Ignition ballast voltage drop question

[2cams70]

Any meter that is either consistently inaccurate or consistently accurate is fine for this job!! It’s only voltage differences that we need to know here. A $10 digital multimeter is perfectly ok.

[dcmarsh]

The frustration level continues to increase. Since I can’t access behind the dash due to my declining yoga position capability, things like jumping the tach, and cleaning connections back there aren’t possible for me. So, I’m wondering if another dreaded owner hack would work, or at least not cause a fire, but still allow the tach to work.

I have replaced the ignition switch as mentioned earlier. I’ve also removed the anti theft switch and soldered the 2 wires together without the one to the horn. (Had to cut a hole in the glovebox to do that so you can tell how desperate I’m getting!). No change in the 2 volt loss somewhere behind the dash on the way to the ballast.

I’m wondering if I can run a new wire from the “running” lug of the ignition switch directly to the coil ballast. That would provide correct voltage to the coil ballast,, BUT it would also backfeed the circuit that has the dropped voltage.

After you have finished rolling on the floor with either groans or laughter, wouldn’t this take care of the issue? At least until I’m willing to spend thousands of dollars and 6 months with a Lotus expert 1-2 thousand miles away? (You Brits are so lucky! At least in regards to Lotus mechanic availability!)

I should add that I’ve done a lot to the car own so I don’t consider myself a neophyte. Pulled the engine and worked on it, repainted it (back when lacquer was available), rebuilt the brakes including lines, twice, Created my own linear actuator replacement for the headlight vacuum system, replaced the donuts with a solid system, replaced the starter, fuel pump, rebuilt the carbs multiple times and tons of other stuff over the years. It’s just these electrical gremlins that have me stumped!

Dave

[billwill]

Don't bother trying to get behind the dash until that is REALLY diagnosed; you can feed full battery voltage from big fat terminal D on the starter solenoid to the input of the ballast resistor, point H, and you can also trigger the starter solenoid by using a short wire from point D to that smallish terminal on the starter solenoid which operates the solenoid. That's the other small terminal (I didn't give it a letter) on the solenoid, not the one that goes to the coil. {it actually comes from the ignition-switch-start position.} Some solenoids actually have a pushable button to have the same effect.

Only make this small-lead jump connection for a short time (like using the start position on the ignition switch). and keep your fingers & tools away from any external moving bits of the engine as you are starting it up!

In other words you can simulate the starting conditions in the engine compartment without getting behind the dashboards. With the ignition switch off there should be no back-feed, though I cant see with the information so far that any backfeed would be a problem.

As previously effectively mentioned: DO NOT KEEP THE JUMPER FROM D to H, IN PLACE FOR A LONG TIME WITH THE ENGINE STOPPED, because if the contact breaker points are closed it could burn out the ignition coil.


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In the photo of your earth-lead in the boot/trunk, the connection between the actual cable and the connection ring looks poor. It is difficult to see, but I would be inclined to either buy a new earthing lead for that or take it off the car (both ends) then pull the copper wires out of the connection ring and give them a good clean and then either solder them back in place or (since you almost certainly do not posses a heavy-duty crimp pliers) use a workbench vice to crush the connector tightly onto the copper wires of the cable.

[Craven]

billwill, I wrote.
It is a simple matter to run the ignition system completely independent of any connection to the cars electrical system.
Remove the + connection from the coil. Connect in it’s place your Power Probe 3 set 9volts. Negative connection to the engine block. Start the car.

[Slowtus]

billwill, I wrote.
It is a simple matter to run the ignition system completely independent of any connection to the cars electrical system.
Remove the + connection from the coil. Connect in it’s place your Power Probe 3 set 9volts. Negative connection to the engine block. Start the car.

Exactly, too much talking and not enough doing here, I could have diagnosed and repaired this in an hour - tops - beer break included.

[Craven]

By simple resistor divider network, 1.5 ohm ballast and 1.5 ohm coil you will only get half the battery voltage at the coil any way.

