PDA

View Full Version : Pos to Neg..... no tach



JohnnyO
09-09-2010, 11:16 PM
Hi everyone!!! Well I converted my 66 Midget 1098cc from pos ground to neg ground today. Switched the ignition coil wires, polarized the dynamo at the voltage reg and at the dynamo. Switched the battery cables. When I went to start it, it acted like it had a dying battery, barely turning over, then not turning over. So I jumped it, and it started right away. Let it run for a bit turned it off, waited a few and tried starting again,...very weak battery, almost didn't start. So I took it out for about a 20 minute drive and that seems to have put a charge in the battery, will be checking that more later.....now for my question.....Is that dead battery symptom normal? In pos ground mode the battery never gave a hint of a low charge. Second, my tachometer isn't working. All other gauges, lights, wipers, heater, are all operational, but not the tach.......any suggestions...

Billm
09-10-2010, 06:53 AM
Probably just bad battery connections on the battery- clean them up and it will probably be fine.
The tach needs to be converted to neg ground, really easy and only 2 wires to solder. The instructions are in most Moss catalogs (near the gauge section) but one of can give them to you is you don't have a Moss catalog handy.
BillM

jvandyke
09-10-2010, 10:57 PM
If a '66 tach is like my '63, this may help;
https://www.mgexperience.net/article/neg-convert.html

and this thread
tach conversion with picture (https://www.britishcarforum.com/bcforum/ubbthreads.php/ubb/showflat/Number/534316/Searchpage/3/Main/52553/Words/tach+conversion/Search/true/Re_positive_to_negative_ground#Post534316)

JohnnyO
09-11-2010, 12:47 AM
Ah yes jvandyke, that's exactly the instructions I used, and to my amazement I FIXED IT!!! It's working fine now....thanks....

jvandyke
09-11-2010, 09:21 AM
Here's the picture I was looking for....after the fact apparently, maybe the next guy can use it. https://homepage.mac.com/jvandyke/tachgroundconversion.jpg

mightymidget
09-11-2010, 10:18 AM
Yes I can use it, Just purchased a used tach to install in modified dash project.

Don't have the Tach yet, Can you tell me how the instrument is wired (which wires go where?)

dklawson
09-11-2010, 10:41 AM
Can you tell me how the instrument is wired (which wires go where?)

The wiring connections are different depending on whether you bought an early RVI type tach or a late RVC type.

Starting with the easy (RVC type).
The RVC senses voltage pulses and has three electrical connections (plus one for the lamp wire). The case is earthed for the ground connection. On the rear insulator plate will be a male spade and male bullet. The male spade gets 12V switched power, the male bullet receives a wire from the "low" side of the coil (the coil terminal that goes to the dizzy. 12V supply is typically a green wire (indicating switched, fused power), the ground wire is black, the bullet connection to the coil should have a white/black wire.

The RVC type tachs are current impulse sensing and are wired in series with coil power. Where they were originally fitted from the factory, the coil power comes off the ignition switch goes through a white wire looped on the back of the tach and then goes off to the coil "high" terminal (NOT the one going to the dizzy). On aftermarket installations the white wire loop goes between the coil low side and the dizzy (replacing the wire that normally goes between the coil and dizzy.) The case gets a black earth wire either using a male spade lug on the case or a ring terminal around one of the tach mounting studs on the back. There should be an insulator plate on the back with a male spade lug. Switched 12V is supplied to that terminal.