low voltage at coil

[roman.lysiak]

This is on a 73 tii USA spec VIN 276xxxx. I've set timing, replaced wires, plugs, cap rotor, and upon further inspection I've found that btw the positive side and negative side of the coil, with the ignition set on run (I can hear the fuel pump running, but without the car running) I only get 7 volts. With the car running, the voltage is all over the map. I've also just put in an AFR guage which I wired to the hot terminal of the coil to the guage and it rarely gets up to temp to operate, also indicating that the voltage is low on this terminal. Engine hiccups at a regular interval at idle but smooths out at 2500rpm, where I can accelerate nicely. At lower rpms, I get a backfire when I shift gears, thinking that not all the fuel is burning with a no spark situ. I had it out last night and running at 3k and up on the highway to try and tune the fuel ration at the tuna can. Ran great at the high rpms... With the higher reving the AFR guage was working and I could dial in the wide open mixture to burn around 14.5. Right now I'm trying to resolve the low voltage at the coil and have looked at the wiring diagram on what provides power to the coil and what to check.  So far I've found that the main feed comes from fuse no 12 with the red green wire, but it also shows a wire going to the starter.  Anyone of you experience this low voltage condition at the coil? I'm thinking that I'm not getting a proper spark at lower rpms giving me my hiccup, but sorts itself at the higher rpms.

[Healey3000]

Hi,

 

Do the tii's use a ballast wire to feed the coil like non-tii's do?  If so, your reading of 7 volts suggests that the bypass circuit isn't working.  Check your solenoid for this.

[coloincaalpine RIP 2021]

Does your car still have the resistor before the coil?

[Einspritz]

Does it backfire into the intake or out the exhaust? On acceleration or deceleration?

 

What is your timing set at? Is your distributor curve within specification? More advance is not more "betta".

 

7 hours ago, roman.lysiak said:

I could dial in the wide open mixture to burn around 14.5.

 

Too lean. S/B ~ 12.6 for best power, or shoot for at least 13 AFR.

 

Then set the mid range, then set the idle mixture.

 

Does your AFR sensor have a heating element? IIRC they use about 5 amps, which will probably overtax your coil feed wires' capacity creating a low voltage situation to the coil. The heating element should be on a separate relayed and fused circuit.

 

An all too common problem is the fact that most wiring harnesses are 40 years old and the corrosion within the harness creates too much resistance; cleaning the outside and reworking the ends is many times insufficient in getting the resistance down, particularly on small wires. IIRC the coil wire(s) are no more than 0.75mm (probably 0.5mm) so resistance is a real issue, regardless of whether you are using the bypass resistor, which yes, will lower your voltage to the coil.

 

Again, IIRC, the wire from the starter is energized when the starter is engaged, providing 12V bypassing the resistor circuit when the key is at "start". when you release the key onto "Run" the relay for the bypass moves to the resistor circuit lowering the voltage through the points and coil. I don't remember what that voltage should be.

[TobyB]

You've got yourself a resistor in the positive wire.  Probably by design.

 

Don't power the afr off that- the heater circuit needs an amp or more,

AND the electronics are going to hate the fluctuating voltage.

 

If you want really cool voltage readings, switch your meter to AC when

the car is running, and meter the primary side of the coil...

 

t

 

[Mike Self]

On 6/25/2019 at 10:46 AM, TobyB said:

You've got yourself a resistor in the positive wire.  Probably by design.

I don't think roundies (carbureted or tii) had resistor wires; they used the ceramic resistors that are located directly above the coil on the inner fender.  But the function is the same, to lower the voltage to the coil (and thus the points) when running so that the points last longer.

 

mike

[2002iii]

Check voltage at ignition switch terminal 15 it's the one with 2 big green wires, it should be 12v. If its anything less than 12v you need a new ignition switch ( I just had this same problem).

[Chris_B]

What you can do as a quick reality check on whether this low voltage condition is causing your performance problems is to install a relay between your coil input (the green wire) and your battery. You can use the the green wire as the trigger for the relay, wire directly to the battery for power, and find a good ground (the bolt that holds down the coil mount might be a good choice). This is a bit crude, but it will resolve your questions about the low voltage to the coil affecting your performance.

[roman.lysiak]

So I’ve checked the voltage at the ignition switch terminal 15. I get 12v. Yes I do have a resistor by design before the coil. 11.5v before it and about 7v after it. This is without the afr attached to it. I’ve relocated the power for the afr to run off of fuse no 11. Gets a full 12v there. I’ve gotten rid of my pints and have the Petronix in my dizzy. Can I just bypass and get rid of the resistor to give the proper voltage to the coil without destroying it?


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[roman.lysiak]

Just looked at the petronix wiring diagram and looks like I’ve had it set incorrectly
All along. The red wire from the ignitor should be connected before the resistor. Will try it that way.


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[roman.lysiak]

What you can do as a quick reality check on whether this low voltage condition is causing your performance problems is to install a relay between your coil input (the green wire) and your battery. You can use the the green wire as the trigger for the relay, wire directly to the battery for power, and find a good ground (the bolt that holds down the coil mount might be a good choice). This is a bit crude, but it will resolve your questions about the low voltage to the coil affecting your performance.

Hey Chris. So what you’re saying is to send full 12v to the coil? Could I just connect the in and out of the resistor without it in place as well( without the relay)?


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[Chris_B]

No, what I was suggesting was a diagnostic technique. If you have the original 02 coil, you should keep the relay in the circuit to bypass the resistor on starting and run the circuit through the resistor in normal running. This all depends on the coil you are using.

[roman.lysiak]

No better. Still has studded on start up. I’ve ordered a new dizzy from Ireland eng. Still have low volatage at the coil. Will see if Chris’ diagnostic check will tell me anything. What’s the voltage you guys get after the resistor?


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[Chris_B]

You should be seeing 8 - 10 volts after the resistor.