Jaguar XJ-S. Service manual — part 141

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What really happens is that, at the instant the contacts break, the voltage spikes high enough to jump the gap. As the
gap between the contacts continues to widen, the current continues to jump it (helped by the generation of ozone), until
the energy has been dissipated in bridging this relatively large resistor. In other words, the energy stored in the
inductive load has been used to vaporize a few molecules off the surface of your contacts. Meanwhile, the high voltage
spikes haven’t done any good for the insulation in the wiring, and it’ll eventually start to break down.

So, you don’t want too much resistance. If the resistance was about the same as the resistance in the inductive load
itself, then the reverse voltage spike would be about 14.4V -- which the electrical system is clearly capable of handling
without damage. In practice, you can always go with a resistance a couple times larger than that of the inductive load
itself, since the wiring and components will be designed to handle much higher voltage than 14.4V.

Note that the amount of energy dissipated in the resistor itself is exceedingly small and brief. Like the diode itself, the
resistor can be tiny. A 1/4-watt resistor or even smaller will do.

Keep in mind that installing a resistor with the diode only makes sense on inductive loads where it is important to
quench the magnetic field quickly, such as on relays. It shouldn’t be necessary on electric motors, since the momentum
of the rotor will keep it rotating several orders of magnitude longer than the effects of inductance will last. Similarly,
the A/C compressor clutch probably doesn’t need to disengage suddenly, since it will spin for a bit when disengaged
due to momentum anyway, and the coil releasing the engagement a couple of milliseconds more slowly probably won’t
make any difference.

Now, note that if the resistor is high enough ohmage and high enough wattage, you can leave out the diode and simply
wire a resistor across the coil terminals! This will waste electricity, because whenever the power is on to the relay coil
it will also flow through the resistor with no diode to stop it. The resistor now needs to be large enough to handle that
current on a continuous basis rather than just the instantaneous spikes -- but resistors are cheap, so this is plausible.
While a resistor with a low enough resistance to suppress the reverse voltage spikes on a large inductive load would
waste a lot of electricity, a resistor across a relay coil could have high enough resistance that the wasted electricity could
be considered insignificant. The scheme has the advantage of making polarity unimportant, so power can be wired to
the coil and resistor in either direction without blowing out a diode.

Bosch appears to have adopted this strategy on later relays, building a resistor right into the base of the relay. One such
relay, having a gray plastic housing and number 0 332 204 159, shows a device wired across the coil in the schmatic on
the case but doesn’t indicate what the device is; it’s shown as a simple rectangle rather than a resistor or diode symbol.
However, a dissected relay revealed that the device was a 630Ω 1/2-watt resistor. At 14.4V, this resistor would waste
only 0.023 amps. Such a resistor would theoretically permit a reverse voltage spike of about 107 volts as connected
across the 85Ω coil in the relay. Presumably, this spike is considered a reasonable balance; a lower ohmage resistor
would permit a smaller spike but would waste more electricity. Using a diode would permit use of a lower ohmage
resistor for a smaller spike without wasting any electricity, but requires care in connecting to observe correct polarity.

There are other ways to deal with reverse voltage spikes in inductive loads, including installing zener diodes (which
will “clip” the voltage spike at a particular level) and capacitors (which will absorb the energy and then send it back
through the inductive load, resulting in a resonance wave back and forth until the energy is dissipated by resistance in
the winding). Craig Sawyers likes the zener diode: “This might actually be an improvement over the resistor, for the
following reason. The reverse voltage is in direct proportion to the rate of collapse of current. With a resistor, the
voltage drops as the energy is dissipated, giving rise to an exponential fall in current and voltage. With a zener, the
current will collapse faster, because it will attempt to hold a constant voltage of (say) 36.7V until the current is truly
minute (microamps).” Zener diodes are cheap, but a bit more difficult to find than simple diodes and resistors.

One application where the simple diode won’t do is on reversable motors, such as electric windows, electric seat
adjusters, automatic antennas, and electric mirror adjusters; the diode will conduct when the motor is running one
direction. One possibility is to wire in a resistor without a diode, as described for relays above; since most reversable
motor applications are only operated for a few seconds at a time, the wasted electricity can be considered insignificant
even if the current through the resistor is significant. You could actually put the diode back in if you want, just to
eliminate the waste of electricity in one direction.

Perhaps a more esthetically pleasing solution would be to install two zener diodes, each with a zener voltage somewhat
greater than the 14.4V that the motors run at, wired in series but arranged with opposite polarity across the motor

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terminals. Any voltage spikes that exceeded the zener voltage plus the bias voltage of the other zener diode (about
0.6V) would be clipped; otherwise, the pair of zener diodes would do nothing. Offhand, going to such efforts might not
seem worthwhile; if you think so, please see the section on window switch problems starting on page 602.

