I have recently aquired a 1959 FHC with 3.8 Engine and electrically operated overdrive. The control circuit includes a throttle switch installed between the carbs and is a simple plunger operated micro-switch.
I understand the intent of the throttle switch but on my car the switch is of the normally closed variety - that is when the throttle opens the switch makes contact and the circuit is closed. Conversely, when the throttle is closed or at idle then the circuit is open. In this case I cannot understand how an open circuit will keep the solenoid engaged if shifted from overdrive to top gear with the throttle at idle. I believe that the micro-switch original but is it possible that a previous owner replaced the original with the wrong switch?? (normally closed as opposed to normally open).

Further, I am not able to follow the Jaguar wiring diagram on the overdrive relay as I cannot fathom how the engergizing coil between W1 and W2 can go to ground when the circuit is closed by the overdrive control switch.

Presently through a previous owner, the throttle switch has been taken out of the control circuit by controlling the solenoid through a simple C1/C2 relay operated by a W1/W2 energizing coil which is taken to ground as per Fig. 6:26, pge. 84 of the Autopress workshop manual.

The overdrive can be made to work without the throttle switch but I would like to get it back to original as per Jaguar. I understand that I would likely need a different relay.

Any advice would be much appreciated.

Tom Doyle
JNCA NW42

Submitted by NW42-44901J on Wed, 10/31/2007 - 00:26

Bill Brady, I received page 1 of your fax being the sketch of the circuit and the schematic of the relay. It is what I have surmised. I will study it but right off the top of my head, there is no way the relay coil can close the C1/C4 contacts as it does not go to ground when the secondary circuit is closed by the OD control switch. W2 must be taken to ground by some route in order for the relay to operate.

I note that you have tried several times to send the fax of 3 pages but on two attempts only 1 page has come through as noted above.

Thanks very much for your help.

Tom Doyle

Submitted by NW42-44901J on Tue, 10/30/2007 - 12:31

William and George and anyone else who is following this thread, I have concluded that my earlier hypothesis just won't hold water. My interpretation of how the OD Relay should work did not consider the case where the car is in 4th. gear and the OD Switch is in the out postion. this creates a circuit from the Switch though the slate/red wire to the Throttle switch and then through the slate/green wire to the W2 Terminal of the relay and then thru the slate jumper wire to the C4/C1 terminal and thus the OD Solenoid is energized whenever the throttle is eased (the throttle switch is closed). This is not what Jaguar intended.

The key to this is getting the correct relay and understanding how it functions. Back to the thinking position. In the meantime I can use the OD by by-passing the throttle switch and making sure I blip the throttle when shifting down - which is what we do anyway.

Tom Doyle

Submitted by NW42-44901J on Mon, 10/29/2007 - 02:21

William and George - Thanks again for your interest and help. I will try to respond to both comments. The Jaguar wiring diagram W549 456 00 does help in that it it clearly shows that the relay is grounded. What I really need is a wiring schematic for the relay itself to confirm the following hypothesis : When the OD control switch on the dash is switched "IN" and the 4th gear is engaged, current is applied to the relay thru the slate/black wire to terminal W1 and thru the coil to ground thus closing the C2/C1 circuit and the solenoid is engaged. Simutaneously, the C4 terminal allows current to flow thru the slate jumper wire to terminal W2 and also thru the coil to ground. Then, when the OD control switch (I assume it is a make and break) is switched to "OPEN", current continues to flow thru the slate/red wire to terminal W2, and the slate jumper wire to terminal C4/C1 and the solenoid remains engergized as long as the throttle switch is in the closed position. That is to say the rotating throttle arm is in the throttle off position and presses up against the plunger on the throttle micro switch which is located on a bracket between the carbs. This means that the micro switch used for the throttle switch is a normally open switch ( when the plunger is not depressed the circuit is open). When the throttle is advanced, the throttle switch opens and the OD is then disengaged. This meets the stated Jaguar objective of picking up engine rpms before the shift from OD to 4th. gear is completed.

If my hypothesis is correct, then it is important to buy the correct relay -- one that is grounding the W1 and W2 circuits and the throttle switch must be a normally open micro switch. Presently on my car the relay is not correct nor is the throttle switch as it is a normally closed micro switch when the plunger is not depressed.

I have all of the wiring diagrams for the car including those published by Jaguar and the booklets published by XKs Unlimited. As mentioned I don't have a wiring schematic of the correct relay for the application.

Again thanks very much for your assistance - please don't hesitate to challenge my hypothesis if you think that I've got it wrong. If you have a sketch of the relay schematic my fax number is 604 896-0155.

Tom Doyle,
Furry Creek,
British Columbia,
Canada

Submitted by bonnettoboot@e… on Sun, 10/28/2007 - 22:13

Hello, what did you find out. I initially thought you might be getting confused with the hill hold. Is your KD switch on the gear lever or the dash? If you need them I could fax you diagrams of both systems. Jaguar affectionado and etc.

Submitted by SE98-32482CJ on Sat, 10/27/2007 - 09:36

Tom you are correct in that Mr. Brady has confused the transmission. You are also correct that Jaguar wanted the driver to have some throttle on prior to engagment of the overdrive. I looked at your drawings on the other site and thay seem to be accurate but suggest you look at the factory wiring diagram (no. W549 456 00) which is a little more friendly and has wiring codes. If you have the correct relay and the switch and bracket (tough to find sometimes) you should be in good shape. The switches are problematic and a few cars I have done I eleminated the switch function by simply connecting the wires on the lower firewall where they bundle. As far as the throttle switch is concerned it will prevent engagment if the throttle is closed but will not disengage the od once it is engaged. Hope this helps but ask again if I have not captured the essence of your problem. Lastly do not let anyone sell you a switch unless they understand the system. I have removed more than one switch that worked on a reverse system--hence never od unless there was no throttle. Good luck. George Camp

Submitted by NW42-44901J on Sat, 10/27/2007 - 00:48

William, Thank you for your response. From your comments I think that you may be referring to the Anti-Creep feature of the automatic transmission. I should have been more specific in that I have a manual transmission with electric operated overdrive. My understanding is that in this case the throttle switch is intended to prevent disengagement of the overdrive while the throttle is at idle - I presume Jaguar intended that the driver should increase engine rpms. when shifting to 4th. gear from overdrive and not backing off on the throttle to touch the brake and at the sametime disengaging the overdrive switch.

Best Regards,

Tom

Submitted by bonnettoboot@e… on Fri, 10/26/2007 - 21:58

The throttle pin switch and the solenoid you describe are part of the hill hold feature on some cars. I operates by feeding the solenoid (this is fitted in a hydraulic brake pipe) when the Acc pedal is released, this prevents the brake fluid from returning to the M/cyl thereby holding the brakes on. As soon as you operate the acc pedal it opens the solenoid and releases the brakes. The kickdown solenoid is attached to the overdrive unit and is connected directly to the switch on the dash. Jaguar affectionado and etc.