Erratic Rev Counter (RVI Type)

[Baldrick]

Hi

First post. I have done a search and found lots of helpful info on rev counters, but not a solution to my problem. :?

Car – 1973 Rover P5B Coupe
Problem – Erratic rev Counter Needle
Type of Rev Counter – RVI – i.e. wired in series between coil and distributer
Occurrence – particularly when the indicators are switched on but can be brought under control if a heavy current draw is introduced into the system – i.e. rear window heater switched on

Before I send off the rev counter for repair I wanted to ensue it was the actual problem. It seems to react very sensitively to current draw in the system, and I am not sure if it is to do with the coil sensing circuit or the 12v supply to the rev counter. Yesterday I removed the rev counter, disconnect the car wiring loom from the coil and distributor and he back of the gauge and ran wires direct from the coil to the gauge and back to the distributor. The problem still occurred, so I can rule out interference in the white/black and white/slate coil to distributor wiring. The next step I will try (which I had thought about it when I had the car opened up yesterday) is to supply the gauge from a spare battery that is not connected to the alternator and car electrics. If the problem still exist, I can be fairly sure it is the gauge and the sensing circuit, if the problem goes away, then it would point to a problem with the power supply circuit either due to a problem in the gauge or the alternator supply. That’s my theory anyway.

Any electrical experts out there that have any thoughts.

 

[chrisyork]

Well done on navigating the net and the forum successfully, and welcome to the forum! Of course we shall demand details and especially pictures of your car as well!

I'm not sure about the P5, but P6's have a voltage stabiliser in the instrument cluster. You may find this is the culprit, it usually also shows up in varying degrees of error in the fuel gauge in particular.

Chris

 

[rp61973]

Could the voltage stabiliser be the source of my own problem? My RVI rev counter in my 73 V8 overreads by around 1000 rpm (too high) if the car has been parked in warm sunlight for a period of time and the dash has got a little warm. The rev counter also deflects wildly (3000-4000 rpm) whilst cranking over.

BTW, I run Lumenition electronic ignition (not sure if this has anything to do with it).

Weird, eh?

 

[Baldrick]

Hi, thanks for replies

I forgot to say that I hooked up a voltmeter to the rev counter volt supply and no voltage deviation was apparent. Also, the RVI part number is current detecting per another post in the forum, hence the "I" after the RV. There is a red box in the engine bay, about the size of a matchbox that is some sort of alternator control, it could be the source of the problem. When I get a photo, I'll post it.

 

[Tor]

My wee knowledge of RVI tachos is that they tend to fail in hot countries - components eventually give up in consistent high-temperature environment. My SA Rover resource has seen them fail all over the show, displaying various erratic readings in different situations including flailing around the red line on startup like rp61973's one. His fix is a replacement item, preferably a (minor) rewiring and upgrade to an RVC tachometer, which is more durable and doesn't tend to 'cook' inside. I created a thread when dealing with just this job. Your wiring might of course be different but the basics could well be the same

 

[Baldrick]

I am upgrading my ignition to the later electronic. I contacted JDO instruments regarding upgrading my Rev counter from the RVI type to RVC as RVI doesn’t work with electronic ignition. When I asked the chap at JDO about the wiring changes to the car, he told me that the RVI type was wired on the +ve side of the coil between the ignition and coil.. I tried to explain that it was on the negative side in series with the distributor and he disagreed. Does anyone have a copy of the wiring diagram that shows the wiring for the RVI as his website wiring diagrams http://www.jdo10.pwp.blueyonder.co.uk/page19.html do not currently reflect the Rover set up. Also, I hate being told that I am wrong, when I am only explaining what has been done by the manufacturer…

 

[darth sidious]

I am upgrading my ignition to the later electronic. I contacted JDO instruments regarding upgrading my Rev counter from the RVI type to RVC as RVI doesn’t work with electronic ignition. When I asked the chap at JDO about the wiring changes to the car, he told me that the RVI type was wired on the +ve side of the coil between the ignition and coil.. I tried to explain that it was on the negative side in series with the distributor and he disagreed. Does anyone have a copy of the wiring diagram that shows the wiring for the RVI as his website wiring diagrams http://www.jdo10.pwp.blueyonder.co.uk/page19.html do not currently reflect the Rover set up. Also, I hate being told that I am wrong, when I am only explaining what has been done by the manufacturer…

RVI's can be wired either between the coil - terminal and the points (P6's up to about 70/71), or between the +12v supply from the ignition switch and the ballast wire [which ultimately leads to the coil + terminal, of course] (P6's up to about 73, I think)

Both are valid because the tacho sniffs out the current pulses; because the coil + terminal is wired in series with the coil - terminal (as are the points of course), and the current is the same in a series circuit, the current going into the coil + terminal is the same (near as darn it!) as the current leaving the coil - terminal, going to the points.

Taking a bit of a liberty here (and hoping nobody gives me a tongue lashing and/or rap on my knuckles!), from viewtopic.php?f=16&t=10034#p75209, courtesy of Tor

Item 37 on that is the tacho, and it's clear it's wired between the coil - terminal (Item 44) and the distributor/points (Item 22). There's a white/slate wire from the coil -, which comes into the car and is terminated by a female bullet (which mates onto the male bullet connector on the tacho), there's female bullet on the tacho which is mated by a male bullet, the male bullet being terminated onto a white/black wire which goes from the car to the distributor/points.