All,

Newb here and first want to say thank you to the many members who contribute to help out all of those with questions! This site is really awesome!

I just purchased a 92 XJS Convertible. Beautiful car outside and in...under the hood it's a mess. I've been reading up quite a bit around here and have been getting prepped to do some work. Unfortunately as I found out, not only is it grungy under the hood, there are several wires that are bare, roasted, whatever...same for a lot of the connectors. I ordered a few new connectors for the coils because they were both awful...the wires were bandaged up in a bad way and the connectors had all but melted. So...ordered some new connectors from Amazon and decided I would just redo the wiring going to the coils. Unfortunately, when I started on the B Coil the wires both broke off. So, now I have no idea which wire was the positive and which is the negative. The wiring diagram shows the positive to be white while the negative is white-black (who woulda thought that would have caused problems later...). When I pulled the the wires apart and started looking the wires crumbled so there is no way whatsoever to tell what wire is what.

Sorry for the long back story but thought it might help ascertain what happened on my end. Anyway...is there any easy way (or any way at all) to figure out which of the two wires is the positive one? I don't think it usually matters all that much if the wires are backwards polarity wise but knowing how finicky these cars are with regards to the ignition system I don't want this to be wrong...

Thank you and cheers!

 

Welcome to the Forums.

When the beer settles the nerves, please do an Intro in the New Members Area, its keeps the "boss" happy.

Your use of COILS, has me assuming a V12???

That would then make it a Marelli car, Jag Speak for the Engine Management System.

I know next to zero about that system, BUT, there are members here that do. When the Planets align a tad more, they will turn up.

Good luck.

 

Hey Greg,

I’ll do the intro…definitely want the boss on the happy side! I’ve already read a few of your posts…very cool info and projects.

Should have been more specific but yes it’s the V12 Marelli. I’ve already done the distributor silicon mod but was make sure I get the wires correct. I THINK the positive may show some voltage when the ignition is switched on per the wiring diagram but not totally sure.

If anyone else has specific info I’m all ears!

Thanks!

NJ

 

Ignition on, the positive, if disconnected from the coil and metered to earth, will have 12 volts. The negative will not.

 

Awesome sir! I’ll give it a shot tomorrow and tell you what I find.

Thank you!

 

So I hooked up the volt meter and one wire reads 12V while the other’s not 0…it’s actually 7 or so.🧐 I’m gonna assume the 12V is correct though.

 

Actually, just retested and BOTH wires show 12V when I turn the key on. Any other ideas?

 

Are the wires disconnected from the coil, or coils, when you test them?

 

@Jag Nigel in addition to the distributor shaft mod, the other thing as a Marelli owner to make certain of is that your spark plugs are correct spec AND have the exact correct gap.

Cheers

 

Greg,

Yes both of the wires are just hanging, as in not attached to anything since they separated from the four row connector. One should be for the positive and the other for the negative sides of the coil. This morning when I turned the ignition on and tested either wire they both showed 12V DC…

 

Mac,

Per The Book I replaced all my plugs and faired them to .025. Interestingly enough I ordered NGK BR7EF plugs and they were gapped correctly and NOT at .035.

Unfortunately I now have bigger issues…

 

Hi Jag Nigel

Don't know if this will help but the Lower Coil the 'Red One' does 'A' Bank and the Upper Coil the 'Yellow One' does 'B' Bank

Get them connected the wrong way round and She won't Start!

 

So if there are two coils, are you testing the two positive leads? A photo would help the Marelli gurus (not me, I am Lucas) to identify the leads you are testing.

 

Hi Greg

I will see if I can find the Photo as many Years ago now you helped me to get my Car Started when we found I had connected the Coils the wrong way round, as it wasn't about testing them as we didn't do that

I just had the wrong Coil connected to the wrong 'Bank' and once I'd swapped them over, She started straight away

This is the Photo I was thinking of Showing the Coils

Nothing wrong with your Memory 'Grant' (This was in 2013)

After doing my very first Job on this Car which was Changing the Spark Plugs that as I remember took me about a Week to do!

She still refused to Start and I was completely out of ideas, until you and Greg came to the rescue, where we discovered that I had the Wrong Coil connected to the Wrong Bank!

Where when we swapped them over She Started Straight away!

I Still can't believe how rusty She looked compared to how She looks now, or the day I bought her, where She was parked right in front of the big Country House of the Owner, on one of the most beautiful days that you could ever imagine, with her Soft Top down and waiting to meet the New Lucky Owner

Which was going to be me!

And I wasn't leaving without her!

Just one look at this 'Ruby' glistening in the Sunshine was more than enough to convince me that this was the Car of my dreams! and still gives me a thrill every time I open the Garage door and see her there!

 

Thanks again guys for all the help...unfortunately I'm out of town right now and can't get back to the car for over a week.

I had another issue. When I tried to start the car one of the harnesses (pretty sure it was the injector harness) tried to catch fire on turnover. I had to leave for my trip so I just shut everything down and left it as is for now. I'm sure I'll be building a new harness when I get back home.

But I'm about 95% sure I had the coils attached to the right spot. I know the one that has the 3 wires for the coil is the one with the with the tach drive, is the A coil and sits basically under the throttle pedestal. Not my car (I wish) but it did look like this plug wire routing wise

Pretty sure the no start issue was directly related to the wiring harness melting.