I’m troubleshooting an intermittent ignition problem.

I’ve converted the car to push-button start since the ignition switch/key assembly failed. This is shown in the diagram. Its worked well for several months.

On occasion, when I activate the ignition, nothing happens. I mean nothing. The started doesn’t even try to turn. The battery is in great condition and the starter works, I’ve confirmed this by jumpering from the right terminal post to the C1 and C4 leads on the starter relay which lead to the starter and the EFI, (shown in violet) and the starter fires every time. So I don’t think the starter is the issue.

What is confusing me is the inhibit relay. As annotated in red on the diagram, when the starter button is pushed the ignition relay is triggered, and power is sent to the inhibit relay. But what action closes the inhibit relay?

I’ m guessing that the purpose of the inhibit relay is to prevent the car from starting if either the inertia switch or open, is this right?

So if I jumper across the inhibit relay (proposed jumper shown in blue) I should be able to start the car by doing the terminal post to starter jumper (shown in violet)? This would just be a test as I don’t want to defeat either the inertia or neutral switches permanently.

Thanks,


John

1987 XJ-S V12

75,000 miles

 

The inhibit switch is a microswitch on the front of the gear change lever, and is accessd by removing the ski slope. What it does is to cut the feed from the ignition switch position III (ie starter position) to the starter relay. The switch prevents the starter being used if the lever is in any position other than neutral or park. If it goes out of adjustment it can prevent the starter activating even in the two start positions.

Assuming you placed your starter button before this switch, which you must have done if you took the feed from the ignition switch loom area, bad adjustment of it is the most likely cause of your problem.

I attach the relevant starter circuit diagram, which I think, repeat think, is still correct for your car. The microswitch I am referring to is numbered 6

 

Thanks Greg!

Is the "Neutral Switch" the one circled in green in the photo below? I see that it may be two switches, one to sense neutral (or park) and another that does something else.

The MY 1987 XJ-S Electrical Guide shows an image of the Neutral Switch with two connections, one being black and green, the other being black. On another page it shows an "inhibit switch" with one wire being yellow and purple, which seems to be the one closest to the camera in the image below. I take it these switches are mounted together, thus explaining the four wires instead of two.

I don't see a way to adjust this switch. In other posts I've read that the factory service manual explains how to do it, but I can't find that document.

Thanks,

John

 

It almost certainly isn't the correct diagram

And the diagram JCR posted might not be correct, either.

At some point Jaguar began using an inhibit relay on cars with oxygen sensors. It's purpose is to put the car into open loop ("inhibit" the oxygen sensor feedback) when the transmission is in "P" or "N". It uses the neutral safety switch (inhibit switch) to do so.

The starter relay cannot operate unless the inhibit relay has been triggered.

Cheers
DD

 

The neutral switch has black and black/green wires, yes.

The other switch is the cruise control inhibit switch ..... yellow/purple and black/white wires

The adjustment requires loosening the tiny screws so the switch can pivot a bit. It's a little fussy. Before going thru the bother I'd remove the black and black/green wires and jump them together. If the starter operates then go ahead with the adjustment procedure.

If the starter still does not operate then we obviously need to dig deeper. If you post the last 6 of your VIN I can post the right diagram as there were several variations of the inhibit relay-starter relay scheme.

Cheers
DD

 

I'm going to make a temporary switch that I can flip to short the neutral switch. The problem is so intermittent that I'll have to drive it for a while to have confidence that the switch is the issue.

Last four of the VIN are 134879.

Thanks,
John

 

This should be the right diagram for you

 

Hi Doug, yep, that diagram looks just like the one I have.

I've put a toggle switch in that will "short" the rocker switch in a no-start situation. Testing this setup certainly indicates that the rocker switch is out of adjustment. I'll give it a week or so and then delve into adjusting the rocker switch.

Thanks,

John