low voltage at coil on cranking

[sclemow]

Hi

I'm getting a low voltage at the coil positive on cranking causing poor starting. Any ideas on what would cause this?

It's running lumenition electronic points replacements.

I'm a bit confused as to what is causing it to drop the volts.
I've tried removing the rev counter from the circuit, changing the coil, disconnecting the electronic ignition, and checking the connections. I can't really think of anything else that would cause it.

S

[rich@minispares.com]

rubbish coil?


I am beset with this problem at the moment

im trying an ancient duciller 12v ex-scrap yard coil at the moment as its proving to be more reliable than any modern offering I have tried so far

[sclemow]

hmm, I guess it could be although I tried a few from ex-scrap cars including ballast ones which should give a better spark. Maybe I should try a few more.

I was wondering about the ignition switch - although I think the voltage is reasonable at the white connection on the fusebox.

[rich@minispares.com]

I was wondering about the ignition switch - although I think the voltage is reasonable at the white connection on the fusebox.

I tried a separate battery going direct to the coil and a wire direct from the '+' on the battery in my car - neither helped with the poor starting or misfire in my car.

[sclemow]

hmm, that doesn't sound great. I take it from your despondent note and fact your using an old coil you haven't found a satisfactory solution either.

My last new coil lasted less than 3k miles. It failed completely while doing 70 down a road on the way back from Goodwood. One of the internal connections must have broken as there was no connection between the positive and negative.

[dklawson]

What are you calling low coil voltage? How low is low?

Before you installed the electronic ignition was your car outfitted with a ballast or standard ignition system? What other wires are on coil (+)? Have you removed the low tension wires from the coil and measured its resistance.

[mikep]

Hi

I'm getting a low voltage at the coil positive on cranking causing poor starting. Any ideas on what would cause this?

It's running lumenition electronic points replacements.

I'm a bit confused as to what is causing it to drop the volts.
I've tried removing the rev counter from the circuit, changing the coil, disconnecting the electronic ignition, and checking the connections. I can't really think of anything else that would cause it.

S

I would take the battery and engine earth straps off and give the leads and mounts a good clean and reassemble, you may find this helps as they tend to deteriorate over time especially the engine one. Also may be worth checking the integrity of the connector to cable joints.
There will always be a voltage drop this is the reason ballast systems were fitted.

[sclemow]

It's dropping to 6.5-8 volts on my dvm so going to be a pretty weak spark.

Thanks for the replies. I've cleaned up the high tension and earth leads, this improved the drop slightly but not enough.

It was built with a brand new loom and new coil 3k miles ago so it shouldn't be deterioration.

The only thing hanging off the coil terminals are the rev counter (I've disconnected and discounted that) and the ignition module.

Next things I can think of are :
change the ignition switch as the white coil feed is supplied through that and it was the original one.
Take out the fuse 3/4 with the brake light circuit in case something is sinking current on that side of the fuse.
Check the terminal connections on the white wires for high resistance and on the brown supply leads to the ignition switch
put a lead from the solenoid to the +ve coil to remove the ignition/fuse circuit.
Put an external battery across it - if that doesn't work I'm a bit stumped!

[rich@minispares.com]

Put an external battery across it - if that doesn't work I'm a bit stumped!

I hope this doesn't fix it, it will be a bit impractical having a second battery bolted through the front grill!!





(joking apart, I had the second battery inside with a pair of bright orange wires going out the passenger window, into the bonnet and direct to the coil, looked a bit odd, but was good enough to do ten miles or so to see if the misfire / not starting was cured)

[minimans]

Make sure the Impedance of the coil matches what is required from the luminition Ign. Unit. also is the volt drop across the coil or is the unit only supplying 6.5 volts on crank? I suspect that the coil is not compatible with the Luminition. If 12v is supplied and your losing it across the coil then the problem will be a bad coil or incompatible coil.

[sclemow]

I've checked the impedance of the coil with Lumenition, they say just use a standard non-ballast coil, which this one is (I've checked it's resistance against the manual). I get low voltage at the positive so it's not dropping it all across the coil resistance.

S

[sclemow]

I hope this doesn't fix it, it will be a bit impractical having a second battery bolted through the front grill!!

I dunno - some of the racing gel batteries are pretty small these days, I reckon you could find room under the bonnet for one!

