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I have an 87 Jag XJS convertible which had a 12 cyl fuel…

Customer Question
I have an 87 Jag...
I have an 87 Jag XJS convertible which had a 12 cyl fuel inj. engine which died.
We replaced it with a Chevy 350 V-8 w/Holly 4 barrel carb, dual exhausts & MSD coil & distributor. Wow for performance. I wired the tachometer cable to negative side of coil & it worked perfectly for one day & just now quit working.Wiring is OK..Did I miss something???
Mike A.
Submitted: 2 years ago.Category: Jaguar
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Answered in 19 minutes by:
2/26/2016
Jaguar Mechanic: Gary, ASE Certified Technician replied 2 years ago
Gary
Gary, ASE Certified Technician
Category: Jaguar
Satisfied Customers: 1,985
Experience: Senior Technician
Verified

Hi Mike,

It is unlikely the 12 cylinder tachometer worked ok with a V8 engine.

You now need a V8 tachometer, for which the factory tachometer will need to be removed and calibrated for a V8.

What distributor, amplifier and coil are you using?

Gary

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Customer reply replied 2 years ago
Hi,Gary,I have a new MS D blaster coil & Msd distributor / calibrated
For v-8 engine.it was wired to negative post on coil for one day & quit after 5-6 hours.whats next, I'm confused.P S engine runs fine.thanx
Jaguar Mechanic: Gary, ASE Certified Technician replied 2 years ago

I would say the tachometer is not designed to be wired to a coil negative.

The RPM signal would come from the ECM.

So engine RPM is read by the ECM via the crank position sensor.

The ECM then tells the Tachometer how much RPM to display.

More than likely the Tachometer is now faulty.

It would have been inaccurate because the pulses per rev would be different.

Gary

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Customer reply replied 2 years ago
Gary: whats the ECM? This Jag had a chevy V-8 transplant when I bought it years ago & it was rewired to the V-8.The engine was really bad shape so I had a new 350 small block installed which runs like a
jack rabbit.The coil has a positive ground,but the chevy transplant people said to wire the tach to the negative contact,perhaps an error? all the systems work fine & instruments like amps,fuel & speedo work .I am trying to connect tach,low oil press & hi-oil temp now. Thanks for your help.Mike
Jaguar Mechanic: Gary, ASE Certified Technician replied 2 years ago

Hi Mike,

The V12 would have been EFI. The ECM is the EFI computer.

Obviously now, no longer in use.

With your MSD, it could have ground on the negative if it has an 6AL amplifier.

Can you tell how many wires leave the distributor?

Purple and green; or red,orange and black?

What colour wires on your coil, positive and negative?

Gary

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Jaguar Mechanic: Gary, ASE Certified Technician replied 2 years ago

Hi Mike,

Do you need any further assistance?

I hope I was of some guidance for you.

Gary

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