Help - a bouncy Tacho....??

[alistair260661]

my 72 Mark 1 previously had electronic ignition in it, but when I bought it it had been returned to points.

I drove it home no problems (200 miles), but then had a coil failure later - I replaced the coil with the Rimmers standard 9 volt - and also did all the points, capacitor etc - the coil burned out again after about 100 miles running.

I subsequently discovered that the carhad been fitted with the 12 volt coil and the ballast resistor removed when the electronics were put in, and this set up left on the return to points............................ so I'veput it all back to how it should be, 9volt, balast resistor......................but,

i've got an on going problem with the Tacho in that it bounces around a fair bit, it also seems to "jump" without changes to engine speed - any ideas???? could it be the tacho itself (how can I check it)?, what about the voltage stabiliser (I haven't investigated that yet (again how do I check it?). Something else?

PS I'm certain that I have it wired as per the book diagram - oh and the other dials all seem to work fine.............:?

[Guest]

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Taking a wild guess....
The tach has a coil of wire inside. It uses amperage to generate the indication. Conventional tachs essentially count the pulses.
The Stag tach was engineered to the lower voltage, and the coil may be burned from its time with the 12 volts.
I am running with a 12 volt coil, but I have replaced all my instruments.

[V Mad]

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The early (Mk1) tachometer is a current impulse sensing type, and counts pulses of current through thewhite wire goingto thecoil. There are three connections to it, two are the white ignition wires and the third is the 12 volt supply to the tach.

When they bounce around it can be caused bythe current pulsesbecoming erratic. The cause of this (in your case) could be an intermittent connection in the ignition (white wire) circuit, bad capacitor (condenser) or bad points.If that is not the case it must be the tach itself. They are not easy to test though as you need a known good source of ignition current impulses.

[GDPR]

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Just to clarify the mk1 has an inductive loop tachometer, and the mk2 is current impulse.

The loop of wire on the back of the tachometer is the sensing loop, and it is very particular about the polarity of the coil (connections reversed) and the resistance of the coil primary winding as that dictates how much current is induced in the sensing loop.

I would suggest you recheck the coil polarity, and if the terminals are not marked try reversing them, and if they are correct then maybe the coil is wrong (I've always known them to be 6v not 9v).

Russ

[Guest]

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Before you get to technical could it be something obvious, poor earth connection to the tacho ?.

Don Kennedy.

[GDPR]

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Good point Don.

I just went down that route because it appeared (though not made clear) that the fault was ongoing from after the coil and ballast work.........

[V Mad]

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kryten wrote:

, and the mk2 is current impulse...........

Russ

You mean voltage impulse Russ :?

[GDPR]

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So you are awake then Chris

Bad day and a case of 'head up arse syndrome'

Apologies to all for misleading

[alistair260661]

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thanks all -



Coil - maybe it is a 6 volt - I can't be certain - I certainly got as for Mk1 Stag from Rimmers - so maybe it is 6 volt.................. it is cetainly on the right way round and the wiring is correct according to the book.

When I first got the car (before the first coil burn out (I think this was a 12 V coil without the ballast resistor) the tacho seemed much steadier. Now it seems much more irregular in relation to the (sound of) the engine speed. Also it suddenly kicks. At start up, while cranking the engine, the tacho is way over to max, then when the engine fires it comes back to more normal readings

Going back to basics for a minute, is there any reason why I could be burning out coils that would also show as the bouncy tacho? Voltage stabalizer?

[GDPR]

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No, the voltage stabiliser is only connected to the temp and fuel gauge - I hope! If the output of the stabiliser is connected to the blade connector of the tacho then it would explain a lot about the erratic tacho but not the coil. It could of course just be a faulty tacho after 38 years!

Coils burn out only for a couple of reasons - poor ratings/quality on the windings due to cheap manufacture, it's the wrong resistance primary winding (wrong coil) or it's shifting too much current due to the feed voltage being too high, usually because the ballast resistor is faulty or the wrong resistance (basic ohms law).

Check this by putting a voltmeter onto the coil +ve terminal with the ignition on and the points closed or the coil -ve shorted to earth. If you see more than 7-8 volts the ballast resistor could be out of spec.

Get yourself an accurate digital ohmmeter and check the coil primary resistance against spec too, or spend the extra and get a genuine Lucas coil.

Russ

[V Mad]

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You first need to make sureall your components are working properly and wired correctly. There are some easy tests if you have a voltmeter?

With ign on there should be 12 volts on one side ofthe ballast resistor. Then with points closed (engine off)the other side of the ballast should be about 6 volts (+/- a volt). Voltage at coil+ same as 6v at ballast. If that all check out OK youshould try a new capacitor, andthen new points as pitted points maycause erratic tach readings especially if the points have been overloaded by having no ballast.

Let us know the results if you are not sure.

[alistair260661]

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thanks guys -

it will have to wait for the weekend (if its warm enough!) for me to have a look at all of this.

I'm actually in the process of setting the whole engine up - so far been changing pipes, rebuild carbs etc................. so the tacho has been a bit of an irritation - main priority is to get the engine running right - so will check out points etc, and confirm right coil, then do timing, then have a bash at the carb mix............ hopefully there is nothing underlying which will screw everything up................ perhaps the Tacho will be resolved too.

will let you know - in a couple of weeks - I've got a bathroom sink to replace too (I cracked the current one............. fun fun)