[E-Type] Overdrive control system

E-Type
1

I wanted to have an easy to use and failsafe control of the
OD in the newly installed gearbox in my 74 SIII.
Using the built in 4th gear switch and some custom parts I
now have a control system that will allow me to have the OD
available at a push of a button in 4th gear only and at the
same time give me the following features:
-Push the button again and the OD disengages.
-Change out of 4th gear and the OD disengages and will stay
disengaged until the car once again is in 4th gear and I
push the button.
-Have a small blue indicator light in case I forgot that
engaged the OD.
http://www.jag-lovers.org/v.htm?1420408820--
Ole-XKE1974
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2

In reply to a message from Ole-XKE1974 sent Mon 5 Jan 2015:

Very nice. The button is a toggle on-off I presume, where
you can’t tell visually without the light whether it’s
engaged or not (apart from 25% lower engine revs on the
tach)?

I’m not an electrical genius by any stretch, but what do
you need electronics for? Your list of features is
completely met by the standard Jag installations that
either used a two-position switch marked on/off and/or an
overdrive engaged light. This is a switches and sire
systrm rather than electronics, or what am I missing? As
you say, once you use the top gear switch you are
guaranteed to get your other features.

Marek may have some competition! Your’s looks strong and
simple although a fraction less ‘James Bond’.

As it happens, I was trying to remove a stock XJ6
overdrive knob this weekend and failing miserably. After
several days of soaking the special ring nut in
penetrating oil it still won’t budge. I will have to make
up some kind of special tool to engage the two slots. I
assume you used a hollow stick but did you just tap the
knob to screw down onto it?

Pete–
1E75339 66 D, 1R27190 70 FHC, 79 S2 XJ12L
Gaithersburg, Maryland, United States
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3

In reply to a message from PeterCrespin sent Mon 5 Jan 2015:

Pete,

indeed less ‘‘James Bond’’ than Marek’s solution.

The button is just providing a pulse to the Volvo OD relay
to operate it. A second pulse will disengage the relay.
Because the relay has the relay engaged as default, I needed
an other relay to invert that, which is also able to handle
the peak current drawn by the solenoid. The 4th gear switch
simply cuts the ground to the Volvo OD relay. I am using
the ground to switch it off to get the disengage function
when leaving 4th gear. Unfortunately there’s a short period
when the ground is connected through the 4th gear switch
where the relay is connected to ground very shortly before
engaging, which would cause the power relay to briefly be
switched on. To avoid that, I’ve included a delay relay to
filter that out. Hence ‘‘all’’ the electronics.
My gear stick is solid, but I expect the leather boot to go
far enough up to cover the black wire coming out of the
bottom of the gear knob.–
Ole-XKE1974
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4

In reply to a message from Ole-XKE1974 sent Mon 5 Jan 2015:

Dear Ole,

That peak current for the overdrive solenoid you refer to is
19amps. As the overdrive engages, it then drops down to 0.8amps to
keep it engaged. Failure to drop down will melt the wiring harness -
I have seen this happen once so do be aware - the risk is not
merely theoretical.

The standard o/d gear lever is hollow and has a broader thread than
the lever on the ordinary non o/d box.

For mine, I used a fused 22RA relay so that no high currents would
be seen by the gear lever which due its movement, will naturally
attempt to wear/fray the switch wiring over time. I also have a
changeover relay wired in so that power to the reversing lights is
mutually exclusive with that to the 22RA overdrive relay.

kind regards
Marek–
The original message included these comments:

Because the relay has the relay engaged as default, I needed
an other relay to invert that, which is also able to handle
the peak current drawn by the solenoid. The 4th gear switch

–
v12 E-type running MS3/3X sequential lpg and petrol
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5

This may be a real bad memory but IIRC the big Healeys with O/D I had
in the 60s had a switch on the firewall that was contacted by the
throttle linkage. I “think” what it did is if in O/D and you floored
it it would kill the ign. momentarily and remove the current to the
O/D relay allowing the O/D to disengage. Sort of like a passing gear
in an automatic. I remember thinking it was pretty clever but it was
a long time ago…
pauls

Pete,

indeed less ‘‘James Bond’’ than Marek’s solution.

