Canadian XJ12 1987 Inhibit Relay

(Gregor Eichenberger) #1

Dear Jaglovers
Had some trouble with my starter and engine (details in the V12 section).
Now my starter relay went down. After measuring all ports, I’ve only
about 2-3 Volt on the W1 wire from the “P” & “N” neutral switch.
It does not change, even if I change the gears to “D” etc.
Because it’s a 1987 model I think there might be a inhibit relay
(which is triggered by the neutral safety switch).
But where is it? Following the W1 wire ends in a fat harness/confusing.

Btw, I have two switches on the gearshift :frowning: and shorting them didn’t
change anything, got still only 2-3 Volt on W1.

1987 XJ12 HE Engine won't crank
(Rob Reilly) #2

One switch on the shift lever is the starter inhibitor and just connects W1 to ground when you are in Park or Neutral; the other works the reverse lights.
I’m looking at the Series 3 wiring diagram in the Haynes manual and there is only the one starter relay. W1 is the wire from the shifter switch. W2 is the wire from the ignition key switch Start position. C1 is the wire to the starter solenoid and also the fuel injection amplifier and cold start relay. C2 is hot positive from the battery. C3 is a wire to the ballast resistor.
If you jumper from W1 on the relay to ground and it starts that means the shifter switch is bad or not tripping, but be careful because it will start in all gears.
2-3V at W1 means it is probably picking up voltage from the invertor.

(Paul M. Novak) #3

Did you follow all the steps in the “Engine Will Not Crank Checklist” that I suggested to you on the V12 List? Again that checklist is found on the Jag-Lovers “Old Site” XJ site at:

This excellent guide, written by list member Doug Dwyer year ago, should lead you into finding a resolution.

BTW, I own a Canadian Market 1990 V12 Vanden Plas, very similar to your XJ12. It has a feature not found on the Jaguar S57 Electrical Guide or in any of the manuals that I have found. When the Inertia Switch is tripped, not only is the fuel pump disabled, but also the starter is disabled. I am not sure why Jaguar did this, and it is not that way in either of my Series III XJ6s or my wife’s 1990 XJ-S convertible. But in my 1990 V12 VdP if the Inertia Switch is tripped neither the fuel pump or the starter will work.


(Gregor Eichenberger) #4

@Rob_Reilly what is an invertor? I’m really wondering fron where W1 takes the voltage. Right, shorting W1 on the starter relay to ground works.

@Paul_M_Novak2 Yes I did follow Doug’s list and also checked the starter relay as discribed in Kirby’s book.
Then I found out that the Voltage on W1 is not enough.

On the internet I found two various schematics. One with the (-) circuit going from the shifter switch to a inhibit relay and the other one directly from the shifter safety switch to the starter relay (W1).
Thougt maybe the older cars don’t have a inhibit relay but mine has?

(Frank Andersen) #5

As Rob says, Gregor; the only relay is the starter relay - and shorting the W1 to ground is the first step…

This will bypass the start inhibit switch - and if the engine then cranks as it should, the fault is in the switch itself (or the wiring to it).

If no cranking; the problem is deeper. You do not mention the voltage readings on the other relay points; W2 should show 12V with the key to ‘crank’ - if not; the problem is at the ign key.

Brown should shot 12V at all times - white/red should show 12V in ‘crank’. The sole purpose of the relay is to connect brown to white/red - to engage the starter solenoid. Shorting these wires bypasses the relay and should crank the engine - or there is a solenoid/starter issue…

There should be no voltage on any relay connections with ign ‘off’ - except brown. And generally; connected wires showing low voltage implies a current flow and/or a bad connection. Checking should be done with wires disconnected…

xj6 85 Sov Europe (UK/NZ)

(Aristides Balanos) #6

Well, you have your answer., the micro-switch is kaput.
You might have luck with some contact cleaner, but I doubt it.
The switch is a generic item and can be found in any electronics supplier, measure the size and the mounting holes.
Don’t remember if it’s NO or NC…

The voltage you see must be coming, most probably, from the relay… or your meter is not that accurate…
If you don’t lose any smoke, all is well !!


