Canadian XJ12 1987 Inhibit Relay

(Carl Hutchins, Jr. ) #22


Eureka, you have the proper schematic. It is a page from the Jaguar S57 Schematic that has guided me for a long time.

Far more accurate and easier to decipher than the one in Haynes. I quit looking in Haynes. Stuff is there, but the organization is awful and confusing.

Go for it, you have a proper course on the proper chart. As sailors say, which I am not, “fair winds and trailing seas”.



(Gregor Eichenberger) #23

Now I found some time to work on my Cat…
The original Inhibit Relay (Bosch # 0 332 014 411) was fried…

So now I bought another relay (old Inhibit Relay Bosch # 0 332 014 411) with a Bosch # 0 986 AH0 135. This Relay has also a Diode inside like the old Relay (Bosch # 0 332 014 411).
As described in Kirby’s Book on page 564 I changed the Terminals 85 to 30 cause the Relay’s are different.
Tried with another Relay with an resistor in it (Bosch # 0 332 019 103), trying with some common standard Relay (without Diode) and get the same results => Now I have the Problem while hitting start I get a rattling noise from the solenoid and the Engine won’t turn.

Shorting the Relay (Inhibit Relay) Socket 87 (WB cable to the W1 Port of the Starter Relay) directly to 30 B EFI Chassis or 85 BG from neutral Switch, that works fine.
The Solenoid hits the Wheel and the Engine is turning as it should be.

Measuring Voltage while Starting from Socket 86 (+) W from Inertia Switch and Socket 85 (-) from BG Neutral Switch works fine.

Does anyone know what’s wrong? I am speechless…

(Paul M. Novak) #24

I suspect that you might have an incorrect relay or two in your engine bay that are causing your problem. I own a Canadian market 1990 V12 Vanden Plas, which should be very similar to your 1987 XJ12. I also own a 1984 XJ6 Vanden Plas and a 1990 XJ-S convertible and the wiring and relays used are different between them.
Please look at the two attached pictures, go out to your car and identify the Bosch # for the relays #1, #2, #3, #4 and #5 in my pictures and write back with what you have in your car right now. There are at least 4 different kinds of realys used in my 1990 V12 Vanden Plas and they look very much alike except for the part number. I want to make sure you have the correct relays in the correct locations in your car. Once you provide me with what you have I will compare it to my car, which starts and runs perfectly. I can’t get to my car right now, but once you write back with your results I will check my car.


(Doug Dwyer) #25

This is bringing back some unpleasant memories.

Paul might well be correct about an incorrect relay. The various relays look alike superficially but there are some important distinctions that can go unnoticed, especially with respect to the diodes.

Complicating this is that the original relays (AGU1070 “main” and AGU1097 “inhibit”) are no longer available from Jaguar and vendors are offering incorrect substitutes! The diode points the wrong way, or is in series with the relay where it should be across the relay coil, or vice-versa.

From some old notes it appears that a Hella 933791091 would be a proper replacement for the inhibit relay:


I had similar problems a couple years ago and, from memory, the problem remained even with the 091 Hella relay. I bypassed the inhibit relay, as mentioned above, with the intention of revisiting the issue. I totally forgot…until this thread came along.

I’ll do some more checking and head scratching…and perhaps will find some additional notes.


(Carl Hutchins, Jr. ) #26


Oh yeah, the diode thing!!! Much travail here as well.

Johns cars helped me. External diodes. sourced from Radio shack. One on each leg of the load circuit., direction unknown to me. Mission clearly to prevent a back feed and crank the engine while it is running!! Don’t ask…


(Paul M. Novak) #27

I just looked at some notes I wrote down regarding the relsys when I recently cleaned up the engine bay of my 1990 V12 Vanden Plas. I believe that the last 3 numbers of the Bosch metal covered relays shown in my previous email are:

#1. 113

#2. 101

#3. 411

#4. 113

#5. 112

If this is not what you have, your problem may be due to incorrect relays and/or one of them may have failed. From a distance they look alike, but the part numbers and electrical schematic stamped on the sides show that there are 4 different kinds of relays used in these 5 locations.


(Aristides Balanos) #28

All the relays have a diagram printed on them.
It’s fairly easy to get a generic relay and modify it.
Add a diode, resistance or bridge as per the original (internally or externally) and rearrange the spades on the relay socket so that they correspond with the original configuration.

I have also added a LEDs on their primary circuit, makes troubleshooting so much easier !!

Relay%20005 .

