Tachometer question again...

C

chris varming

Member
Gentlemen, my tacco is not working after I have fitted an electronic distributor. I have read the threads on Tachometers, but still have some questions.
1)What does RVI and RCV actually stand for?
2) I have a few tachometers all from P6. RVC 2412/00 (0->70 RPM) and RVI 2832/00A (0 ->60 RPM) which one will fit?
3) on the rear there are 2 grey connections, the upper one is for the bullet wire but the lower is not connected? what is the purpose for the lower?
4) will any of the 2 Tachometers fit directly into my P6 or do I have to buy a converting unit as shown in the thread?
Thanks regards, Chris Varming++
 
Chris

RVI is a current triggered tacho, RVC is a voltage triggered tacho (don't ask me what the specific letters are). Have a read here http://www.mgb-stuff.org.uk/tach.htm and here http://www.gomog.com/allmorgan/smiths_tachometer_conversion.pdf for some general and specific information.

Essentially an RVI, or current sensing tacho needs to be connected in series with the trigger at the coil. This is what the 2 bullet connectors are for. To connect this for correct operation with points ignition you would run a wire from the coil negative to one of the bullet connectors on the back of the tacho, then run a wire from the other bullet connector to the distributor. From a circuit point of view the tacho then sits between the coil and the distributor with the current flowing though the points acting as a trigger to a small sensing coil in the tacho.

An RVC, or voltage sensing tacho is connected between the coil negative and the vehicle earth. In this instance it's the small voltage generated by the points opening and closing that triggers the tacho.

This is generally why RVI tachos don't work with electronic ignition. If there are no points, there's no current flowing across the sensing coil to trigger the tacho into operation.

Bizarrely, and I haven't figured out why yet, when I fitted electronic ignition to my '72 3500S with an RVI tacho, it worked fine with no conversion needed.

Hope that helps.

Dave
 
chris varming said:
2) I have a few tachometers all from P6. RVC 2412/00 (0->70 RPM) and RVI 2832/00A (0 ->60 RPM) which one will fit?
Click to expand...

The 0->7000 rpm is from a 4 pot, while the 0->6000 will be from a V8.
If you fit the first to a V8 it will read aproximately double the actual rpm, if you fit the second to a 4 pot it will read half the actual rpm.

Dave3066 said:
This is generally why RVI tachos don't work with electronic ignition. If there are no points, there's no current flowing across the sensing coil to trigger the tacho into operation. Bizarrely, and I haven't figured out why yet, when I fitted electronic ignition to my '72 3500S with an RVI tacho, it worked fine with no conversion needed.
Click to expand...

Dave, also the lumenition magnetronic in my TC caused no problems to the RVI tacho.
 
Thanks Dave and Demetris for your response. I've now found a tachometer among my spares with the designation RVC 2812/00 (0 -> 60000) guess that will fit without conversion? Regards, Chris Varming
 
6 years ago, after replacing the points with a Pertronix Ignitor, the RVI tacho in Occie continued to work just fine on the original ballast setup.

Prior to this, when I was looking into it, Marvin Grebow at Pertronix had been of the opinion I'd have no problems, but if necessary a 10K 0.5W resistor fitted in line with the tacho and coil negative would fix any problems. However, I didn't need to do this.

Early last year I fitted a new electronic distributor from simonbbc and converted to non-ballast. The RVI tacho, as expected, didn't work but I ran the car for a short while with the inactive tacho. However, I wasn't really happy with the distributor and went back to a Lucas 35D8 with the Ignitor, retaining the non-ballast setup.

Tacho is working again, and I'm much happier with this setup.


Stan
 
Stan,

What kind of problems did you face with the Simonbbc distributor? I have one in my P5B for about 8 month now and it is working well, Did you change the coil also?
Peter
 
Peter,
Yes, fitted the Lucas electronic coil that was part of the package.

Was running well for a couple of hundred miles but the non-Lucas module then failed leaving me stranded. Replaced under warranty, but failed again. I'm not convinced these modules are man enough to stand up to engine bay heat.

Got a Lucas module but, having somewhat lost confidence in the setup, I reverted to the tried and trusted Pertronix and fitted a Flamethrower coil from Martin at Distributor Doctor.

No problems now.

Stan
 
Stan,

I believe they supply these now standard with the Lucas module. I have now done 8000km with the ignition and use my p5b daily. it is living outside and runs on LPG and even on cold winter mornings it started very well. I was very surprised about the low price of the set as it included a complete new distributor.

Peter
 
Glad to hear you're pleased with the setup, Peter.

Interesting that they're now supplying the Lucas module as standard...I was told, quite adamantly, that there was no difference in quality between that and the pattern part.

If, for peace of mind, the module had been an easy roadside change, I'd have probably persevered with it a while longer (using a Lucas module) but I'm quite happy now with (and confident about) the Pertronix mated to the Flamethrower coil.

Cheers,

Stan
 
vaultsman said:
I was told, quite adamantly, that there was no difference in quality between that and the pattern part.
Click to expand...

Sometimes, there just aren't enough of these......... :LOL: :LOL: :LOL: :LOL: :LOL:
 
You must log in or register to reply here.
Back
Top