Tach pegging to redline

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MilesnMiles
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Joined: Fri Nov 04, 05 8:40 pm
Location: Cornwall

Tach pegging to redline

Post by MilesnMiles »

Hi all, today my ‘66 Formula S factory tach suddenly decided to peg to the redline with motor running. Turn ignition off and it returns to base. Any ideas on this one?
Using original points ignition system.
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Dave999
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Joined: Thu Feb 24, 05 10:31 am
Location: Twickenham,London, England

Re: Tach pegging to redline

Post by Dave999 »

mine does this every now and again

not worked out why, hence everything below is a guess and suggestions based on that guess but it usually goes away when i

1) unplug and replug the connections in the chain up to the coil. i.e at ballast and to the coil bulkhead etc
2) arrange my plug leads so they don't run too near my electronic module or right across the coil mounting area on the engine block
3) blast out my igntion switch with contact cleaner (use one with lubricant, not circuit board cleaner)

potentially none of that fixes it

this started out as just the above apologies it got very long.. i was bored this morning.... this was the salve that maintained my sanity through 3 meetings :)


but ultimately i think its due to an aged capacitor, in the guts of the tacho. capacitors can collapse inside and short out, and can also self heal when they get charged sympathetically up to a proper operating voltage again. I presume the action of disconnecting and reconnecting discharges a capacitor somewhere and it gets a chance to dig itself out of the error state it is in

depending on type, capacitors have a life, in years usually, 10 or so, or usage hours 3000 being the minimum for high voltage electrolytic used at their maximum heat and voltage capability (nobody does that, you always over specify based on usage)

can more or less guarantee that any in your car will be well past it

believe a tacho has a circuit to protect it and potentially 1/2 rectify the flyback pulses from the coil
and a timer circuit made from resistor and capacitor in order to do its rpm counting job.
and a circuit to drive the pointer and keep it steady.

A capacitor stores charge and a resistor dictates how fast that charge can flow in to it and back out again hence its a simple timer

When the tacho pins the pointer to the end stop, its ability to count pulses per unit of time has gone up the Swanee, which would suggest its ability to do timing aint busted, its just wrong due to the capacitor not working right. i.e the capacity of the capacitor has reduced due to collapse of its internals. it needs to self heal and un-collapse its inner gubbins

this is potentially caused by a fly back pules of massive size from the coil or alternator (both big coils of wire) when you switch on the ignition, due to the position of the points or trigger in dizzy or a fault with the ignition switch.

the action of turning the key can switch ignition coil or alternator on off on off on as you go from ON to Run to Start and back to Run. depending on how crap your ignition switch is, i mean it shouldn't do this, but dirty worn contacts play a part.... and every time it powers the coil up and down it tries to make a spark and will fire a pulse at the tacho.

a poor capacitor in the tacho can get abused by this.

this timer circuit also produces pulses in a tiny coil near a magnet that moves the pointer against a hair spring, and that pointer driving circuit will have an inductor (electro magent) in it to act as a damper to the signal, so the pointer doesn't BUZZ and vibrate and snap, it averages the pulses out to let the pointer stay still at a specific RPM otherwise it would just look like a blurrrrrr.


and finally the trouble is this tacho circuit will be potted in epoxy resin which will be rock hard so you can't see whats in the circuit, removing epoxy uses chemicals that will melt it and most of the circuit contained within.

Hence given the high cost of replacement for mine (Aussie/Mopar/ R/T tax) i haven't had the guts to investigate further...i'll leave that until it properly dies

Try switching on with the points disconnected
or the coil diconnected
or the alternator disconnected
see if the tacho stays at zero.

insulate any disconnected wire to avoid sparks

that would help point to maybe igntion switch/coil or alternator blasting the tacho and causing a capacitor short

if one of them is
a zenner diode in the lead to the tacho may help

1 end to coil - and crimp on a connector to the other end so you can plug in the tacho wire
wrap in cling tape or cover in heat shrink

you will have to try it 1 way and then the other, a diode is a directional device that stops current 1 way and lets it flow the other, and i can't remember which way round it needs to be.

wrong way round and the tacho won't work.

IN4004 zenner diode would be ok but get one, good for say 400V cost about 30p worth a try

we don't know which way the errant pluse is going, so this guards against backward stuff hitting the tacho, a sure fire way to collapse the internals of a capacitor is to hit it with a voltage going the wrong way.. this is why many have the negative lead marked with a -

the diode allows the GODD but stops the BAD and the UGLY

Dave
The Greater Knapweed near the Mugwort by the Buckthorn tree is dying
MilesnMiles
Posts: 7309
Joined: Fri Nov 04, 05 8:40 pm
Location: Cornwall

Re: Tach pegging to redline

Post by MilesnMiles »

Ha ha, brilliant. You were bored eh :lol: ?
Thanks though, that does make a lot of sense. My ignition switch is pretty tired.
What your explanation does is help,explain what is now actually an intermittent problem.
Sometimes it works fine
Sometimes it pegs
Sometimes it read at 4,000 when I’m probably doing about 2,500rpm
It’s all old. Connections look good, but I don’t trust the ignition switch and by luck have a spare one for a ‘68 Charger that I keep in the boot.
Closest I’ll ever get to a ‘68 Charger🙂
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Dave999
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Joined: Thu Feb 24, 05 10:31 am
Location: Twickenham,London, England

Re: Tach pegging to redline

Post by Dave999 »

i saw an opportunity to occupy myself with something that seemed more worthwhile than a load of old guff about process and procedure :)

and the symptoms you describe are exactly the same as mine
an issue that has annoyed me off and on for the 18 years i have owned the car.

either pinned to the end stop, at the point of key movement
or just takes a bit of a funny turn and double counts for no apparent reason when driving along
switching off and on makes no difference .

does it for usually about a month and then no problem for 9 months to a year

i have had many ignition switches ...well many ignition switch rebuilds... things always seem better after everything has been un plugged and re plugged

and i mean everything....it seems to replace the odd 1/2 volt missing here and there and stuff just works better :)

Dave
The Greater Knapweed near the Mugwort by the Buckthorn tree is dying
MilesnMiles
Posts: 7309
Joined: Fri Nov 04, 05 8:40 pm
Location: Cornwall

Re: Tach pegging to redline

Post by MilesnMiles »

Yes, messing with electrical stuff is always..interesting.
Two day ago my mates Belvedere had all indicators on at the same time. Or none, or some. Much thinking and fettling later everything is now normal. It really is often the smallest of details that gets it done
Or blind luck 😀
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