[minimans]

Std coils can range from 3 to 8 Ohms..........I don't know anything about Luminition but Petronix are very sensitive to coil impedance...........

[ianh1968]

Someone else on here had a problem when trying to start a car
a while back... I wonder if it is the same problem as we have here?

Could it be the same car, and the same engine, perhaps?

I made a post about how to check for volt drop in the main
battery wiring: (See the second post on the page below).

viewtopic.php?f=3&t=8536&start=10

EDIT:
The general method described here will detect volt drop
IN ANY PART OF THE CIRCUIT - TRY THE MAIN CABLES FIRST

Next, connect one meter lead to the solenoid where the main
cable comes up from the back, and the other lead to the
coil +ve. OK, with the ignition ON, you will have 2 x "plus"
connections and you will also have zero volts showing.

However, when you crank the engine over, the meter will show
the VOLT DROP between the solenoid and the coil. If this is an
appreciable amount, check the wiring here and/or ignition switch.

This might sound complicated, but the first time you see it happen,
it will be a "Eureka!" moment...

Ian

[dklawson]

I get low voltage at the positive so it's not dropping it all across the coil resistance.

Understood. Out of curiosity, have you connected a voltmeter elsewhere and measured if the whole system voltage is dropping vs just the coil supply voltage?

[sclemow]

Hi Ian,

Thanks, I have looked at the voltage drop across the system as you suggest. This is how I found that it is dropping in the low tension circuit to the coil.
It's narrowing it down from here that is causing me pain!

Hi Doug,
It's at the coil that you notice the difference. At the white connectors on the fusebox you don't really see a drop, this should point at the coil or the wire between the coil and the fusebox however they seem ok, and I've changed the coil.

[ianh1968]

It's narrowing it down from here that is causing me pain!

This is always the fun part, but a methodical approach always beats
"pissing in the wind" any day!

Hi Doug, It's at the coil that you notice the difference.
At the white connectors on the fusebox you don't really see a drop,
this should point at the coil or the wire between the coil and the
fusebox however they seem ok, and I've changed the coil.

"They seem ok" is not the same as "they have been checked and they ARE ok".

You probably know this already, but there is NO FUSE on the ignition circuit...
A brown goes to the ignition switch and comes back as a white. This goes
directly to the ignition circuit, the voltage regulator, oil pressure light feed,
and the ignition light feed. This white also goes to the the feed side of a
fuse which has greens coming from the other side of it.

I am not sure what the exact setup will be on the "white side" of the
fuse box, but there are likely to be TWO whites, basically using the
fuse box terminal as a convenient joint-box.

You have already said that you are getting 12v at the whites at the fuse box.

To check the white wire between the fuse box and the coil, use the
method I previously described.

i.e. Connect one side of your voltmeter to the +ve white at the fuse box.
Connect the other end to the +ve on the coil. You will have TWO positives
at the meter and it should show zero volts.

Crank the engine.
The meter shows the VOLT DROP UNDER OPERATING CONDITIONS
between the connection at the fuse box and the +ve on the coil.
If this is an appreciable amount, try connecting a wire directly from
the whites at the fuse box directly to the coil. If this helps, you have
proven that the fault is between the fuse box connection and the coil.

You can also disconnect the whites from the fuse box and the coil.
With the ignition OFF, check the resistance across the ends of this
wire.

The wire is definitely white and not "pink and white"?
Was the loom made incorrectly?

YOU MUST NOT CHECK THE COIL VOLTAGE ACROSS
BOTH THE COIL TERMINALS!

This will give a "lumpy" and lower reading on the voltmeter as the
coil will be switched on and off by the points/ignition trigger.
This will give a sort of crude/average/lower/RMS rating...

If you have an alternator, does it make any difference if this is
disconnected? Could be diode problems... Remove the fan belt
as it is said that spinning the alternator with it disconnected is
NOT GOOD. (Anyone know why?).

Ian

[sclemow]

Just to post an update on this. Thanks everyone for the ideas. I think I have solved this by tracing the voltage drop through the ignition switch and connection at the fusebox.

I now have a voltage of just over 10v while cranking at the coil which seems respectable to me.

Here's hoping it solves the tricky starting more permanently!

Simon