Tachometer reading low

[plc]

Hi All,

My tachometer seems to be reading low. While my bike is in need of a tune up the bike never idles above 600 rpm according to the tacho, but it feels faster than that. Also my generator light goes out at about 800 rpm or slightly lower which I think is wrong.

I know that tachometer gets its signal from the coil so I cleaned off the connector that plugs on to the coil, but no difference (new Dyna coil fitted May 2009). Last year the tacho was serviced and the board was re-soldered and had no cracks, yet the problem has remained. I checked the resistance between the triple clamp and the engine earth and the reading was 8 ohms so I assume the front of the bike is well earthed.

I am puzzled. On Thursday I will check the plug but I think from memory it is OK. Does the tacho rely on a varying voltage from the coil or is it just detecting the amount of sparks per minute? If it is just detecting rate of firing then surely resistance from corroded connectors makes no difference.

So I am baffled and need some advice from those with much more experience. What is going on?

Thanks in advance,
Paul

[Barry]

I checked the resistance between the triple clamp and the engine earth and the reading was 8 ohms  

I think that 8 ohms is a high reading and would expect something closer to zero maybe a fraction of 1 ohm at most.

There is no specific voltage measurement involved with the tacho  it just counts the number of impulses from the coil primary connection which is the same as saying the number of sparks.  If it isn't an earth problem sometimes fitting a condenser at the coil terminal can resolve problems with erratic tacho readings by cleaning up the pulse.  Your new coil may not have polarity markings on the terminals but the tacho takes it's signal from the earthy side of the coil that is the black wire connection that normally goes to the points.

[Dizerens5]

But there are two coils on the R65. Does it matter which one?

[Barry]

Yes if there are two it's always the earthy side of of the front coil which should be terminal 1 if the coils are correctly wired or which ever terminal this black wire was connected to on a dual output coil.

[Dizerens5]

Thanks Barry. I'll lift my tank and have a look. I've always been leery of the tacho readings.

[plc]

Hi All,

Barry thanks for that. I rechecked the resistance between the engine earth and the triple clamp, and it seems to be around 1 ohm, I must have had a poor connection to the test lead.

Mind you looking at the LS wiring diagram it appears that the front wiring connects to a frame earth, so i guess I should check the earth there to be sure all is OK there.

I am now going to test the resistance between the coil and pin 5 of the tacho plug to see if the wire has unnecessarily high resistance. Rick from Motorrad Electrik told me the Dyna Coil has no polarity so it doesn't matter which terminal I use. That said the old BMW coil with the terminals labeled still gave low tachometer readings.

It is indeed a mystery.

Will keep investigating,
Paul

[nhmaf]

Various weirdnesses have occurred with instrument panels due to a cracked or intermittent ground wire in the wire harness, resulting in the instrument and headlight circuits having to find ground return to the frame through the steering head bearings - this can cause issues and over a long time, bearing wear from electrical conduction.   One quick&easy thing to try is to take a 14-16 gauge wire with crimped on ring terminals and make sure that there is a good solid ground connection (NOT insulated with paint, etc) from the  headlight bracket or triple clamp back to a point on the bike frame, such as where the horn bolt is installed.   This way, if you have an intermittent ground in the wiring harness there is another solid ground (not through the head bearings).  The head bearing electrical conduction path will vary in resistance as the grease moves around in there and as the handlebars are turned.   This may or may not be related to what you are observing with the tach, but bad grounds to the instrument/headlight area cause lots of odd things at times.

[Bob_Roller]

All of the electrical circuits on the front of an R65, have their own ground or earthing wires, these are the solid brown wires, the frame, steering area is not a designed return path for electrical power .

On the '81 and later year bikes, all of the grounds/earths are ganged together at one point under the tank around the relay area, by one bolt threaded into a threaded lug on the frame .

On '78-80 model year bikes, the main grounding/earthing point is the front ignition coil mount, quite prone to cracking, and giving odd intermittent electrical problems .

Remove that bolt/screw, and check for corrosion on the threads  on the frame lug .

[plc]

Hi Guys,

Thanks for all the advice. I pulled off that tank today and found the earth wires attached to the voltage regulator mount. I sanded the earth ring terminal with 180 wet and dry to get rid of any oxidation/corrosion. I also cleaned rust off the frame lug and the regulator mount bolts that hold the earth in place.

I then cleaned the terminals in the tacho plug that connect to the coil and earth. Put a multimeter between earth and tacho earth pin, around 0.5 ohms. Checked the resistance between the coil and tacho plug, also 0.5 ohms. Then I checked the resistance between the transmission breather bolt and the tacho earth and it was 0.6-0.7 ohms.

So I think I have eliminated any poor earth effects and yet the bike still shows an idle speed of 500-600 rpm, yet it sounds much higher. At 80 km/h (50 mph) in 5th gear my tacho reads approximately 3400 rpm, how does that compare to other bikes? I am wondering if it is around 600 rpm below on all readings.

So that only leaves the Dyna coil which was fitted last year in May and the tacho which was overhauled last year in March.

If anyone has any other ideas, please let me know.

Still baffled,
Paul

[Barry]

Assuming a 3.44:1 final drive 50mph calculates out at 3335 rpm. So given a speedo usually reads high and your observed 50 mph is actually in the high 40's I would say your tach is reading a little on the high side rather than low.

That doesn't help much does it. Very odd

If you have an accurate figure for the rolling circumference of your rear tire I can substite that in the calcs. I've assumed 26" dia = 81.69" circumference.

[plc]

Hi All,

Barry, thanks for that. But how the plot thickens. Yes my final drive is that ratio, I don't have exact rear circumference, but I have Metzeler ME77 4.00 rear tyre. I think Snowbum says the approx circumference of them is 80". So your calcs are probably dead on Barry.

So my tacho reads low at idle and a little slow at 80 km/h.  :-?  Great. I should add it never flickers and moves really smoothly on increasing and decreasing revs. Only get the tinniest flicker when turning the ignition off before starting.

So except for its mild inaccuracies it operates well... if that makes sense.

Best to all,
Paul

[Barry]

Paul

80" rolling circumference calculates to 3406 rpm which would be spot on if by some chance your speedo was close to accurate. So maybe it's not so bad after all except for at idle.

I've attached the spreadsheet which will calculate revs vs mph in every gear.

[Bob_Roller]

I brought the R65 into work today, here's a few rpm to speed readings .

45 mph: 3500 rpm, 60mph: 4500 rpm, 70 mph: 5200 rpm .

My speedometer reads 2 mph high, these are indicated speeds on the speedometer .

If this helps at all .

I'll get a 50 mph reading on the way home this afternoon .

At 50 mph indicated, rpm's are 3800 .

I also have the 32/9, 3.55 final drive, so I guess my observations, are not accurate due to your different final drive ratio .

[plc]

Barry, thanks for the spreadsheet, I have modified it for km/h (just easier in my head) and it seems like my tacho isn't too bad, so maybe the idle is the only place it is badly out.

Thanks Bob for your numbers too, they were helpful as a guide too.

Best wishes,
Paul