can find any instrument cluster for the old Honda twins you can mix and match most of the instruments look at red line for the same as you have lots of luck :) also I may have your oem replacement tach, get back to me to find out, i will watch to see

Answer 4

The cable broke.

Answer 5

Yes..its the cable...

Answer 6

better off buying a new or used one. They can be complicated.