• Hi Guest!
    If you appreciate British Car Forum and our 25 years of supporting British car enthusiasts with technical and anicdotal information, collected from our thousands of great members, please support us with a low-cost subscription. You can become a supporting member for less than the dues of most car clubs.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Getting a tach to work--im at a loss

Share this page

E

eurobiketrash

Member
Offline
I removed the tach today from my 66 3000 to find the wire that is the induction loop cut on both ends. So I begin to try and find the rest of the wire that should go to the ignition switch and to the coil.As I keep trying to trace down everything I start to think that the wire was reconnected together just bypassing the tachometer, and one end of the wire is at the coil and the other is on the ignition switch.
My problem is this.Im not a whiz with electrical troubleshooting, however; I know that my battery is connected in positive ground configuration with positive cable to the battery shut off then from shut off to chassis. When I check the coil with the ignition on, the positive side of the coil is showing voltage and the negative side is basiaclly showing zero----shouldnt this be the other way around?? The negative side should show voltage shouldnt it???
 
The loop should be hooked to the white wire from the ignition switch to the coil as you surmised, since it just runs off a pulse, and you are just splicing a piece of wire back in line you won't harm anything by hooking it into your white ignition wire, and it may even make it work! (although I wonder why the wire would have been cut if everything was working OK to start with, you may have some troubleshooting to do)

Here is a link to a late 3000 wiring diagram.

https://www.healey6.com/Technical/BJ8wiringdiag.pdf
 
eurobiketrash said:
... When I check the coil with the ignition on, the positive side of the coil is showing voltage and the negative side is basiaclly showing zero----shouldnt this be the other way around?? The negative side should show voltage shouldnt it???
Click to expand...

How are you measuring voltage? You can only measure voltage across an open circuit or a resistance. I'm going to assume you have points ignition: when the points are closed, you have essentially a dead short to ground and there will be essentially no voltage except some across the coil terminals due to a little resistance(1-3ohm) in the coil windings (don't leave the switch on with the points closed for very long; that's why coils get hot and burn out). With the points open, you should be able to measure battery voltage--about 12.7V--at any point along the primary ignition to chassis ground (engine block, etc.).

An electronic tach measures the speed and duration(?) of the pulses going through the loop (aka 'pickup'). It's essentially a simple frequency counter. If you look at the schematic that Greg provided, note the green/blue lead from the #2 terminal on the ignition switch--I think it's actually a white wire--that goes to the little rectangle in the tach labeled 'RPM' to the coil then to the distributor. As the points open and close with the engine running pulses of current go through the pickup, through the coil and distributor and to ground ('earth') as the points open and close.

I believe the little block with 'RPM' above represents the loop pickup on the back of the tach. IIRC, you need one or two loops around the pickup; unfortunately, the direction of the loop is usually found by trial-and-error (the tach will work one way and not the other).
 
Back
Top