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MGB MGB Tachometer

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R

RETNJ

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