Ignition Coil Voltage

[Sir Paolo]

Hi all,
I’d really appreciate some advice, as auto electrics are something of a black art to me.
Having established that the fuel pump voltage is low (as per a separate thread), I also checked the coil voltage.

With the engine running and multimeter probes on the +’ive and -‘ive terminals of the coil, I’m getting fluctuating readings of around 1.5 volts.

However, if I put the -‘ive probe on the engine, the reading still fluctuates, but goes up around 13v.

So my questions are- should the readings fluctuate like they do, and does it also indicate a coil earthing problem (if the voltage goes up, if I move the probe to the block)?

The coil is brand new Bosch btw.

Many thanks

[Steve_D]

When the engine is running the negative side of the coil is being repeatedly earthed in order that it generates a spark at the plugs so testing across the coil will show a pulse.
Your connection to the block will show the supply voltage.

So all good.

Steve

[Sir Paolo]

That’s brilliant

Many thanks, Steve!

[Polly Grigora]

The coil supply voltage can only be measured correctly by having the coil load on/in the circuit

A bad supply or a supply through a ballast resistor will still measure as 12 volt when the coil load isn't on/in the circuit

You need to remove all cables from the ignition coil negative terminal

Use a jump lead to connect the coil negative to battery negative or a known good earth

Now switch the ignition on and measure the voltage at the coil positive

Don't leave the ignition on for too long as the coil could overheat, 10 to 15 seconds isn't a problem

You will have then measured the supply voltage to the coil while it is under coil load (coil is drawing current)

[Sir Paolo]

Thank you Polly.

I’ll do this

If I’m checking the voltage at the coil +’ive, do I put the multimeter -‘ive probe on the coil -‘ive or the block?

Cheers

[Polly Grigora]

Sir Paolo said:

Thank you Polly.

I’ll do this

If I’m checking the voltage at the coil +’ive, do I put the multimeter -‘ive probe on the coil -‘ive or the block?

Cheers

As posted above Use a jump lead to connect the coil negative to battery negative or a known good earth

When you are sure the negative from jump lead to coil negative is good, multimeter -ive can be connected to coil -ive

[Sir Paolo]

Thank you Polly

Sorry for the questions that must be obvious to someone who knows what they’re talking about.

You have no doubt already guessed that I’m a complete novice with electrics.

I’m good with the oily bits, though

I’ll report back on my test!

[Polly Grigora]

Sir Paolo said:

Thank you Polly

Sorry for the questions that must be obvious to someone who knows what they’re talking about.

You have no doubt already guessed that I’m a complete novice with electrics.

I’m good with the oily bits, though

I’ll report back on my test!

Complete novice?No way

Once you've carried out the above test and proven something you will have joined the elite in automotive electrical fault finding

The same test applies to all circuits

Always measure voltage throughout a circuit with the load on the circuit being tested

Easy innit once you know the correct method

Joys of the internet and all that

[Sir Paolo]

That’s very kind of you to say so!

I’ve only just graduated from “complete idiot”, to “complete novice”

Really appreciate all your help, I’ll test as you mentioned, and report back

[Polly Grigora]

Sorry, I have failed miserably

Hope this post isn't too late

You will also need to measure and note the battery voltage just before measuring coil voltage, by this I mean measure battery voltage when ignition is on and coil negative is connected to earth with jump lead

[Sir Paolo]

No, not too late at all.

Work gets in the way of me playing with my Chim!

I’ll add this to the diagnosis list.

Many thanks

[blitzracing]

Dont get too bogged down in this. The supply to the coil and fuel pump take a very different route so it's very unlikely a voltage drop to the pump will show up as a voltage drop in the coil at the same time. Do you have a dash voltmeter showing the overall voltage from the back of the ignition switch?

Edited by blitzracing on Friday 17th June 09:04

[blitzracing]

Polly Grigora said:

Complete novice?No way

Once you've carried out the above test and proven something you will have joined the elite in automotive electrical fault finding

The same test applies to all circuits

Always measure voltage throughout a circuit with the load on the circuit being tested

Easy innit once you know the correct method

Joys of the internet and all that

One can always tell Penelope's writing style .

[Polly Grigora]

Sir Paolo said:

No, not too late at all.

Work gets in the way of me playing with my Chim!

I’ll add this to the diagnosis list.

Many thanks

Very good then

The volt-drop throughout the whole circuit is measured battery voltage minus measured coil voltage

[Belle427]

The fuel pump circuit is known to be a common one for voltage drop due to the number of connections in it, the important ones to check for corrosion are in the passenger side b pillar where the central locking solenoids live.
Moisture in here leads to corrosion.

[Sir Paolo]

blitzracing said:

Dont get too bogged down in this. The supply to the coil and fuel pump take a very different route so it's very unlikely a voltage drop to the pump will show up as a voltage drop in the coil at the same time. Do you have a dash voltmeter showing the overall voltage from the back of the ignition switch?

Edited by blitzracing on Friday 17th June 09:04

Hi Blitz,
Thanks for this. You’re right of course, I just want to rule out any potential contributing factors with the misfire/hiccup I’m experiencing.

[Sir Paolo]

Belle427 said:

The fuel pump circuit is known to be a common one for voltage drop due to the number of connections in it, the important ones to check for corrosion are in the passenger side b pillar where the central locking solenoids live.
Moisture in here leads to corrosion.

Hi Belle,
Thanks- the connectors in the B pillar are all good.
They seem to have been covered in Vaseline(?).

[Polly Grigora]

Sir Paolo said:

Hi Blitz,
Thanks for this. You’re right of course, I just want to rule out any potential contributing factors with the misfire/hiccup I’m experiencing.

As daft as it may come across as, there is much logic in checking the ignition supply voltage @ the ignition coil from time to time