Tacho inoperative

[jeremybarnes]

Looking for a little troubleshooting advice, before the dive-in...

Went for a drive to a friend's new year's party yesterday, and on the way home, the tach died. I was doing about 70 mph on the freeway -- call it 3100 or so rpm -- when traffic slowed. I dropped to about 40 mph, accelerated around the incident and back up to speed. Pulling hard in 3rd at about 3500 rpm, I heard a small "pop" from behind the dash and the tach dropped to zero. No difference in the way the car ran, but the tach remained at zero.

I did a key cycle while driving (the Lucas "reset" button), but nothing. Everything else on the dash worked normally, car continued to run perfectly. If it matters, it was about 8pm and the headlights were on. Stock wiring, had had zero issues up to this point.

Got home, checked to see if the wire on the tach generator might have fallen off, but no such luck.

So... Recommendations on where to start? Has the tach died and I need to do the Spiyda conversion to bring it back to life (this thing: http://www.spiyda.com/M/tachometer-elec ... dules.html). Is it something as simple as a fuse internally? Or...?

Thanks all, and wishing a wonderful new year full of successful and safe E-Type motoring!

Jeremy.

[christopher storey]

Your car has the AC driven tacho, so it does not form any part of the cars electrical circuits, but is a self-contained AC circuit driven by the tach generator behind the inlet camshaft . The Spidya conversions are for the later S2 type electronic tacho only. I do not really like the sound of the pop from behind the instrument although i cannot think what it would be, and there are no fuses etc to be considered. First check ( it sounds as though you have already done this ) that one of the two wires has not come adrift from the tach generator. If that has not happened, then the next thing to do is to put an AC voltmeter across the generator terminals and with the engine running you should see about 10 volts per 1000 engine rpm . If you get voltage, then you will need to look at an instrument failure/disconnection. If you have no voltage, then it is a generator failure and the commonest mode of this is that the drive has become deranged. I suggest you search these boards for tach generator failure or similar, as there have been plenty of threads discussing this and what to do

[jeremybarnes]

Sorry for the delay -- update:
- with a volt meter set to AC volts, when I approach the engine compartment, I'm showing voltage on the meter. It's like there's voltage in the air. Yeah, that's weird, I know...
- anyway, with the meter on the two prongs on the generator, I get voltage, although it's shaky from 6-10 volts
- inside the car, I get the same voltage from the two wires that go to the back of the tach (strangely, the voltage I get "in the air" by the running engine, I don't get inside the car. Still weird...)
- when I tap on the tach, the needle bounces, so I know the tach isn't frozen or something strange

So... Dead tach? Generator not making enough voltage? Something else weird?

Thanks all.

[christopher storey]

It does sound like a dead tach from your description. The circuitry is fairly simple - it is essentially an AC moving coil voltmeter and Ray Livingston described it this way

"The S1 tach consists of a number of components: the tach-generator, the gauge itself, and inside the gauge a selenium bridge rectifier and a series inductor. The tach-generator outputs roughly 1V/100 RPM. Doing a little quick math, you'll see that's almost 60V at 6000 RPM. The gauge movement needs only a few volts to reach full-scale. The combination of the series inductor and the gauge resistance forms an AC voltage divider. Since the tach-generator is really a permanent magnet alternator, the frequency of it's output increases linearly with RPM. The impedance of the series inductor increases with increasing frequency, so the voltage drop across the inductor increases at higher RPM, limiting the voltage applied to the gauge movement to a safe level. The current load on the tach-gen is highest at max RPM. What tends to happen to these things is the magnets get weak from heat, vibration and age, which causes the voltage output to fall off and/or the ability to output enough current to provide the necessary max RPM output. The result is, quite consistently, when a S1 tach is off, it reads low. Sometimes *very* low. Mine was off by nearly 25%. Replacing the tach-gen may or may not really solve the problem, since the new ones seems to be essentially un-calibrated, so there is huge variation from unit to unit. My solution was to build a microprocessor board that uses the tach-gen only as a frequency source, not caring about the voltage or current levels, and drives the gauge as required to achieve about +-25 RPM accuracy at any RPM. Personally, I think your best bet is to consider having yours converted to a modern ignition-driven gauge, or slip in a S2 tach. I believe MoMa will do this, among others. Other than that, about all you can do is buy a new tach-gen, but be prepared to return/exchange it if you find it is off-calibration. Also, *never* trust the S1 mechanism at high RPM, unless you've recently verified its accuracy. I'm sure more than one E-type owner has blown an engine because of the lousy tachs."


If you take the instrument out and take the works out of the canister you should be able to see and check the various components . One failure I have come across is that the solder which secures the spring of the moving coil breaks and thus breaks the circuit

Mr Tesla ( Nikolai not the car people) would be interested in your voltage in the air

[jeremybarnes]

Spoke with the people at Nisonger here in the US, and from my description, they suggest it's a tach issue too. I removed the gauge from the dash last night and will be sending it off to them today. They say a 2-3 week turnaround, so I'll update this thread when it returns (and hopefully works!). Will pull the speedo out while I'm at it, and give it a thorough clean so that it'll match the tach when it comes back.

They also suggested sending the tach generator in to them, as they'll clean it up and check for accurate operation as well, so I'll be doing that too.

And lastly, while I have the tach out, I've pulled the clock off the back and will be sending it off to Mike Eck for one of his conversions. In for a penny, in for a pound, eh?

By the way, I opened the tach up and looked at all the wiring and all the solder connections inside, but I saw nothing to suggest any of the wires had come loose or broken, and nothing looked burned, blackened or singed either. Will be most interested to see what Nisonger finds.

So, stand by for updates in a few weeks...

Jeremy.

[cactusman]

The tachometer contains a selenium rectifier, a choke and a moving coil meter. The meter is very unlikely to have failed. Even less so the choke. My money would be on the rectifier especially if it has shorted to ground somehow inside....I am guessing the rectifier is extinct as a replacement so they will have to use silicon diodes and 're calibrate...

[jeremybarnes]

Hi all, wanted to finally close the book on this one for future forum sleuths...

I sent the tach and the generator off to Nisonger here in the US. Got both units back today, (6 weeks) put it all back together and it works perfectly! The tach came back fully rebuilt and cleaned, and looks brand new. They even clear-coated the housing, it seems, and it looks as good behind the dash as in front now. Honestly no idea what was wrong or what they did inside either unit -- the invoice says "tested and required rebuild" on both.

With the tach looking new, the speedo was letting the side down a bit, so I took the time to remove it, remove the glass and thoroughly clean that too, and it now matches the tach. I had Nisonger send me two new gaskets that go between the gauges and the dash too, and it made a nice touch to reassembly. Took the time to take the other four gauges apart and clean the glass and faces on those too.

Not sure what they did to the tach generator, but it too looked brand new -- cleaned and greased, with a little white paint mark on one of the screws that hold the electrical connection piece to the metal body (which I know wasn't there when I sent it off), and they included a new o-ring on it too.

All that said, and while it's fixed (and, let's be honest, ain't nothing cheap on an E-Type anyway...), it was not an inexpensive proposition -- the tach rebuild was $185 and the generator redo was $110. Add postage and it was a $325 project. Still, couldn't be happier at the moment with the outcome.

And lastly, I sent the clock (which didn't work) off to Mike Eck for his innards update, and it works perfectly -- it's so cool to sit in the car in the garage, in the quiet, and hear the clock tick like it did back in October 1967 when my car first drove out of the showroom. Who knows if it'll keep time, but it does sound cool!

Hope this helps someone down the road.

Jeremy.