(Rob Reilly) #7

Starter inhibitor switch, in industry it is called a micro switch, many brand names to choose from, I like Cherry and Honeywell.
NO = normally open; NC = normally closed
I think it is normally open and closes only when the shifter is in Park or Neutral.
The invertor is in the panel light circuit and I think operates with the panel warning lights for brake fluid level, brake pressure differential, handbrake and oil pressure.
My Haynes manual was printed in 1980 so if there is an inhibitor relay in later cars I don’t know about it. Not clear to me why it would be needed.
I don’t think you are on the right track looking at voltage at W1. 12V positive comes from the #1 terminal on the key switch to W2 and through the W=Windings inside the relay to W1.
BTW the C terminals mean Contact.
Could it be something as simple as the inhibit switch out of position, not closing when in Park?
Try removing the micro switch on the shifter and operating it by hand. Shifter in Park, key on, listen for a clicking, if you hear something else clicking it may be your mystery inhibitor relay.
Then holding the switch closed, try the starter.
If no start, try shorting across the switch terminals and see if it starts.

(Carl Hutchins, Jr. ) #8

I recently had my adventures with the starter on my car. But, in an exactly opposite way. It activated the starter with the engine running !!! Starters and the battery suffered. I off f lucky in that the teeth on the flywheel survived

Issue was two fold

  1. Bad ignition switch did several odd things, including combining crank and run!!!

  2. The start relay as wired in my car needs two diodes. Just why, a bit fuzzy to me. But, seen as a back feed gate!! The new relay I installed had none!!! Replacing the one with them along with a new switch, starter and battery and all went well.

Think relay and how they work. Here we have a load circuit. Volts in and volts out.

And the trigger circuit. Prove power to one lega nd the other to ground sand the load circuit is triggered. Starter, in this case cranks.

Control here is via the inhibit switch. Some jags have it on the BW transmission Dual function switch. Also for reverse lams. Otyhers mount the same or similar switch on the switch tower and function the same way. And yet another use the micro switch at the front base of the shift tower. In that case, the micro only is involved with the starter.

So, here, I do not understand a volt reading on the ground leg of the start relay. Issue in the relay with a minor short ?

When into my adventure, I added an emergency/convenience crank wire. One end at the solenoid atop the starter. The other end up top with the bare end capped. Thusly, I can tough that bare end to battery +, which is near by, and the engine will crank. It cares not as to the position of the ignition switch. Beware, if in run, the engine will fire. In gear, not good…

All before quite useful. My vote s for a bad or misaligned micro switch…


(Gregor Eichenberger) #9

Measuring the safety gear switch I get 12 volt in gear P and N as it should be un the black green cable in the picture.

Now, the W1 port for the Relayis white blue and I get only 3-4 volt. Is there really no other relay in between gear saftey switch and starter relay?

(Frank Andersen) #10

The sole purpose of the start inhibit switch is to connect starter relay W1 to ground in ‘N’ and ‘P’, Gregor…

In ‘crank’; 12V is applied to W2 (white/yellow) - which is connected to W1 (black/green) through the internal relay coil. In any other key position there is no power to W2. With W1 grounded, the relay operates - connecting C2 (brown) to C1/3 white/red, operating starter solenoid. The wire from the start inhibit switch to ground is plain brown

Where is that white/blue - it seems irrelevant to a cranking problem. The white circuit is powered with ign ‘on’; and as Paul says - late V12 may have features not common. But the starter relay connections should be identical, though white circuits are indeed cut by the inertia switch…

For the cranking problem; have you tried jumpwiring W1 directly to ground and tried cranking…?

(Aristides Balanos) #11


If I understood correctly from your previous posts, when you ground W1 directly the starter works ?
Nevertheless, first test the micro switch, it should be Closed only in P and N and Open on all other positions.
With new wires, ground one side of the switch and connect the other side to W1.
If it works there is something wrong with the original wiring…
If not then there is something wrong with your starter relay.

Alternatively you could also use a generic relay.
There lots of info also in Kirby’s book.


(Gregor Eichenberger) #12

Yes, grounding the W1 Port on the Starterrelay makes it possible to start the Jag. On W2 I get 12 Volt + while starting/turning the Key to start.
The white/blue is the W1 cable cause it has a rubber Ring written W1 on it.

I don‘t understand why I have a black/green cable on the safety switch and W1is white/blue? wherecare they connected and why they got the diff. Color?

(Rob Reilly) #13

According to my 1980 Haynes manual:
white/blue is supposed to go from C3 to the ballast resistor.
black/green is from the safety inhibit switch to W1.