(Frank Andersen) #29

In this case, for some reason; one relay cancels the action of the other, Gregor - for some reason. One relay engages the solenoid, and as it does, the other relay activates to cancel the first ( or the other way around) - and the solenoid doesn’t know what to do…

Diodes are sometimes used to prevent such mutual interference. However, as Paul and Doug implies you may have incorrect relay(s) for the diagrams connections to work. Which may be solved by different connections…

Original set-up; the starter relay connects brown (C2) to solenoid white/red - it’s the sole function of the starter relay.

The starter relay solenoid is powered in ‘crank’ white/yellow (W2) - though in your schematic indicated white. But relay will only engage when coil is grounded white/black (W1)…

…which is done through the inhibit relay; connecting white/black (82) to black (30). This connection is made when the inhibit relay engages…

The inhibit relay coil has power in ign ‘on’; white (86) via the inertia switch. Inhibit relay operates when black/green (85) is grounded - through the "neutral’ (aka ‘start inhibit switch’ in old speak).

Shorting inhibit ‘87’ to ‘30’ simply connects starter relay W1 directly to ground - and the starte relay then engages as key is turned to crank (powering W2). Black/green ‘85’ to ‘30’ does the same; it grounds W1 (providing the gear lever is in ‘N’ or ‘P’)

The first ‘jump’ bypasses both the inhibit relay and the neutral switch. The second bypasses only the inhibit relay - in fact establishing the earlier ‘inhibit switch only’ set-up. Which you can use while sorting out the problems…

As the wires used in testing seems properly attached to their destinations - the problem is likely wrong relay(s), not connecting the destinations as intended. Keep in mind that black wires are direct ground, while black/something indicates an indirect ground…

Relays can be tested to find out what they actually do - and different connections can then be considered. Basically all relays do the same; connecting two (or more) terminals when the two others (relay coil) is respectively powered and grounded…

You do have power on inhibit white with ign ‘on’…?

xj6 85 Sov Europe (UK/NZ)

(Doug Dwyer) #30

It can be used for a lot longer than that, if my experience is any indicator :slight_smile:

One has to wonder (although not very deeply, in my case) why the “inhibit relay” exists at all.

In the “EFI and Emissions” section the same relay is shown but is called the “Feeedback Inhibit Relay”, presumably referring Lambda feedback. But, I cannot see where it serves any such purpose.

Anyhow, speculation as to “Why does it exist?” doesn’t get us any closer to solving the problem. Sorry for the side track.


(Gregor Eichenberger) #31

First of all thank you Guy‘s for your replies, I appreciate that.

Yes, No. 3 (411) is the Inhibit Relay which fas fried. All other Relay‘s are correct and I didn‘t touch them.
@Paul_M_Novak2 Thank you Paul for the pictures and Notes.

Also I took off fron the + Terminal Post the fat cable to the Starter to go sure if it is the Solenoid which is rattering.
Without Power to the starter I hear one klick, the solenoid hits the Wheel correctly.
Now I don‘t know what rattles. With Relay it rattles and the fan turns 1/2 inch for and backwards.
As said, shorting the Inhibit Relay as marked in my post from Yesterday works fine (Starter works great and solenoid works fine).
Also measuring the Power for the Inhibit Relay for the Coil of the Relay while turning start is ok and doesn‘t break.
The white Cable from the Inertia Switch to the Relay Coil has always 12V + whin ignition is on.
The Diode in the new Relay is in the same Way as in the old Inhibit Relay (see Picture below).

@Doug_Dwyer4, I was also wondering why they made such a thing… see below what I found in Kirby‘s Book on Page 568:

OTHER SWITCHES AND RELAYS INVOLVED IN STARTING: If you are trying to use the EFI wiring diagram in the ©1982 Supplement, page 19-1, or Figures 13.10, 13.12, or 13.93 in the Haynes repair manual to diagnose starter problems, you may be getting very confused. These diagrams indicate that the only thing that disables the starter relay is the “start inhibit switch” (on the shifter, also called a “neutral switch” or “automatic gearbox safety switch”, item 75 on Jaguar schematics) via the “feedback monitor relay” (also called an “inhibit relay” or “feedback inhibit relay”, item 355 on Jaguar schematics). Why is a starter-disabling relay called a “feedback monitor relay”? Judging from the diagrams it appears that the neutral switch on the shifter was originally intended to prevent starter operation in gear, but with the emission-controlled Digital P EFI it’s also needed to switch the EFI from open-loop to closed-loop operation – and the neutralswitchonlyhasasinglecontact. So,insteadofaddinganotherswitchorprovidingonewithmorecontacts,they installed this relay. Since the relay likewise only has a single contact, it is used to control the starter, while the signal to switch the EFI – being very low current – is taken through the relay coil. When the neutral switch is closed, the BG wire is grounded, the relay closes, and the EFI signal input sees 0V; when the neutral switch is open, the relay opens and the starter won’t work, while the EFI signal input sees 12V in the ungrounded BG wire.
In the “UK & Europe” diagrams on page 19-2 of the ©1982 Supplement or Figures 13.9 and 13.11 in the Haynes, there’s no such thing as closed-loop operation and therefore no switching it on and off, so the feedback monitor relay is omitted and the starter relay is simply grounded through the neutral switch. The starter wiring circuit on page 86-7 of the ©1982 Supplement shows this non-US layout with a simple connection from the starter relay through the neutral switch to ground.

