kenmorgan
Rover Rookie
Posts: 6 
Location: Ashford, Kent
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Post by kenmorgan on Jan 25, 2016 20:04:42 GMT
Can anyone help? My Rover P5 coupe rev counter on start up/tick over goes up to 1500rpm. on touching the throttle it was then races off the gauge! Can anyone help with this problem? Many thanks
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Post by johnwp5bcoupe on Jan 25, 2016 20:57:30 GMT
Can anyone help? My Rover P5 coupe rev counter on start up/tick over goes up to 1500rpm. on touching the throttle it was then races off the gauge! Can anyone help with this problem? Many thanks Hi Ken there are lots of things it could be but more than likely the Capacitors etc in the Rev Counter have passed their sell by date the best way would be to get another from David Green or get yours repaired! |
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Post by Ken Nelson on Jan 28, 2016 1:26:48 GMT
I think the tachometer problem is pretty universal when the electrical components get this old. Many posts written about this. I finally solved my tach problem by sending it to Nissonger Instrument repair here in the US and it works perfectly now. Cost was about $150 US. I'm sure there is a similar repair service in England to avoid shipping costs.
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Post by djm16 on Jan 28, 2016 15:00:38 GMT
Not readily fixable without an oscilloscope, circuit diagram (they are available on Google) and some electronic know-how.
A long shot. Have you recently changed / upgraded your ignition system?
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kenmorgan
Rover Rookie
Posts: 6
Location: Ashford, Kent
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Post by kenmorgan on Jan 30, 2016 16:00:05 GMT
Thanks for your replies. Will look into possible solutions suggested. Ignition has recently been upgraded but problem was occurring well before the upgrade.
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Post by djm16 on Jan 30, 2016 23:04:47 GMT
The way your speedo TACHO works may well be as below. The earlier speedosTACHO [thanks guys.  ] were triggered by current pulses drawn by the ignition coil, later ones by voltage pulses. The current supply from the ignition switch to the coil passes the tachometer, where one turn of the wire is wrapped around a soft iron core on the back of the tach. Inside the tach is a secondary coil making up the other half of what is essentially a transformer. Each time the contact breaker opens the current through the coil suddenly changes. The sudden change in current through the single winding on the back of the tach causes a secondary current to flow through secondary winding inside the tach. This current is then amplified / buffered by a single transistor (from memory) before being fed to one side of a monostable pair of transistors causing a state change, and a current to flow through the armature coil of the tach needle. The monostable remains in the ON state well after the curent pulse from the ignition coil has passed. The duration of the ON state determines how far over the tach needle moves. The duration of the ON state is determined by the time constant of the R / C combination that makes up the monostable. Typically the capacitance C remains the same or goes down, but old capacitors can develop an electrical short circuit making R smaller, and the time constant shorter. The commonest repair therefore is to replace that capacitor. I have just convinced myself that the usual failure mode is for the tach needle to under read not over read. A failure of the R resistor is vastly less likely. A poorly soldered jt at either end of R is possible. Spurious extra current pulses coming from the coil would also cause an over-reading as would break-down creating noise in the buffering transistor. Hence the need for an osciloscope to locate the source of the extra currents. |
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Post by johnwp5bcoupe on Jan 31, 2016 9:40:20 GMT
The way your speedo works may well be as below. The earlier speedos were triggered by current pulses drawn by the ignition coil, later ones by voltage pulses. Better edit these 2 typos while the post is still fresh. No he didn't mean Budgie Smugglers Warwick  |
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Post by johnwp5bcoupe on Jan 31, 2016 9:47:59 GMT
I'm glad I haven't got one!I think Stephen Hawking would be struggling with that explanation! It's a good description of how a rev counter works Richard, I have a scope if you want to borrow one when you fit yours and it reads incorrectly  |
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Post by johnwp5bcoupe on Jan 31, 2016 10:41:15 GMT
It's a good description of how a rev counter works Richard, I have a scope if you want to borrow one when you fit yours and it reads incorrectly I've got a saloon John and fitting the hazards taxed my tiny brain to the limit re electrickery matters! I did say "when" Richard |
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Post by Ken Nelson on Feb 3, 2016 21:13:00 GMT
I'm glad I paid the $150.
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] were triggered by current pulses drawn by the ignition coil, later ones by voltage pulses.
