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Tachometer and Speedometer Failure

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Twstnshout

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Took my project 55 survivor for a first drive today. A little chilly here in New Hampshire with the little windscreen and no top. The frost heaves were probably not in the design plans either. But everything worked after sitting for 55 or so years; even the overdrive.

The speedo and tach worked and then they didn’t. Are they both mechanical? I can’t find any troubleshooting mention in the Service Manual. It’s hard to follow 70 year old wires as they are all brownish black.

Any thoughts on where I should start?
 
They are both mechanical. Wiring to them would be for internal instrument lighting, high beam indication, and generator not charging. You can unscrew the cables from the instruments and see if they rotate when the engine is running (tach) and when driving (speedo). If the cables rotate, the problem is in the instruments; if they don't the problem is in the cable or drive gears.
 
It's odd that they both worked, then quit. It's possible, but not likely, their cables both snapped, though I've never had one snap. The speedo is driven by a right-angle drive off the O/D housing; it can be problematic. The tach on my BN2 bounces around, and I haven't been able to fix it. Next step is send it out for a rebuild. The speedos have a worm gear that drives a couple other gears for the odometer and trip meter; again problematic. The driven gears can crack radially and cause the needle to 'blip,' proportional to speed. Parts are hard to come by, so I picked up a couple used speedos just for parts.

As suggested, the first test is to remove the cable and housing from the back of the instruments and see if the cables rotate; if they do, the problems are likely internal. Fortunately, the tachs and speedos on Big Healeys are easy to remove and replace. I have a short piece of old speedometer cable whose last inch or so I squared off with a grinder so that it inserts into the drive mechanism. To test, I mount the tach or speedo in a vise and spin the inserted cable with a Dremel with RPM readout to see if the instrument works; you can roughly calibrate them this way.

I've attached the 'goto' for Smiths speedometers; the tachs work the same except, of course, they don't have odos or trip meters.
 

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  • Speedo_Repair.pdf
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Twstnshout said:
What does tach run off? Not the generator
Click to expand...
OK, I couldn't get a photo. The tach is driven off a shaft that drives the oil pump, but there is an angle drive on the side of the block, under the carbs and exhaust manifold. It's in the center of the block, and because the carbs and exhaust manifold are in the way, one can't see it, or photograph it, but one can reach under there and feel it. Because it can't be seen, I would be reluctant to remove it. It might be visible from underneath.
 
I got the tach drive out it was broken about 12 inches inside the sleeve. There is a tube hanging down off the bottom of the intake manifold not attached to anything. Like a vacuum tube what is that for. It isn’t connected to anything. This car has never been restored and only was driven till 1969 so the nooks and crannies are really oily and dirty.
 
The tube attached to the intake manifold is probably a excess fuel drain.
 
You can test the instruments with a 1/8" square drive bit in a battery electric drill (spinning backwards, CCW, if I remember right, from orientation behind the head of the drill)
 
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