(Doug Dwyer) #32


I think I have it now.

My mistake was thinking that the ECU simply relies on a ground signal to inhibit feedback. This obviously wouldn’t require a relay.

What the ECU is looking for is 12v to allow feedback.


(Doug Dwyer) #33

I add the following with some reluctance as I’m not sure if it will help or confound the situation…above and beyond the side-tracking I’ve already done.


Are we working with the correct diagram here?

In refreshing my memory on this circuit (as I haven’t visited it in a couple years) I was mulling the diagrams and something hit me. It was roughly 1987-ish when Jaguar added two idle control relays, and two inline diodes.

In this arrangement, the Black/green wire from the neutral safety switch does not go directly to the inhibit relay as shown in the diagram posted above. Instead it goes to the “Idle Override Relay” and then on to the “Inhibit Relay”…with a diode in between.

Before going any further into the weeds let’s make sure you do (or don’t) have this later circuitry.

The idle relays are as follows:

Idle relay, black base, has Green/brown, black, pink/white, yellow/orange, and yellow/black wires.

Idle Override Relay, blue base, has white, black/green, black, and yellow orange wires.

I think the pics posted by Paul earlier on shows these two relays.

If you don’t have these relays then ignore everything I’ve said.

if you DO have these relays then we’re working off the wrong schematic.


(Paul M. Novak) #34


I believe that you are correct that Gregor is working off of the wrong electrical schematic.
I own a Canadian market 1990 V12 Vanden Plas, a 1990 XJ-S convertible (5.3L V12) and some Series III XJ6s. I also have a lot of technical documents about these cars including the Service Manuals, Repair Operations Manuals, and the S57 Electrical Guides.

Since getting my 1990 V12 Vanden Plas in 2011 I have learned that I have to use a combination of the XJ6 and XJ-S documents in order to address issues, particularly electrical ones. That is because as good as the Series III XJ6 S57 is, it doesn’t cover some of the things that I find in my 1990 V12 Vanden Plas. I have not located an XJ12 S57 Electrical Guide that specifically covers my car. A good example is this relay issue where some of the relays in question are not found in XJ6.

I think tgat Gregor needs to be working from an XJ-S Electrical Guide from about 1987 to see if it matches his car.


(Doug Dwyer) #35

I agree.

And even then there are choices to be made. My '87 XJS S57 Guide has three variations of “EFI and Emissions”.

As good as the S57 guides are, the starter circuit section doesn’t show that differences in the emissions circuits can change the way the starting circuit operates.


(Gregor Eichenberger) #36

Yes indeed I have the Idle Override Relay, right next to the Inhibit Relay, see Paul‘s picture.
The Idle Override Relay ends with the number 101 as posted before.
I measured the Black/Green cable and its the same in both Relays.

And still I can‘t imagine how this Relay could disturb the starting Process?

(Doug Dwyer) #37

I’m thinking the diode (on the black/green wire) which resides between the Idle Override Relay and the Inhibit Relay.


(Doug Dwyer) #38

To clarify, when the gearshift in “P” or “N”, the black/green wire at both relays shows continuity to ground? If so, that’s as good as it gets as far as that goes and the diode I mentioned would be OK


(Frank Andersen) #39

The Lambda system is in open loop until the O2 sensors reaches operating temps, some 400C - and until then the fuelling is regulated by the basic engine setting. During cranking, hot or cold, the Lambda is disabled, open loop, to allow cranking fuel enrichment.

In closed loop, the Lambda tries to maintain ‘Lambda = 1’ - which is OK only for steady pace…

xj6 85 Sov Europe (UK/NZ)

xj6 85 Sov Europe (UK/NZ)

(Doug Dwyer) #40

And, on V12s of this vintage, the system is in open loop whenever the gearshift is in “P” or “N”.


(Carl Hutchins, Jr. ) #41

As relays and their use fascinate me, I interject the following question.

What can a diode do on a ground wire ? No back feed from ground to prevent!