Intermittent rev counter

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Herzeg
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#1 Intermittent rev counter

Post by Herzeg » Tue Dec 01, 2020 11:18 am

Morning

My rev counter sometimes stops working. I can be happily driving along, look down and it's at zero. I've seen it happen four times now over a period of months, but the weird thing is that if I turn off the engine and re-start it works.

Anyone got any idea what could cause it? Nothing else is wrong, and there are no symptoms. when it stops the car is fine. Could it be something intermittently going wrong in the rev counter, or could it be something at the other end? As it only happens occasionally I'm not going to fret but would be nice to know in case it goes for good

John
1969 S2 OTS

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Tony
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#2 Re: Intermittent rev counter

Post by Tony » Tue Dec 01, 2020 11:49 am

If you read my EZ power steering topic in the general section I had the same problem with my rev counter. The answer is normally with the Rev counter generator on the rear of the cam shaft on the inlet side of the engine. One cause could be a worn dog washer that fits into the end of the camshaft. Another option is that the shaft going into the generator gets play in it and it comes away from the camshaft, so does not spin. In my case the shaft going into the generator started slipping in the rotor which generates the singnal. The good news, assuming it is the generator is that it is easy to get too (three bolts) and you do not need to take off the camshaft. When out on the bench spin the shaft with a drill and see if you are getting a voltage reading which will increase with speed. If no voltage when out then either the shaft is spinning in the rotor or it is caput. Worth trying to fix as they are about £40 to £ 50 to renew.


PS : just noticed yours is a series two so not sure if they have the same generator as the series 1. Easy to find out though.


Tony

Good look and keep us informed
Tony (E typed)

Tony

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Tbob
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#3 Re: Intermittent rev counter

Post by Tbob » Tue Dec 01, 2020 1:46 pm

On my S2 the tach (in “Yank-speak”) gets its its tick signal from an ignition control wire running through a coil on the tach. My car won’t run if this wire’s disconnected, so if your car is running fine while this happens, I’m pretty sure the fault must be internal to the tach. That’s my guess.
Bob t

LHD '69 OTS. (Former) basket case

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ralphr1780
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#4 Re: Intermittent rev counter

Post by ralphr1780 » Tue Dec 01, 2020 2:17 pm

John, check the fuses on the fuse block.
The same fuse that feeds the rev counter feeds as well the rear stop lamps and the radiator fan.
With some corrosion on the fuse contacts, when these heat up they could disconnect for a short while until cooled down.
I had the same happening...
Ralph
'69 OTS + '62 OTS - Belgium

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christopher storey
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#5 Re: Intermittent rev counter

Post by christopher storey » Tue Dec 01, 2020 3:10 pm

And also check the earth lead to the instrument - usually attached to one of the fixing posts

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Herzeg
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#6 Re: Intermittent rev counter

Post by Herzeg » Fri Dec 04, 2020 6:06 pm

Thanks for all the comments. It's too cold to drive right now so when I next go out I'll check the brake lights if it stops again. Otherwise I suspect it's internal in the tacho but that's beyond my skill set so I'll just wait and see what develops.

John
1969 S2 OTS

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christopher storey
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#7 Re: Intermittent rev counter

Post by christopher storey » Fri Dec 04, 2020 6:34 pm

John - you cannot rely on the brake lights still being on the same fuse as the tacho moving coil feed, so its best to check and/or clean all the ( from memory ) green wires . Just pulling them off the fuse blocks and replacing them usually will resolve a bad connection

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Herzeg
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#8 Re: Intermittent rev counter

Post by Herzeg » Sat Dec 05, 2020 11:52 am

Thanks Christopher

I'll have a little delve around bhind the dash during the week and see what I can find.

John
1969 S2 OTS

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MarekH
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#9 Re: Intermittent rev counter

Post by MarekH » Sat Dec 05, 2020 8:41 pm

I'd look for a poor earth connection from the tacho board inside the tach to the tacho body and then to the outside world. This is probably via one of the studs that hold the circuit board inside the tacho, rather than by a wire with a decent solder tag and star washer to the chassis. After so many years, the soldering may need to be refloated for some of the components on the circuit board inside the unit also.

Engine bay wiring via those Lucar connectors can also become unreliable. I'd undo and do these back up after treating with a Deoxit type product and retightening once the female connector has been pinched with some pliers to make sure it contacts properly.

In general, this type of electrical failure happens most frequently at moving parts, followed by connectors, followed by heat cycled parts.

kind regards
Marek

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