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MG MGB Technical - Is there a way to test a tachometer...

In my restoration I've converted my '67 B to negative ground. I followed all the instructions for rewiring my tach, which is not working at this time. Trouble is, it's been so long since I drove car, I can't remember if the tach worked before! (no comments about old age, please). Is there a way to test the tach's circuitry before I pay some instrument shop to have a go at it?
Thanks, Dave
David Steverson

David,
The Tiger group on yahoo has developed a replacement board for the smith's electronics. Allows use of any ignition system's tach output signal and has other advantages as well. Should be available about now. Price for the board is about $35 or about $80 to send it to them for rework.

Jim
Jim Blackwood

Jim, I am having a problem finding the Tiger group on Yahoo. Can you point the way?
John Hubbard

The Tiger home page is at http://www.classictiger.com/default.htm
However, I could not find anything about a new circuit board for the tachs on their site. The only thing regarding the tachs is Olson's write up of the tach and how to calibrate it. Good luck - Dave
David DuBois

If the engine is running then that is the obvious way and compare it against something else like the rpm scale on an auto multi-meter. If not running rig up something to spin the dizzie from a drill whilst connected up to a coil and a 12v battery, and compare as above.
Paul Hunt

Sorry, guess I could have been more specific. Try this link. I've signed up on the Tiger list and am getting emails but now I see it isn't listed in my yahoo groups.
Anyway, try this link: http://www.classictiger.com/techtips/motach.html
for details of the modification. The tiger list is: owner-tigers@autox.team.net

Jim
Jim Blackwood

So what exactly does this have to do.
The G

I did read an article once on the use of a 12v battery charger to test rev. counters. The output from a battery charger is 100 htz. This means that 6000 pulses are sent each minute which is identical to 1,500 rpm on your rev.counter. Apparently all you need to do is connect the charger to the input and output sensor cables on the back of the rev. counter. Having said that I have not tried it but it sounds sensible
Harold Carrington

Harold,
That sounds interesting. I might give it a try. I mean, the tach's not working anyway, so I can't "break" it anymore!
Dave
David Steverson

G, it has todo with a way to fix his tach. And probably make it better than new. However, if the movement is burnt out it's a goner. Tigers, as you should know, use the same tach. In fact all the Smith's tachs are the same with minor differences, so you can swap parts and such to make a good one from just about any you can find. The original circuit is nothing special, uses obsolete transistors (big surprise) that are no longer available, and the Tiger bunch being relatively progressive, have designed a new circuit. Give it a look, it really is an improvement. Quite amazing what can be done with electronics these days you know.
Jim Blackwood

Harold's suggestion does sound feasible. The tach will need a smoothed 12v supply i.e. from a battery to its 'green' and 'black' terminals, and the current-operated type will need a coil or similar in series with the inductive loop and charger, but all doable.
Paul Hunt

Jim,
I have looked at all the links you supplied and still have not found any mention of a new circuit for the tach. Any further direction would be greatly appreciated.
John Hubbard

Sorry, did I post this link? http://members.shaw.ca/tsmit/tachmod/tachmod.html
Also: http://members.shaw.ca/tsmit/tachmod/ for the directory. I think most of that info is in the first link though.

Jim
Jim Blackwood

Thanks, Jim. That should save me some money, if not printer paper, and provide some entertainment doing the mod. Joe
Joe Ullman

Thanks for the link Jim. Very interesting.
John Hubbard

Everything you need should be in these links:

http://www.mgcars.org.uk/electrical/body_tach.html
http://www.engravers.com/tiger/tech/stoa_tech_9904.pdf
http://members.shaw.ca/tsmit/tachmod/tachmod.html
http://members.shaw.ca/tsmit/tachmod/tachmod0.html
http://members.shaw.ca/tsmit/tachmod/tachmod2.html
http://members.shaw.ca/tsmit/tachmod/tachmod3.html

There is a pot inside the tach for adjustment, Theo’s address is on the tachmod0 page. Details of the battery charger calibrator are on the 1st link.

Jim
Jim Blackwood

Harold's suggestion seems a good one except that with respect I think 6000 pulses per min equates to 3000 rpm and not 1500 rpm as there are 2 ignition impulses per rev on a 4 cyl 4 stroke engine

Chris
chris

While we are on the subject of electrical stuff can anybody tell me if there is a website that I can get complete, accurate, printable wiring schematics for British cars, especially MGB variety. Thanks.
Kevin

You can find MGB wiring diagrams in color at, http://www.mgbexperience.com/ Scroll down to DIY/Service and look for wiring diagrams. Clifton
Clifton Gordon

You can see them broken down into circuit elements at http://www.mgb-stuff.org.uk, click 'Spanners', 'Electrical System' and 'Schematics'.
Paul Hunt

This thread was discussed between 01/12/2002 and 13/12/2002

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.