• Tiny
  • smax
  • 1986 Jaguar XJS

Engine Mechanical problem
1986 Jaguar XJS V12 Two Wheel Drive Automatic 150000 miles

hi-i have strange problem with a customers v12 jag
he drove car to my shop but ran poorly
changed plugs wires-filters now won t start-i checked rotor arm position at tdc and it was 120 degrees out!
i pulled distibutor to reposition butstill wouldn t start if I move it another 120 deg it starts but rns really flat but is detonating
if I try n retard it kills motor
if I advance it picks up a little but worsening detonation
motor has compression and firing order correct-i suspect jackshaft problem as dist gear ok
if I have compression presuming timing chain and camshafts phased ok-we have good spark and injectors are pulsing-am I missing something?

Monday, November 9th, 2009 AT 8:24 AM

1 Answer

  • Tiny
  • Dave H
  • Expert
  • 13,547 posts

As distributor shaft turns, reluctor turns with it. Refer to Fig. 2. As each of the 12 teeth pass across the pick-up coil's pole piece, the magnetic field strength around the pick-up winding is intensified. This creates a voltage in pick-up coil winding.
The rise and fall of this voltage is sensed by the amplifier module, which then sends a signal cutting off voltage to the primary circuit of the ignition coil. This causes a voltage surge in the secondary, firing the spark plugs at the proper time.
Normally, air gap will not need resetting unless it has been tampered with. Always use a plastic feeler gauge when checking or setting air gap. See Fig. 2. Gap must be checked at each reluctor tooth.
To check air gap, align reluctor tooth with pole piece of pick-up coil. Insert proper plastic feeler gauge. On XJS, set air gap at.007-.015" (.18-.38 mm). Check gap at each reluctor tooth.
To adjust air gap, loosen pick-up coil mounting screws. Align reluctor tooth with pole piece of pick-up coil. Insert proper feeler gauge and move pick-up coil against gauge. Tighten mounting screws.
Fig. 2: Adjusting Reluctor-to-Pick-Up Coil Air Gap
Check battery specific gravity. Specific gravity between cells must not vary by more than.040 in any cell. Connect battery/starter tester and load test battery.
Disconnect high tension wire from center of distributor cap. Hold wire about 1/4" from good engine ground.
Crank engine. If a good spark is obtained, check spark plug wires, spark plugs, distributor cap and rotor. If no spark or poor spark is obtained, perform component checks.
Turn ignition on. Connect voltmeter positive lead to ignition coil positive terminal. Connect negative lead to ground. See Fig. 3.
Reading should be 12 volts. If voltage is less than 11 volts, check wiring back to ignition switch and battery.
Fig. 3: Checking Ignition Coil Voltage (Check at both terminals)
Connect the positive voltmeter lead to ignition coil negative terminal. See Fig. 3. Reading should be 12 volts.
If a zero reading is obtained, disconnect amplifier module lead from coil negative terminal. If voltage is still zero, a faulty ignition coil is indicated.
If a zero reading was obtained in step 3), but a 12-volt reading was obtained after disconnecting amplifier module lead from coil negative terminal, amplifier module may be faulty.
Turn ignition off. Disconnect pick-up coil connector from amplifier module. See Fig. 4. Set ohmmeter to x100 scale. Insert ohmmeter leads into 2 pick-up coil harness terminals.
Resistance reading should be 2200-4800 ohms. If not, replace pick-up coil and harness assembly.
Fig. 4: Checking Pick-Up Coil Resistance
Connect a 12-volt test lamp to negative terminal of ignition coil. See Fig. 5. Ground test lamp. Crank engine, but do not start. Test lamp should flicker slightly.

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Sunday, January 10th, 2010 AT 7:18 PM

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