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MG MGB Technical - XR700 and Inoperative Tachometer

Sorry to bring it up again.

Confusion reigns on this subject. Tach will not work.

I have more schematics than I need, but here goes:

Note: (with/without Ballast Resistor-depends on coil internal resistance).)

1. Some wire from switch (with no indication that the B+ goes through the double winding on tach input transformer.with tach to negative. Does not work, kills ignition, even with switched b+ going to +coil.

2. I have used the original wiring from the switch through the tach loops to the +coil, flickered and quit, car still runs but no tach and has less power.

3. Currently wired from the switch through the tach loops to the +coil, with additional B+ lead to +coil.
Car runs good, with more power. Running through the tach loops alone makes less voltage=less power.

I know I am ranting, but this has me stumped.

Haven't tried running from _coil to tach loops to ground. This would make a circuit, but effects unknown.
James Huggins

i purhased a xr 700 from us automotive last year and had nothing but problems .the ignition system would not work with out the vacume pipe being blocked.the system is very complicated so i changed the dizzy to a aldon 101by2 and fitted a very simple unit which replaced the points .this system is used by most of the guys who race there cars

What type of tach? 72 and earlier had a current pulse operated tach, with RVI on the face-plate. 73 on had a voltage pulse operated with RVC on the face-plate. The current type are known to have problems with electronic ignition, the latter type usually not, although it will depend on the type of ignition. Only rubber bumper cars had ballasted ignition originally, although some aftermarket electronic ignition systems imply that a ballast must be used.

With the current pulse type the ignition supply must go via the pickup on the back of the tach or the tach will definitely not work. If the supply goes direct the ignition should still work. The tach loop should have zero resistance as it is just a single loop of standard guage wire, not many turns of fine wire for example. If the pickup is internal to the tach case and it brought out to male and female bullet connectors on the back of the case for connection of the harness wires then it is possible that there is a bad connection in the circuit. But this should affect points ignition more than electronic, electronic is supposed to be *less* susceptible to low voltage than points, not more!

With the voltage pulse type the ignition supply goes direct to the coil +ve then a different wire comes back from the coil -ve to the tach.

Both these are regardless of whether a ballast resistance is in series with the coil or not. Note that a coil measuring about 3 ohms should *not* have an additional ballast resistance in series (or you get weak sparking), but a coil measuring about 1.5 ohms *must* have a ballast of a similar resistance in series (or you burn the points and overheat the coil, and can overstress an electronic trigger).

Don't know what you mean by 'B+', this is usually a terminal on the alternator, which should *not* be connected to the coil, tach or ignition switch, only the ignition warning light and battery.
Paul Hunt 2

What type of tach? Current RVI 1433/00, double loop internal pickup

I picked up a thread somewhere that states removal of one loop, and possibly add resistor in series. I think this is probably the cause. I did not think of over current in the tach, and it is probably blown. Installed another used one, that is when it moved (jerked) a little and went dead. Too much current.
Can't afford to keep buying tach's, think I'll tear into the electronics. The thread shows a double transistor circuit for a monostable multivibrator circuit, but mine only has a single transistor. Half as much to trouble shoot. Wish I could remember more of what I learned in tech/engr electronics school.

No bad connections, but is the wiring correct? Single wire from ignition switch, through the loop, to the coil +. No one shows schematically how the tach is actually wired for the electronic system. Conversations with Crane techs, one says tach wire to + and the other says tach wire to - coil.

My B+ = your +ve (samo-samo) means the voltage supply from the battery.

Did MG (Smith) make the voltage pulse type tachometer?
Are they available? I really think that would be a better unit with less faults and failures.

My ballast is in the coil, 2.8 ohms on my meter. No additional ballast required, and car runs pretty good. No heat from the XR700, runs very cool. Better when B=(+ve) wired direct from switch to coil+.

However, on retrospect, I believe I have the answer to the quandry, too much current going through the primary side of the tach transformer, thereby, destroying something in the tach circuit. Will continue to trouble shoot. I think less current is the answer.

Thans to everyone that tried to help. Call on me anytime. I'll help if I can.

James Huggins

*Definitely* don't add a resistor to a current pulse tach pickup wire, or it will drop the voltage to the ignition. Making a single pass through the pickup (i.e. a half turn instead of a full turn) is often mentioned as a fix for a *high-reading* tach.

Making other 12v connections to the ignition will definitely confuse a current pulse tach, they are designed for coil current to flow through them to trigger the electronics. I have never heard of the electronics failing because there was too much current flowing through the pickup. 2.8 ohms coil primary is only slightly less than the standard 3 ohms, unless you are supplying other circuits from the tach supply and not just the ignition I can't see too much current causing a problem. Supplying other circuits though *will* confuse the electronics, even if it doesn't blow them.

It depends on what the electronics are doing. Some simple types like pertronix and Magnetronic are just like the early 45DE4 factory electronic system in that the trigger simply replicates what the points do, right down to maintaining the same dwell pulse to the coil, and these should be OK with a current pulse tach. But the more the electronics gets sophisticated with variable dwell, capacitive discharge etc. the more the current flow through the electronics and the coil departs from the original, and the less likely a current pulse tach will work.

Tach pickup to coil -ve is for the *voltage* pulse tach, as I mention above people often have to replace the current pulse tach with a later voltage pulse tach to get them to work properly with electronic ignition systems.

As far as I was aware Smiths (or their agents) always made the tachs for the MGB. If yours shows RVI etc on the face-plate then surely it shows Smiths as well. But if you say yours only has one transistor in the circuit (which doesn't seem right in any case) then maybe someone has swapped out the internals for some reason.
Paul Hunt 2

This thread was discussed between 22/01/2007 and 25/01/2007

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