There are a few specific inductive loads in the XJ-S you don’t need to worry about. One is the injectors. Bywater:
“The old V12 D Jetronic amplifier has a 'snubber circuit' on each of the injector drives consisting of a 6.8 μF capacitor
and an 11 Ohm resistor in series. This has the effect of softening the induced voltage spike but still allows the reverse
voltage to build up to oppose current flow. It might be expected that this arrangement would promote oscillation but
that does not seem to be of much consequence.

“The 6CU and later V12 systems have 47 volt Zener diodes which clip the induced voltage spikes. This is a good
solution because it provides rapid build up of reverse voltage, protects the output transistor from an excessive voltage
spike, yet does not provoke oscillation.”

Another non-concern is the fuel pump relay. Bywater again: “Both 6CU and 16CU have a protection diode on the
pump terminal already. Failure is usually caused by either a dry joint or a component failure and has nothing to do with
the relay.”

If you have the earlier solenoid type door locks, you don’t have to worry about them either. They are never connected
to system voltage! They are operated by power from discharging capacitors, so their current drops off gradually as the
capacitor discharges.

If you have an early car with the solenoid-park wiper motor, there’s one place you do need to install a diode. See page
620.

One final note: Even with diodes, resistors, zener diodes or whatever in the circuit limiting the reverse voltage spikes, it
is still unwise to have a single set of contacts operating both an inductive load and an electronic device. Alan
Heartfield: “The insulation will handle it, not being polarity sensitive, but if it finds its way back to any collector-
emitter junctions, they will not handle any reverse voltage spikes. Neither will the input circuitry of many integrated
circuits. And the effect can be cumulative. A semiconductor will deteriorate due to overvoltage and/or reverse voltage
effects.” If you must have a switch or relay control both an electronic device and an inductive load, it is advisable to
select a double-pole control device and use a separate contact for each.

Relays

RELAYS: There are relays all over the XJ-S. On the ’83, most are a Bosch 12V 30A SPST relay number
0 332 014 113, and are a small metal box with four spade terminals labeled 30/51, 85, 86, and 87. 85 and 86 are the
coil connections, 30/51 is the common contact, and 87 is the Normally Open (NO) contact. The typical layout of these
terminals is shown in Figure 39 on page 677.

These relays apparently conform to a standard, and are readily available at any auto parts store. Often, the aftermarket
relays are labelled for use in controlling driving lights, and may be found among the driving light kits instead of under
general electrical components. They are usually entirely black plastic, and they often have an integral mounting lug.
And of course, an aftermarket electrical device is likely to be as good or better than a British original (although not all --
this author found a particular type relay made in Italy and sold at AutoZone that wasn’t worth a damn, three in a row
failed quickly).

If you are installing or relocating relays note that relays are not watertight, even when they appear to be. Bosch relays
have a distinctive little hole in the bottom. Their durability will be greatly enhanced if you will install them with the
spade terminals pointing downward, so that dripping water can’t get in and any moisture that does get in can drain out.

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Unfortunately, the XJ-S is covered with exceptions to the common relay description above. Following are descriptions
of several components that look the same and will fit in the standard socket, but are not the same and exchanging may
cause problems or even shorts.

FIFTH SPADE TERMINAL: Many relays have a fifth spade terminal centrally located in the base in the middle of the
other four; Figure 39 on page 677 shows a terminal 87a in this location. When replacing relays, it is of considerable
importance that the new relay has the same type terminal in this central location. Common designations for this fifth
terminal include 87, 87a, and 87b -- and these are not interchangeable. In most cases, a relay with a fifth spade terminal
can be used to replace a relay with only four, as the socket or plug will usually have a hole or slot for the unused spade
to protrude through, and the basic four are nearly always the same regardless what the fifth terminal is.

SECOND 87 TERMINAL: Some relays in the XJ-S (and some of the aftermarket generic equivalents) have a second
terminal 87 in the center of the base. This terminal is connected internally with the first 87; it merely serves as a second
connection to the same contact. Although internally the same, one should be careful about replacing a relay with two
87 terminals with a relay having only one; the socket may have a wire that connects to the central spade, and it will not
be connected if the terminal is not there. At this point, the solution is usually a simple matter of trading one relay with
another to get a relay having both terminals where it is needed. It would also work to simply splice the wire going to
the second 87 terminal into the wire going to the first 87 terminal.

SPDT RELAYS: The radiator fan relay, SRB411, has five spade terminals on the bottom, and the terminal in the
center is labeled 87a. This relay is bright red -- Lucas’ way of indicating “Hey, dummy, this relay is different!”. A
close inspection of the schematic on the housing shows that this is in fact a SPDT relay, and the 87a is a Normally
Closed (NC) contact.

A SPDT relay might not be quite as common as the typical SPST relay, but it’s close. Finding substitutes is not
difficult, although a generic driving light relay won’t serve here. If you don’t wish to buy the Lucas original, you can
look for a Bosch, Hella, or Potter & Brumfield. Per Bob Whiles, the part number for the Bosch is 0 332 204 105 and
for the Potter & Brumfield is VF4-45F11; this author suspects Bosch numbers 0 332 204 109, 0 332 204 125, and
0 332 204 159 would work as well. Per Volker Nadenau, the Hella part number is 4RD003 520-13. All of these will
plug right into the red socket.

