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MGB MGB Tachometer

RETNJ

Member
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Seller of my newly-acquired '74 MGB told me that the tach was "intermittent." Following advice here, I cleaned and tightened the leads from the coil but it is still inop.
I've been carefully organizing and reading a large stack of invoices from the restoration of the car and found a note that "Tach is shorted and must remain disconnected" from 2006. Not so intermittent...
Anyway, is there a source for a replacement tach or a good place to have this one rebuilt over the winter?
Thanks.
BT
 
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Does the tach have an internal short...or is the short in the wiring TO the tach?

I have a tach from my 1974-1/2 MGB/GT available if you need one.

PM me offlist.
 
OP
R

RETNJ

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The notation indicates an internal short, but I don't know for sure.
It's a new harness assembly so doubt the problem is there. Probably should get behind the tach and look, but if wire is off the tach there, I should assume that it is because of the mechanic intentionally removing it. As soon as I figure out how to get to the back side of the tach I'll let you know what I see.
 

fogliner

Senior Member
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https://www.classictiger.com/techtips/motach.html

Here is a link that I found useful,there is a schematic and a ton of info on Smiths Tachometers.

I believe the gent was even rebuilding them for a while but not sure if he still is or not.

As for getting at it.....have fun and welcome to our nightmare.. :cheers:


Mark
 

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DrEntropy

Great Pumpkin
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Mark!!! You're doin' it wrong! Your feet need to be on the seat back, separated by the headrest! And yer nose rubbin' the bottom side of the brake pedal. :smirk:
 

fogliner

Senior Member
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Thanks Doc.,I was wondering why I always get a kink in my back. :thumbsup:

Actually I think the position you described is probably the ONLY way to get at the nut on the choke control.
Seems to me I had to do it from the passenger side though..go figger.
 

Bruce74B

Jedi Knight
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Oh no...working from the passenger side, where there is no steering wheel or pedals to get in the way, is definately CHEATING! :nonono:
 
OP
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RETNJ

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Last night I tried to get behind the driver side dash to inspect back of tach.

Oh my.

I could look or touch, but not do both at the same time. Got the front bezel & glass off & the tach is loose in the panel, but I don't know what holds it from behind so don't know what to feel for to get it free. I would appreciate some help with that.

I can see a loose wire with a green spade connector and an empty spade connector on the back of the tach, but can't reach in there to connect the two. Having found a mechanic's notation that "tach is internally shorted," I'm not sure that I want to. I just want to pull it back out of the dash so that I can test it and decide whether to repair or replace.

BTW - I could also see a disconnected demister tubing, but no idea how I'll get in there to do anything about it. Perhaps I should remove the drivers seat before working behind the dash?

Didn't invent any new works, but did put some old ones together in new ways..
 

DrEntropy

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It comes out from the front. There are (or are supposed to be) two knurled round nuts holding a stand-off bracket against the inner face of the dash. Flashlight and small mirror should reveal the setup.
 
OP
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RETNJ

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I purchased a used tach to replace the inop one that came with the car, but am having a problem getting it to work.
The inop tach was wired with two black ground wires connected via a ring connector to one of the tach mounting posts and a yellow wire to the positive spade connector.
That was clearly incorrect.
The wiring diagram shows a white & black wire from the coil to the tach, a green wire from the voltage stabilizer to the tach, and a ground wire.
Looking behind the panel I found a green wire with spade connector and a white & black wire with a bullet connector. Attached the green to the positive spade connector and the B&W to the male bullet connector on back of tach. No luck.
Tested the voltage on the green wire and it is steady at battery voltage. Tested the B&W and the yellow and had continually changing voltages and erratic readings.

1. Can I test a tach by wiring it directly to the coil and ground?
2. Testing with voltmeter, what should I see on the black & white wire from coil?
3. Does a constant voltage on the green from the voltage stabilizer indicate that the voltage stabilizer is bad? If so, where will I find that behind the ’74 MGB dash? Is the voltage stabilizer critical to the operation of the tach?
4. The only yellow wire that I can find on the diagram is for overdrive switch and transmission interlock switch. The overdrive is working fine so I suppose this is the transmission interlock switch, so I assume that I should be looking for an empty spade connector on the overdrive switch – right?
Thanks for the help.
 
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