Wedge ignition question tr7

[philman]

finally finished on my '76 tr7 dhc intake manifold gasket r/r, now it runs only as long as the starter is engaged. release the key and the engine dies. got fuel at carbs, swapped out sparkies and wires no difference. previous owner had installed an allison electronic ignition and lucas sportcoil. any ideas? i read about 1.5 volts on the coil "+" terminal with key on (motor not running). is that normal? thanks

 

[Geo Hahn]

I think you should read full battery voltage there. Check the resistance across the coil terminals... a Lucas Sport coil is about 3.7 ohms.

The sport coil does not require a ballast or ballast wire -- I do not know how the TR7 is set up but seems likely it has a ballast wire. If so I would expect some wiring changes are needed to use this coil though if it was running okay before perhaps those changes were made at the time the Allison ignition was added.

 

[sammyb]

My TR7 appears to have a ballast.

 

[philman]

well, the funny thing is i drove it from missouri to california, trailered to oregon, drove in the garage where i made repairs including replacing the alternator, starter, intake manifold gaskets, new battery. now i don't seem to get full voltage to the coil (i checked wife's 68 mustang with pertronix for comparison) unless the starter is engaged. so maybe i'll run a new wire from the ignition switch.

i also noticed the previous owner seems to have used usa spec fuses in the fuse block, so that will be rectified today.

thanks for the help guys. have to be driveable next weekend for annual british car club run to florence.

 

[Geo Hahn]

If your coil is the typical Lucas Sport coil then it is what some call "internally ballasted" though I think what really happens is that the coil is built to not require any ballast.

If the original TR7 coil was "externally-ballasted" then it would have had an external ballast or a ballast wire designed to add additional resistance during running.

The idea in the latter system is that the coil has a lower resistance and gets a full 12V during cranking (to aid starting) as the ballast resistor is bypassed. Once the switch is off the 'start' position then the ballast resistor is the current path and voltage to the coil is reduced.

If an "internally ballasted" coil is substituted into this arrangement it will get the voltage it really needs only during cranking and once the switch is off the 'start' position then the voltage will be reduced.

Why did the car run before? Well I have never tried an "internal ballast coil" on a system that had an external ballast or ballast wire but... it may be that it still ran until a later coil or ballast failure put you where you are now. That you can measure some voltage and get fire while cranking suggests you have not had a total coil failure, but perhaps a partial failure of the coil or a total failure of the ballast.

Again, the simple first test I would do is measure the resistance across the coil (should be 3.7 ohms). Then check the ballast resistance (should be 2.7 ohms) if there is one and you can find it. My guess is that either the coil number will be higher or the ballast number will be infinite. I would also review the ignition wiring comparing it to a wiring diagram to see if any accomodation has been made for the use of the sport coil.

A simple bypass would be to hot wire directly from the - post of the battery to the - side of the coil. Based on what you have said I think the engine would run with that set-up but you may still have a problem that needs addressing.

 

[Dale]

Could the ignition switch be suspect? Not that a Lucas switch could ever be faulty.

 

[philman]

Good News!

ok, got it. i used geo's method and ran a wire from the fuse panel headlamp circuit directly to the coil '+' connector (removing old wire). fired and ran as long as parking lamps were on. then i redirected the wire to the ignition on circuit (requiring a connector multiplier on the fuse panel). runs good, balanced carbs (i went over to twin webers). so then i thought i would drive around to the front and wash it. bad move; i had left the wiring hanging down during troubleshooting and it wound around the steering column. now i need an ignition switch. hello wedgeparts?

thanks for all the input and help. i think that somehow the ballast wire from the ignition switch to the starter relay opened up while troubleshooting a starter relay problem.