(Gregor Eichenberger) #14

Ok, now, I got the black/green cable (W1) and measuring I get 12 Volts in Gear P and N. Putting that together it works. On C3 I have already a white/blue cable connected.
The black/green in my hand, I have always 12 Volt (-) on it. On W2 I‘ve the white/yellow cable and W1 black/green which now works fine.
I always thougt the white/blue is W1 and the white/black on W2 as written on the rubber (see added picture below, ends in my hand) and for what is the black green cable in the picture?

(Rob Reilly) #15

Seems like you need to get a wiring diagram.

This one is dated 1980 so I don’t know if everything would be the same as '87.
On here item 194 is the starter relay, 4 is the starter solenoid, 5 is the starter, 3 is the battery, 38 is the key switch, 75 is the shifter safety inhibitor switch, and 164 is the ballast resistor.
wire color codes are
B = black
U = blue
W = white
G = green
R = red
Y = yellow
K = pink
P = purple
N = brown
O = orange
L = light
S = slate
I think the letters GR on so many wire labels must mean Ground not green/red.

(Aristides Balanos) #16

As the black/green cable is connected to ground through the inhibit switch its voltage is irrelevant.
You should measure it’s continuity to ground.

(Frank Andersen) #17

To recapitulate, Gregor;

At the starter relay there are W1 (black/green) to start inhibit switch - should show 0V in ‘P’ or ‘N’. Switch should ground W1…

W2 (white/yellow) - should show 12V in ‘crank’ - otherwise unpowered.

C1/2/3 is coupled together when the relay is engaged in ‘crank’ - W1 powered. Normally one connection is brown - always powered. One, white/red is connected to starter solenoid when the relay is energized in crank The third is ‘spare’ not usually used - but may be used to operate something in ‘crank’

If your connections are different, ie white/blue it may be related to Paul’s remarks on different set-ups for late versions of the V12…

But what connections do you actually have on your relay set-up - by colour and number…??

xj6 85 Sov Europe (UK/NZ)

(Carl Hutchins, Jr. ) #18


  1. The crank relay has four posts. It has two circuits. Two posts for the load circuit and two for the trigger circuit. Here the trigger circuit is enabled only when it receives power. done through the crank position of the ignition switch But only when the other post is to ground And that is where the inhibit switch is involved. Only closed with the transmission is in N or P.

  2. That diagram might resemble that for the later cars. I suggest you track down a copy of Jaguar Schematic S57. Easier to decipher. And true to your car.

  3. If you can’t find one, contact me PM and I’ll see about scanning a page for you. I’d do it here, but Att’s are not always easy for me on this version of J-L. OTH, that is the way I did it on the old J-L.

Daughter was over yesterday. Delicious meal. Comfort +++. T bone Steak, mashed potatoes,
roasted veggies. and biscuits, Southern style. And a nice adult beverage to wash it down…
Nice exchange of gifts. And “played” with
my smart VIZIO TV!!!

Today, I’ll visit with son for more XMAS!!!

Coco had a three bone night!!! Delighted.
Billy gets his XMAS treat later. He is up and looking!!



(Gregor Eichenberger) #19

Hi Guy‘s

That‘s propably the Plan for my XJ12 with the inhibit Relay. Also the Colors of the Cables for the Starterrelay Ports fit.
The inhibit Relay is on the left side of the Starter relay on my XJ12. In the Haynes Book I couldn‘t find that.

Propably my Inhibit Relay smoked away while I had trouble with my old starter.

(Frank Andersen) #20

Because it isn’t there, Gregor…

…it’s a later addition, and may be used on the xj40 and
model Jaguars - which is why it is not within the S1 - SIII ken. So you were right - there was an additional relay, which indeed may be the source of your problem. Adhering to the wiring diagram, and possibly changing the inhibit relay, should solve the issue. but do check the inertia switch…

The white/blue is slaved to the white/yellow - in the original set-up the latter was connected directly to the ECU. But serves the same purpose - to inform the ECU that cranking fuel enhancement is called for…

For what it is worth; it’s likely an extra safety layer. With the inertia switch tripped, as a result of a collision or your upside down in a ditch - it will prevent cranking. However, with the inertia switch tripped, there is no pump action, no ignition, no injector action - and the ECU is unpowered…:slight_smile:

xj6 85 Sov Europe (UK/NZ)