Low tension circuit questions

nickm · Post by **nickm** » Thu Nov 22, 2018 2:59 pm

I am investigating an annoying misfire at low speed/tickover on my SE5.
I measured the voltage at the +ve lead going onto the coil and I'm getting 12.5 volts.
I thought this a bit odd as I assumed it would have a ballast wire fitted in the loom.
When I fit the wire on the coil with the ignition on I get 5.5 volts,but when I open the points
or remove the -ve wire from the coil I get 12.5 volts again.
Any thoughts?
Nick

Post by **philhoward** » Thu Nov 22, 2018 3:22 pm

Sounds like your coil is too low a resistance compared to the ballast resistor (wire).

nickm · Post by **nickm** » Thu Nov 22, 2018 3:45 pm

Shouldn't I be getting around 9v from the coil +ve wire?

Post by **AJL Electronics** » Thu Nov 22, 2018 4:11 pm

9V will be the average Voltage with the engine running.

nickm · Post by **nickm** » Thu Nov 22, 2018 4:16 pm

Oh I see.
Will check it with engine running.

Post by **AJL Electronics** » Thu Nov 22, 2018 4:18 pm

nickm wrote: ↑
Thu Nov 22, 2018 4:16 pm
Oh I see.
Will check it with engine running.

Not with a digital meter please, unless you strap a big electrolytic across the terminals of the meter.

Post by **David Tew** » Thu Nov 22, 2018 4:32 pm

The 12.5v is a bit of a red-herring as that is your full battery voltage. With the coil +ve connected and either the coil -ve disconnected or the points open you will get full battery voltage on the coil +ve even with the ballast in circuit as there is no circuit back to the battery -ve.

5.5 volts from the ballast sounds a bit low to me. Phil's suggestion of the wrong coil is a good one. It could also be a high resistance ballast/eureka wire.

Post by **AJL Electronics** » Thu Nov 22, 2018 6:40 pm

With the engine not running, the Voltage is of academic interest only. The points dwell angle will determine the Voltage that appears when running and that Voltage is the only one that needs to be concerned about. The exact Voltage is less important as it will vary with terminal Voltage on the battery.

Post by **scimjim** » Thu Nov 22, 2018 6:46 pm

I found a problem with my dizzy cap simply by putting a timing light on each lead in turn - showed an intermittent pulse on several cylinders.

nickm · Post by **nickm** » Thu Nov 22, 2018 8:23 pm

So using a digital multimeter with the engine running is a no no?
I will try a timing light for each cylinder and see what I get.
The engine runs perfectly when some throttle is applied,but it runs like a pig through town
when I am hardly touching it.
I even swapped my twin carb set up for a single as I thought it might have been a carb issue
and dropped the pressure on my fuel reg,but it made no difference[apart from losing a chunk of performance].

Post by **AJL Electronics** » Thu Nov 22, 2018 8:51 pm

The response of a digital meter is too fast for a square wave source. The needle of a proper meter integrates the pulses and gives an average.

Post by **AJL Electronics** » Thu Nov 22, 2018 8:54 pm

I think you need to go back to basics with a dwell angle and timing check, plus looking at plugs and HT leads. Then a brake cleaner test on the manifold and carburettor gaskets. A compression test would also be interesting.