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8 & 10 cyl Bristol cars Type 407 onwards - restoration, repair, maintenance etc

Rev Counter Polarity

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  #1 (permalink)  
Old 08-08-12, 11:34 AM
Join Date: Nov 2010
Posts: 11
Default Rev Counter Polarity

I converted my 407 to negative earth last year to accommodate a new starter motor (thanks to Greg and Dave for helping me source that) and i simply disconnected the positive earth rev counter at that stage. I thought i'd now get around to converting the Smiths RVI 2810/10 rev counter to negative earth so i can get it back in use and have found several sites on the internet which talk about simple polarity conversion of Smiths rev counters from the early 60s,(although not specific to this actual model). However,although the pictures of the "workings" appear almost identical to the innards of mine, they all talk about attaching the green earth wire to the resistor currently attached to the large spade live connector. i don't have a green wire ! Does anyone know what colour wire currently earths the unit please (or is it a solder circuit connection as i can't see any obvious wire for earthing) ?
Any input most gratefully received.
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Old 09-08-12, 12:51 AM
Join Date: Mar 2011
Location: Hopkinsville, Kentucky
Posts: 188
Default Change in polarity on electricals

This may be of some help, and it was successful on my Aston Martin that had positive ground. I wanted to replace the radio,as the original could not be fixed economically,so I decided to buy a negative ground unit. The swap was really simple. I merely used the chassis as the positive side, and insulated everything else from touching,and it worked just fine. I suspect this might work for you. I know, it is an odd arrangement, but it worked for me. All the best. ron
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  #3 (permalink)  
Old 09-08-12, 11:01 AM
Join Date: Nov 2010
Posts: 11

Thanks Ron
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  #4 (permalink)  
Old 09-08-12, 07:51 PM
Join Date: Jul 2008
Location: Renfrew Scotland
Posts: 71
Default rev counter polarity change

I had the same problem,compounded by installing electronic ignition.
I found some interesting info on the mg forums re adapting the rev counter.
However, if you are contemplating electronic ignition, it will not work!
The rev counter counts the pulses by utilising a CT. The pulses with electronic ignition cannot be counted.
I sent my unit to JDO instruments Home page in Yorkshire. I found them very helpful and the service was first class. Other companies can also provide the same service.
Good luck!
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  #5 (permalink)  
Old 09-08-12, 09:32 PM
Join Date: Apr 2009
Location: York
Posts: 808

Why do you need a rev counter in an automatic ?

Anyway -- either get one from a 411 / later V8

Or this may be useful Positive to Negative Ground Conversion

Electronic Tachometer Wiring Reversal.

First, read the front of the tachometer. If it IS marked "Negative Earth" it probably IS negative earth already. But then, you probably wouldn't be swapping your car's electrical system, and wouldn't be reading this. If it is marked "Positive Earth", and there are no other labels on the back housing (notice this after removal of the tachometer from the instrument panel) indicating that it has been changed over, follow these instructions.

Remove the tachometer from the instrument panel. Look at it carefully. Draw a picture of the orientation of the connections for wiring, attachment screws, note markings etc. and... most importantly... note and carefully draw a picture of the orientation of the loop of white wire running through the plastic holder. This will be reversed later in the instructions.

Now turn the tachometer over, glass face down, and locate on the sides of the tachometer, the bent tab edges of the chrome (or black) bezel. Use a very fine-blade screwdriver to wedge between the tachometer body and the bezel tabs. Bend them out ever-so-slightly one-at-a-time, turning and rotating the tachometer, regularly trying to rotate the bezel free from the tachometer body. Go around and around. The very least amount of bending of these tabs is best. When the bezel finally rotates free, turn it until it disengages from the metal body, remove and set aside. Carefully... very carefully, pry off/out the metal shroud and glass from the tachometer body if it did not come off with the bezel. Be really REALLY careful with this glass and shroud, as it is easy to damage either or both. Also, be careful to take care of whatever seals you find here, as replacements are not available...

Set the bezel, glass, shroud and seals aside to clean carefully before later replacement.

Once the glass has been removed, be very careful not to disturb the needle, or leave fingerprints on the face of the tachometer. Turn the tachometer face down, looking at the backside. Refer to the drawing you made earlier. Locate the 2 slotted screws holding the mechanicals of the tachometer inside the outer housing. Unscrew them carefully, slowly, until they are almost free. Pick up the tachometer in one "soft" hand covered with a tissue to prevent fingerprinting-about-to-happen, and complete removing the screws. The tachometer "guts" will fall out into your tissue-covered hand, hopefully, and not onto the floor.

Look carefully at the internal wiring connections. Look for the small 1/2 inch long brown, striped, diode. It is connected to the metal tab that provided electrical power into the tachometer. Nearby, look for the small green wire, soldered to a metal lug that connects to the metal internal frame of the tachometer, providing a ground. Unsolder these two connections, reverse the connections and solder in place. Do not use any more solder if you can avoid it. Just heat, remove, swap/replace, let cool. Now the small brown diode connects to the ground lug, and the thin green wire connects to the "power in" lug. This is now correctly wired for "Negative Earth", as is the picture below.

Take the time to carefully dust off the tachometer face (a small artist's paintbrush is ideal for this). Replace the "tachometer guts" into the metal housing, taking care that any rubber shrouds for the marker lights are in position as the "tachometer guts" are lowered into position. Align the screw holes, and replace the screws. Clean the glass, shroud and bezel, and replace. CAREFULLY re-bend the bezel tabs to grip the tachometer body.

Look carefully at the loop of wire that runs through a plastic holder on the back of the tachometer. This must be reversed, so that electricity "flows" the other direction. You can... follow the wires until you find connections and reverse them at that point. Or... you can carefully un-thread the white wire from the plastic loop holder, and re-thread it correctly, reversed "flow".

Attach a small label to the backside of the tachometer housing stating the "modification to negative earth", Date it and initial it.

Replace the tachometer into the instrument panel, re-attaching all wiring.
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Old 09-08-12, 09:36 PM
Join Date: Apr 2009
Location: York
Posts: 808

P.S. I probably have a 411 one spare
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  #7 (permalink)  
Old 10-08-12, 11:22 AM
Join Date: Nov 2010
Posts: 11
Default Rev Counter Polarity

Thanks David for the JDO link.
Greg, I know a tacho isn't important but it's nice to have working if installed and it would help setting tickover/timing on tickover etc.
The conversion article you included is quite detailed and i had found it , but it's one that talks about the green wire being swapped from earth lug to power lug.....and the RVI 2810/00 just doesnt have a green wire or earth lug next to the power lug - hence the frustration of me and my soldering iron.
I'll measure the face of my tacho and if you have a RVI tacho of the same size in negative earth from a 411 that i can buy from you that may be the easiest answer.
Thanks everyone for your input and suggestions
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