My 1986 automatic Series III 4.2 has recently been intermittently failing to crank. I thought I had made a diagnosis of heat affecting the solenoid + starter motor—having been stranded several times after a 30 minute journey—and replaced the Lucas starter motor with a Mazda one. Then it happened again! Just a whining sound, with a click beforehand.
Randomly I think I have found a working solution, though it makes little sense to me. If I have locked the car, (central locking is installed), which I only do when away from home, and then unlock it, the car fails to crank. But if I move the selector into D, then attempt a crank, and then move it into N and attempt a crank: hey presto, it cranks! I discovered this having been stranded yet again. It has since then worked twice in this way after displaying the same symptoms.
XJ fan,
It is possible that the Neutral Safety Switch is out of adjustment. This switch is a safety interlock that prevents engagement of the starter in any transmission position other than Park or Neutral. This switch is on the left side of the transmission is depicted in the S57 Electrical Guide and in the wiring diagrams in the Series III Service Manual.
You are likely to find more information in the Jag-Lovers archives if you search “neutral safety switch”.
Perhaps someone has worked on your transmission and knocked this switch out of adjustment.
Somewhere, either on the transmission or shift tower is an interlock switch On in park or neutral and allosws a cran there. It does not allow a crank in D or R.
maladjustmentand it may crank in r or D or not at all.
Verify, by Identifying which wire on the relay goes t that switch. jump that wire to ground and try tocrank. if it does, you have the issued identified,\Ipen the console and fid the swtch and its connectrs. Fix.
Carl, XJ fan,
Attached is a picture of a 4.2L Jaguar XK engine with a BW66 transmission on my engine test stand during one of my XJ6 engine removal and replacements.
The Neutral Safety Switch, with it’s electrical harness, is circled in red on the left side of the transmission… The switch needs to be properly adjusted to ensure starter engagement only in the Park or Neutral shifter positions.
Since this was written 20+ years ago I’ve learned that some of the early transmission mounted switch were not made with an adjusting slot. In that case (not likely on an '86) the hold-down hole will have to enlarged to allow the switch to ‘swing’. Even the later switches may require that the existing slot be elongated a bit
It’s likely the start inhibit switch - as others have said. The whining sound is likely the fuel pump running in ‘crank’ as it should. The ‘click’ could be the main relay engaging - which should engage as ign is turned ‘on’. Only the starter relay should operate in ‘crank’, which it won’t if the start inhibit switch fails - intermittently it seems…
Connect the starter relay black/green connection to ground. This will bypass the safety switch - if the swich is at fault; the engine should crank…
The sole purpose of the inhibit switch is to prevent cranking/starting with the engine in gear. By ensuring the car is in ‘P’ or ‘N’ while cranking, it is ‘safe’ to keep the grounding until the safety switch problem is solved…
I found that the shift tower on the XJS was similar, but, it was designed to accompdate the inhibit/backu switch on the tower. The TH400 transmissin has no provsion for the switch.
My lump kit had provisons for that. A added micro stch at the base of the tower for the start inhibit. and XJS fpr the tower. The latter for the back up lights.
After a while, I decided to install a XJS shift tower and accomodae both on it.
I do have an emergency crank wire under the bonnett.
Thank you for the suggestions and help. Today we tweaked the Neutral Safety Switch, without really knowing whether it would make a difference because today was (no longer) a day when it cranked every time the key turned … until now.
I don’t have the confidence to set up an earth and can’t work out how to set up an ‘emergency crank wire’ which is what I obviously need. Can anyone give a diagram and step-by-step please?
(the working solution I thought I had found doesn’t work today. It was obviously a coincidence)
The starter relay coil is connected to the (Jaguar) white/yellow circuit - powered by the ign switch in ‘crank’. The ground is provided, through the start inhibit switch, by the (Jaguar) black green wire.
If the connector at the relay is not readily available you can remove a bit of the insulation and wind/crimp the ground wire securely to the bared black/green - and secure the other to a good ground, The current through the relay coil is small, so a thin wire is adequate…
The main point is just testing if the start inhibit switch is the cause of the problem - or if it is elsewhere. The
‘modification’ is perfectly harmless apart from enabling the engine to crank/start in gear. Which is a safety issue, so in principle, the ‘mod’ should be removed when the problem is sorted…
If the wiring is non-Jaguar standard; you just have to identify the proper wire for the modification…so…?
I could have mentioned that the grounding could have been done by disconnecting the black/green at the switch, and grounding the wire - but I was a bit usure of the switch’s layout…