[billwill]

By simple resistor divider network, 1.5 ohm ballast and 1.5 ohm coil you will only get half the battery voltage at the coil any way.

Only when the contact breaker is CLOSED.

My answers may tend to be long-winded but I try very hard to avoid giving confusing advice such as your sentence because with the engine stopped in an arbitrary position there is a higher probability that the contacts are open rather than closed.

A 'beginner' doing engine electrics for the first time would be totally confused by the most-probable results of trying to do the measurement you describe.

[2cams70]

What type of ignition coil has been fitted?? You don't want one of those low resistance sports coils or a low resistance coil designed for use with an electronic ignition ignition system. You need the correct coil with standard resistance designed for a ballasted points type ignition system.

If the coil resistance is too low it will draw excessive current and therefore cause a higher than expected voltage drop across the ballast (V= I X R remember). In this case V is the voltage drop across the ballast, I the current flow through the circuit and R is the resistance of the ballast. The start circuit on the other hand not having any ballast resistance will not have the same voltage drop effect even if the coil current draw is excessive (this aligns with what you originally determined through measurement). Swapping the coil is something you can easily do and not have to get behind the dash. If you could measure the current flow in the circuit perhaps someone else could provide a figure measured from another car for us to compare.

As previously effectively mentioned: DO NOT KEEP THE JUMPER FROM D to H, IN PLACE FOR A LONG TIME WITH THE ENGINE STOPPED, because if the contact breaker points are closed it could burn out the ignition coil.

No need to worry about this. If the correct coil is fitted it is fail safe in this mode. It was very common to inadvertently leave a car with the ignition in the "run" position whilst listening to the radio when stopped back in the day and the worst that happened was a flat battery!!

things like jumping the tach, and cleaning connections back there aren’t possible for me. So, I’m wondering if another dreaded owner hack would work, or at least not cause a fire, but still allow the tach to work.

Not possible I'm afraid. You can't bypass the tach without accessing the back of it

[Billmack]

What you suggest is no problem (this to car owner). Teach may not work 100 percent but you've had the car a while and I am gonna say you know what an overrev sounds like so just don't do that... I have driven crapbox British cars for 40 years and you want to not let the perfect get in the way of the good. Are you in the northeast u.s.? I am. If close by I'm happy to help.

[Billmack]

Tach not teach

[Billmack]

P.S I saw where you are. Too bad 'm not closer.

[2cams70]

I have driven crapbox British cars for 40 years and you want to not let the perfect get in the way of the good.

A bit off topic but.......

I'm not British but I have to step up in their defense here and say that the best Mechanical Engineers were all British. The Industrial Revolution started there after all!!

I think you Americans take the prize for the best Electrical/Electronic engineers though!!

[Hawksfield]

Hi Dcmarsh

To gain access to the Tacho you can remove it by first removing the air vent adjacent to the Speedo then remove the Speedo next the Tacho they are just held in by spacers and knurled thumb nuts. I can do this in under 10 mins.

I believe this problem was initiated by a poor engine starting. If this is the case why don't do as suggested by other knowledgeable posters, connect a 12v direct supply and try to solve the starting problem as you are delving into area's that may not be necessary.

I have done a quick test of voltage drops on my 1971 +2S

Static 1.Solenoid =12.55v, 2. resistor primary =10.79v 3. coil =5.91v

Engine running 1. 12.64v 2. 11.40v 3. 7.84v

My engine starts first time

Hope this is some help good luck

[dcmarsh]

John, this is a bit eye opening. Your voltages are close to mine and I’ve been assuming that the voltage drop at the resistor primary is a problem, and that the low voltage at the coil likewise. So maybe this is all normal? And we’ve generated 4 pages of great discussion to, as they say here in Tennessee, “fix something that ain’t broke”!

[dcmarsh]

Thanks BillWill, for the comment on the boot ground cable, and all your other great specific recommendations. That could be the source for other electrical weirdness like the random accessory and headlight operation.