Moss Motors, Ltd

I have a problem that my tach reads almost double what the actual RPM is. I can see its driven by impulses from the coil but what could be wrong?

What am i missing?

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Paul Macerone

1968 MG Midget

Hey Paul,

Not that i have an answer for you but.... since my tach isn't working at all, i'm curiuos how you've determined it's 2x's the actual rpm? 

I would guess "Davey" would be able to help - he's an electrical genius....  If you repost this under the "General" topics section, he'll likely see it.

anyway, good luck with yours! 

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Lou
'76 MGB MKIII

Lou, I put my Sun Tach and dwell on the ignition coil and set the idle speed at 1000 and the in dash tach read 2000

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Paul Macerone

1968 MG Midget

The earlier electronic tachs can go significantly out of kilter. This is because their calibration relies on the value of ageing electronic components. As these things age, absorb moisture, corrode etc. their values change. The worst offenders are capacitors, which become electrically leaky, allowing direct current to pass, and also shift hugely in value.

The good news is that these parts can be replaced with modern components. I did this with my MGB's Smiths tach, recalibrated it and it is rock solid. - It was also reading something like 2x over!

A little knowledge of electronics, and a handful of inexpensive parts can save a lot compared to having the gauge repaired professionally, if it is still in otherwise good condition.

Ed

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I want my MGB

Hey Paul,

Any luck yet?  I can't help but share this link Ed Holland sent me yesterday - after following the instructions, my little honey has a workin tach again!  Anyway, there's some good info here in these links - hope they help.  First one is all about converting from positive to negative earth.  The second one is more on "how" they work.  (Major kudos to Ed!)

http://www.lotus-cortina.com/electric/convert.htm

http://www.classictiger.com/techtips/motach.html

ciao all,

Lou

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Lou
'76 MGB MKIII

Capacitors are renowned for failing (short) especially after and extended period of non-use. The dielectric material inside dries up and crystallizes. It's the crystals that are actually conductive.

Before you go about performing surgery, have you tried adjusting R4 yet? Resistors too go bad, especially the old carbon composition ones used in the tach. They usually go bad by changing their value (mostly due to heat and humidity). R4 is there to compensate for resistor tolerances.

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On their death bed, nobody ever said, "Gee I wish I'd spent more time at work!"

Hey Davey!  as usual, the doctor is in....  :-)    that's some good info there.....

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Lou
'76 MGB MKIII

thanks to everyone with great feedback this will be my winter project

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Paul Macerone

1968 MG Midget

Lou,these are great links and very detailed and wiith my electronics background very understandable

 

Thanks

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Paul Macerone

1968 MG Midget

Davey, Now that I read how to take the bezel off i will try the R4 adjustment.

 

thanks Paul in NY

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Paul Macerone

1968 MG Midget

yeah, but without an electronics b/ground, how would a neophyte like me go about "adjusting" R4?  How would i know it's set properly?  I'm guessing R4 is the one with the plastic dial (with a flat-blade screwdriver slot in it) that you turn, right?  I didn't touch mine, but i'm wondering how accurate my tach is reading....

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Lou
'76 MGB MKIII

Lou, yes you are correct R4 is the white trim pot with the blade slot in it. I just took mine apart and measured where the R4 adjustment was located and drilled a hole in the metal cover so I can adjust the tach with a pocket screw driver when its assembled and installed. I will then hook up my Sun tach and dwell on the ingition coil start the engine and set the idle at 1000 rpm and compare the readings on the dash and hopefully the R4 has enough adjustment to get them to match. If not i will do surgery based on the links you provided. 

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Paul Macerone

1968 MG Midget

Davey, the R4 adjustment was made today and it worked perfectly!

What I did was take the tach apart and locate where the R4 trim pot was then drill a hole in the steel case so it could be adjusted when reassembled and hooked up my Sun tach and dwell and set both to 1000rpm

 

Thanks 

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Paul Macerone

1968 MG Midget

I love it when a plan comes together....Good going maceropa!

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On their death bed, nobody ever said, "Gee I wish I'd spent more time at work!"