Ignition Timing Light (Strobe) - Positive Earth

(Timothy M Fox) #1

Hello Again,

I’d like to be able to use a timing light to set the timing and check the advance on my positive earth cars. It seems that I’m unable to use my digital version, which I’ve used successfully on my negative earth cars, as I’m told the reversed polarity will damage the digital functions. What do people use with positive earth cars?

Kind regards,


(Andrew Waugh) #2

I have a cheap Equus one with an inductive pickup that you clip on the HT lead (a bit like a clamp on ammeter). As long as you connect the power leads which feed the internals of this gun correctly it works just fine.

The inductive pickup is marked with a sparkplug symbol on one side, indicating that you’re supposed to connect it with that side towards the plug. I’ve tried reversing it (to compensate for the pulses going the wrong direction), but it shows the same regardless of which way the pickup faces.

(Timothy M Fox) #3

I suppose that I could simply use a spare battery to power the timing light now that I think about it!

(Peter Scott) #4

There no great problem in converting your car to negative earth. I did it so I could install a tracker. You only need to change the battery terminals and swap the battery end for end.
If you want the ammeter to read the correct way around then you will need to swap its connections too but I haven’t bothered. Using negative earth also gives you a wider range of LED lamps that greatly helps for driving at night with heater and wipers running in addition to headlamps. See LED Lamps


(Timothy M Fox) #5

Thank you, Peter.

I think that I’ll just do what you have suggested.


(Timothy M Fox) #6

What about the polarity of the SU fuel pump?

(Andrew Waugh) #7

Unless someone has added a diode, or upgraded it to an electronic pump, changing the polarity won’t make any difference to the pump.

And the heater and wiper motors will also be fine without any modification.

(Timothy M Fox) #8

Okay, thank you Andrew. I was going to fit an electronic SU pump at some stage, so now it can be negative earth!



(Andrew Waugh) #9

You may need to repolarise the genny (search for “Flash” or “Repolarise” in the archives).

(Geoff Allam) #10

When I had my 64 coupe converted to negative earth by a mechanic there was a procedure of “flashing” the generator that was carried out after the cables were reversed. (Don’t worry. Everyone kept their clothes on). Also the leads to the in tank fuel pump were not changed and I got less than half a block down the street before I ran out of fuel and had to switch over the wires to get it going again. Maybe the exterior pumps are different.

(Geoff Allam) #11

Just noticed the comment you posted as I was typing Andrew.

(Andrew Waugh) #12

It’s fairly easy to check if a component has had a diode fitted. Disconnect both leads, and use your multimeter set to diode check to see if it conducts in only one direction.

If someone has fitted a diode to your fuel pump (to protect the points), then you can either reverse the wires (if it’s not earthed through the body), or open the cap, find the diode and reverse it.

One other thing @Timothy2963 will need to do is swap the wires at the coil.

(Geoff Allam) #13

Mine is an in tank pump. I don’t believe there are any points in those. The motor runs constantly when the ignition is turned on. I wonder if it is a permanant magnet motor in which case it would run in reverse if the polarity is switched? Yes I forgot about the coil switching.

(Andrew Waugh) #14

Yes, if it’s a permanent magnet motor then it will run backwards if you reverse the polarity.

(I need to be more attentive when responding to pre-XK questions, as it’s only really the XK Jaguars that I am familiar with.)

(Geoff Allam) #15

And I should probabably not have been commenting on the pre xk forum as my car is much newer but I did want to let Timothy know that in my case it was not as simple as just reversing the battery leads and that there was a possibility of more needing to be done to complete the job.

(Peter Scott) #16

Well actually it is just as simple as swapping the battery connections. The complications only arise if modern components, such as a pump with a diode have been fitted instead of the original equipment. Even if you have a modern pump with a diode and you do nothing about it, it will simply turn itself into an original because the diode will blow leaving you with the pump as original.

As to flashing the dynamo there is nothing scary about this and it is only needed if you find that the dynamo is not charging when you first start up after the change. I can’t remember but I don’t think I needed to flash mine.


(Andrew Waugh) #17

I worked with large DC motors for about 20 years. I’ve only once had to flash a motor (it had been unused since the '40s), and never had a genny need flashing after a polarity change. But… perhaps that’s just the luck of the draw.

(Timothy M Fox) #18

What about changing the polarity of the leads to the ignition coil?


(Rob Reilly) #19

Yes, you would have to change the coil leads.
Back to the original question, I use a light that just plugs into the spark plug wire in series so it doesn’t care about polarity.
My SS, 120 and Mark V are all still positive ground.
The Lucas engineers once wrote a service manual in which they defended their reason for positive ground, having to do with corrosion and spark.

(Timothy M Fox) #20

Interesting article, thanks Rob.

What aren’t post mid-sixties cars still positive earth given the advantages mentioned in the article?