rev counter polarity

Post by **callyspoy** » Wed Oct 03, 2007 3:52 pm

just a quickie, is it possible to change the polarity on a rev counter? seen one i LOVE(please no "don't bother with a rev counter" comments!!) but it is positive earth...
cheers
*Callum*

Post by **callyspoy** » Wed Oct 03, 2007 4:37 pm

oh, and obviously by that, i mean my car is neg, rev counter is pos!

Post by **alex_holden** » Wed Oct 03, 2007 4:56 pm

It's not really practical unless the electronics were designed to have switchable polarity. There are plenty of negative earth rev counters on eBay - I fitted a nice Smiths one from an MGB.

Post by **callyspoy** » Wed Oct 03, 2007 4:58 pm

oh, and obviously by that, i mean my car is neg, rev counter is pos!

Post by **callyspoy** » Wed Oct 03, 2007 4:59 pm

yea, but this is THE one...!! damn my insistence!

Post by **bigginger** » Wed Oct 03, 2007 5:03 pm

Insist all you want, but as Alex says, if there's no switch and you can't re-build it, forget it

a

Post by **callyspoy** » Wed Oct 03, 2007 5:11 pm

tis a shame, but you have answered my question fantastically! thanks for the bad news guys! ;-) you could've just lied to me, so that i could just look at it and weep like a little boy!

Post by **bigginger** » Wed Oct 03, 2007 5:39 pm

You can still do that OK - or convert the polarity of the car back

Post by **Alec** » Thu Oct 04, 2007 7:45 am

Hello Cally,

It is possible on some, the older 'current sensed' type it was just swapping two 9soldered) wires within the instrument. I would imagine being positive it is an older one? I think it would be harder with the later voltage sensed ones. i.e. the coil wire is connected directly rather than being a coil of wire looped at the back.

Alec

Post by **paulhumphries** » Thu Oct 04, 2007 10:01 am

It's possible.
I did it many years ago on my 1964 Sunbeam Rapier when changing from pos to neg earth.
I followed instructions given in club magazine but basically I just opened up the tach and swopped the wires on the circuit board to casing and power tag over.
I suspect pos earth tacho's are getting rare so you might find, however, you can easilly sell yours for more than it'd cost to buy exactly the same in neg earth.

Paul Humphries.

Post by **callyspoy** » Thu Oct 04, 2007 11:44 am

i see...so a bit of swapping and chopping and it could possibly work...well if i buy it, i may just be sending some pics to the pair of you so you can help me in my quest!

Post by **callyspoy** » Sun Oct 14, 2007 12:48 pm

hello guys, as promised, i've got pics of the rev counter that needs swapping over to negative earth(if possible) would love your help if you can. so you know, the black and white ones are for the light, green is -ve and red is +ve. please help!
http://farm3.static.flickr.com/2141/156 ... 91.jpg?v=0
http://farm3.static.flickr.com/2372/156 ... e0.jpg?v=0

Post by **alex_holden** » Sun Oct 14, 2007 1:00 pm

Does it use current sensing or voltage? Ie. does it have two terminals that go in series with the coil or a single terminal that connects to the distributor side of the coil?

Post by **callyspoy** » Sun Oct 14, 2007 1:04 pm

two come, out, one is, i assume to earth it, the other is the power i suppose...hope that helps...

Post by **alex_holden** » Sun Oct 14, 2007 1:07 pm

Do you mean there are two power terminals and two sense terminals?

Post by **callyspoy** » Sun Oct 14, 2007 1:10 pm

sorry, my explaining leaves something to be desired. coming out of the back of the counter are FOUR wires. TWo of these are for the light, so can forgotten. the other TWO are marked on the rev counter as +ve and -ve, red being +ve and green being -ve. please dont get angry with me!!

Post by **alex_holden** » Sun Oct 14, 2007 2:21 pm

I'm not sure how you would wire that. Is it possible one of the light wires doubles as an earth connection?

Post by **callyspoy** » Sun Oct 14, 2007 2:49 pm

it doesnt say anything about that no. supposing it does act as the earth, what would you then say the ones marked +ve and -ve woudl be for? also, again, supposing it does act as earth, what would you do!!

Post by **alex_holden** » Sun Oct 14, 2007 6:21 pm

I think I'd try to reverse-engineer a circuit diagram from the PCB and figure out how it works. I'm totally guessing here but it might be that the + and - terminals are current sense in and out, one of the other two might be the negative supply, and the case is the positive supply.