DPST RELAYS: Some relays, including Bosch number 0 332 015 006 and 0 332 015 012, have an 87b terminal. This
connects to a second NO contact. Note that this is not the same thing as having two 87 terminals; while the relays with
two 87 terminals have both terminals connected to the same contact, this relay actually has two separate contacts.
Here’s the distinction: when the relay is energized, the same connections are made as in the relay with two 87’s, but
when unenergized, the 87 and 87b terminals are not connected to each other. In some instances, this may make no
difference, and perhaps a relay with an 87b terminal can be used to replace a relay with two 87’s, but be very careful
replacing a relay with an 87b with a relay with two 87’s -- something might backfeed through the 87 terminals on the
relay and cause malfunctions.

In addition to variations in the fifth spade terminal, there are other variations among relays -- and things that look like
relays.

EFI MAIN RELAY: On the Digital P EFI system diagrams, item #312 is the “main relay”. This relay is mounted in
the trunk near the ECU, right alongside of the fuel pump relay. Don’t mix up the two relays; the fuel pump relay is a
standard relay, but the main relay, Bosch #0 332 014 112, looks like a standard relay but it has a diode installed

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internally in series with the coil. This difference is clearly indicated on the little schematic embossed in the side. They
also paint a colored diagonal stripe across the top to indicate it is unusual.

The purpose of the diode in this relay is reportedly to protect the expensive EFI system from boneheads hooking up the
battery backwards. If the battery is connected backwards, this diode will prevent the relay from operating, so the ECU
will not get any power.

In this case making a special relay was totally unwarranted; a normal relay can be used by simply adding a suitable
diode in the wiring to the relay.

FEEDBACK MONITOR RELAY: On the Digital P EFI system diagram for the North American “Emission A” spec,
item #355 is the “feedback monitor relay”. In the Electrical Guide, it’s called a “feedback inhibit relay” on the EFI
diagrams and it’s called an “inhibit relay” on the starter circuit diagrams. Whatever it’s called, you’re going to be very
interested in this relay when you try to start the car and nothing happens; this relay must close for power to get to the
starter relay nearby.

The feedback monitor relay is mounted under the black plastic cover at the right rear corner of the engine compartment,
alongside the starter relay and the cold start relay. The feedback monitor relay is the one farthest forward on the
author’s ’83, although there’s no telling if that holds true for other model years.

The cold start relay is a standard relay, but the feedback monitor relay is anything but -- don’t mix them up. On the
feedback monitor relay, Bosch #0 332 014 411, the connections on the base are rearranged, with one of the coil
terminals (#85) and one of the contact terminals (#30) being interchanged compared to standard relays. There is a
diagonal paint stripe across the top to indicate it is unusual.

The feedback monitor relay also has a diode mounted internally in parallel with the coil to absorb voltage spikes and
protect delicate electronics (see page 557) -- long before it became popular to provide such protection in most relays.
In fact, that may explain why the terminals are rearranged -- to make sure nobody substituted a standard relay without a
diode. Of course, the feedback monitor relay is illustrated in most Jaguar schematics as a normal relay; no diode
shown. The schematic embossed in the side of the relay itself shows the diode clearly, and it is also represented in the
diagrams in the Electrical Guide.

Even before such spike protection became common, there was never any need for a special relay here; a normal relay
with a diode wired into the harness would have worked fine. Hence, if your feedback monitor relay is acting up (the
starter fails to respond sometimes), it is recommended that you do not seek out the oddball relay to replace it. Rather,
use a jeweller’s tiny screwdriver or something similar to pop the 30 and 85 terminals out the back of the socket,
exchange them, and pop them back in. Then plug in a common relay (there’s even an unused hole in the socket for a
fifth terminal), making sure to use a modern one with built-in spike protection -- usually a resistor rather than a diode,
but either will work fine. If you want to make sure to protect the EFI ECU from someone later installing a relay
without such protection, hardwire a diode into the harness. Of course, you might also want to stick a label inside the
plastic cover over these relays explaining to Jaguar mechanics that this application no longer requires the oddball relay.

ELECTRIC FAN DIODE PACK: The electric radiator fan diode pack is the blue block at the left edge of the engine
compartment that looks like a relay, but isn’t. It’s described on page 224.

As an engineer, I have to express an opinion here: the guy who decided it was a good idea to make several totally
different and non-interchangeable components all fit in the same socket should be dragged away and shot. There is
simply no excuse for this level of incompetence.

LATE MODEL RELAY PROBLEMS: Michael Neal warns: “I just wanted to advise the list of a known problem with
late model Jags, roughly ’93 and later. They have several Hella brand relays in various places for different
components. Underhood usage of them seems to be the worst problem. The XJ-S and XJ6 use them extensively on