The button is just providing a pulse to the Volvo OD relay
to operate it. A second pulse will disengage the relay.
Because the relay has the relay engaged as default, I needed
an other relay to invert that, which is also able to handle
the peak current drawn by the solenoid. The 4th gear switch
simply cuts the ground to the Volvo OD relay. I am using
the ground to switch it off to get the disengage function
when leaving 4th gear. Unfortunately there’s a short period
when the ground is connected through the 4th gear switch
where the relay is connected to ground very shortly before
engaging, which would cause the power relay to briefly be
switched on. To avoid that, I’ve included a delay relay to
filter that out. Hence ‘‘all’’ the electronics.
My gear stick is solid, but I expect the leather boot to go
far enough up to cover the black wire coming out of the
bottom of the gear knob.
<<<<<<<<<<<From: “Ole-XKE1974” o.wurtz@hotmail.com
Subject: Re: [E-Type] Overdrive control system

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6

In reply to a message from Ole-XKE1974 sent Mon 5 Jan 2015:

OK, got it, thanks.

Jaguar used no relays for the overdrive and the standard
wiring is no heavier gauge than, say, sidelight/rear light
wiring. I just used the top gear switch to limit
engagement to that gear and force overdrive drop-out once
I went to any other.

With a push-button I can see the logic of a lamp I guess,
although since it’s only ever possible to be engaged in
top, and the rev/speed difference is pretty clear in that
gear, I never felt the need.

My problem on my first D-type replica was having wired to
be usable in any gear, which was all very well until you
forgot it was in use and reversed the car. I did that
three times, very briefly, but the overdrive survived
(which was a pleasant surprise and NOT recommended!).

I look forward to seeing you car some time. As you say,
the wire should be totally unobtrusive with a decent
length stick and matched gaiter.

Pete–
The original message included these comments:

The button is just providing a pulse to the Volvo OD relay
to operate it. A second pulse will disengage the relay.
Because the relay has the relay engaged as default, I needed
an other relay to invert that, which is also able to handle
the peak current drawn by the solenoid. The 4th gear switch
simply cuts the ground to the Volvo OD relay. I am using
the ground to switch it off to get the disengage function
when leaving 4th gear. Unfortunately there’s a short period
when the ground is connected through the 4th gear switch
where the relay is connected to ground very shortly before
engaging, which would cause the power relay to briefly be

–
1E75339 66 D, 1R27190 70 FHC, 79 S2 XJ12L
Gaithersburg, Maryland, United States
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7

In reply to a message from MarekH sent Mon 5 Jan 2015:

A simple precaution to avoid the melt down is the addition
of a fuse to the solenoid power feed – I used an 8 amp
Slow-Blow.

The brief surge of 20 amps does not bother it but if the
pull-in coil every sticks on the fuse will blow in time to
save the harness.–
The original message included these comments:

That peak current for the overdrive solenoid you refer to is
19amps. As the overdrive engages, it then drops down to 0.8amps to
keep it engaged. Failure to drop down will melt the wiring harness -
I have seen this happen once so do be aware - the risk is not
merely theoretical.

–
Geo Hahn 1969 OTS 4.2
Mt Lemmon, Arizona, United States
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8

In reply to a message from Geo H sent Mon 5 Jan 2015:

Marek, Pete and Geo,
it may not be clear, but there’s an in-line fuse in the
feed to the box. I plan to use a 20 or 25 Amp fuse and the
relay can handle that. I also like the idea of the lower
rating slow blow fuse, to be sure that the harness is
protected.
I did think for a while to have a kick down function as
suggested, but decided to do without for now, since I
prefer to ease off of the accelerator when operating the
OD. A nerdy way of saving the OD internals that I used on
my 1965 Volvo P1800.
The whole reason for this contraption is to ensure that the
OD does not engage when shifting into 4th if I forget to
switch off the OD and as Marek also mention, avoid high
current through the switch. And I think the gear knob looks
the part. :-)–
Ole-XKE1974
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9

In reply to a message from Geo H sent Mon 5 Jan 2015:

That’s exactly how Jag wire the Compact Type A - an in-
line fuse.–
1E75339 66 D, 1R27190 70 FHC, 79 S2 XJ12L
Gaithersburg, Maryland, United States
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10

In reply to a message from Ole-XKE1974 sent Mon 5 Jan 2015:

Yes, I think a kickdown effect might suit a weedy Healey
but not a V12 and easing of makes perfect sense. I forgot
your button might not be rated as high as the chunky brass
contacts of the old-school switches, hence the relays.

Pete–
1E75339 66 D, 1R27190 70 FHC, 79 S2 XJ12L
Gaithersburg, Maryland, United States
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