Overdrive Throttle Switch

[Rob Glasgow]

20 years ago I had issues with the Overdrive Throttle switch so rather than replacing it, I simply bypassed the switch by connecting the two wires to the same terminal on the switch. I drove it for years and switched into OD by flipping the dash switch and out of OD by depressing the clutch when flipping off the dash switch. Everything worked perfectly. I just finished a 3 year cosmetic restoration of my BT7. I did no work to the transmission or OD. I did install a new wiring harness and a multi-fuse panel to replace the original two fuse unit. I now have a dedicated fuse for the OD.
Since I didn't replace the throttle switch, I bypassed the unit in the same manner I had been using for 20 years i.e. connecting the white/purple and white/green wires to one terminal.
During the first road test, as soon as I flipped the dash switch into OD the fuse blew, not only taking out the OD but killing the engine. I installed another fuse and the same thing happened. I began to test all the OD wiring, switches and solenoid for shorts and continuity. Everything checked out fine. I finally decided to separate the two wires from the throttle switch and covered the ends of both with electrical tape. Problem solved. Switches into and out of OD using the dash switch as before.
I have studied the schematic of the OD system, and while I understand the principal behind using the throttle switch to prevent the OD unit from disengaging unless some throttle is applied, I can't say I fully understand how all that works.
Does anyone have a suggestion as to what is going on? I don't care if I use the throttle switch but I would like to understand why the OD used to work with the wires joined together, and now that situation causes a blown fuse.

 

[GregW]

Not sure how a "dedicated" overdrive fuse blowing would stop the motor. The overdrive engaging draws a lot of amps initially. You may have chosen a fuse that is either to low in value or you need to use a slow-blow fuse.

 

[Rob Glasgow]

I cant understand why it would kill the motor either, but it did both times I tried the switch with the wires tied together. I was using a 20 amp fuse for the OD circuit as recommended by Charlie Hart, the fellow who made the new fuse panel. Ill try a slow blow type and see if that makes a difference.

 

[Keoke]

Not sure how a "dedicated" overdrive fuse blowing would stop the motor. The overdrive engaging draws a lot of amps initially. You may have chosen a fuse that is either to low in value or you need to use a slow-blow fuse.

I agree
The solenoid draws about 19 amps [ Power Wise it is Jules ] for a very short period of time when initially engaging.
Further it is an inductive load that also creates a large transient, so it requires a 10 ampere "Slow Blow" Fuse for safe protection.

 

[BJ8Healeys]

Rob, the operating coil of the overdrive solenoid has about 0.5 ohms resistance. From Ohm's law (Current = Voltage/Resistance), with a 12 volt system and resistance of 0.5 ohms, the current drawn when the operating coil does its thing is 24 amps.
Once the operating coil attracts the solenoid plunger, a set of contacts are opened that switch to a "holding coil" that is about 12 ohms. From Ohm's law, that reduces the continuous current to the solenoid to 1 amp. So, it is the initial operation of the solenoid that causes the fuse to blow.
Originally, the overdrive circuit had no fuse to blow. Where in the circuit did you locate your fuse?
Unless your O/D fuse is located between the battery and the ignition switch (duh!), I don't see any reason for the overdrive fuse to stop the engine. I spliced in a 20A fuse between Terminal C2 of the overdrive relay and the white/green wires. The short-term operation of the operating coil has not yet caused the fuse to blow.

 

[TimK]

Just to add to this conversation, my random, rare experience when adding a fuse had a real benefit. I had added an in-line 10 amp fuse to the OD circuit. Everything worked fine until I had to replace the solenoid. As a shortcut, I only installed the new plunger and coil, not the upper part of the assembly which contains the cutoff switch to reduce the current to the 1 amp holding level. When I went to operate it for the first time, it blew the fuse. I somehow figured out that not using the whole new assembly had caused a mismatch between the old part and the new part -- the new part did not reach far enough up to switch from the pull circuit to the hold circuit, thus causing a continuous high amp draw. Without the fuse, I would have had a wiring disaster. The blown fuse made me reflect on the problem and solve it with no damage other than to the (cheap) fuse.

 

[CraigC]

Since I didn't replace the throttle switch, I bypassed the unit in the same manner I had been using for 20 years i.e. connecting the white/purple and white/green wires to one terminal.
During the first road test, as soon as I flipped the dash switch into OD the fuse blew, not only taking out the OD but killing the engine. I installed another fuse and the same thing happened. I began to test all the OD wiring, switches and solenoid for shorts and continuity. Everything checked out fine. I finally decided to separate the two wires from the throttle switch and covered the ends of both with electrical tape. Problem solved. Switches into and out of OD using the dash switch as before.

The fact that the problem of the fuse blowing went away when you disconnected the throttle switch wires is an indication that the problem is not with your fuse choice.
I highly suspect that you have made a connection error at the overdrive relay. One particular mistake will cause a dead short every time you switch on the overdrive. Look at the two terminals W1 and W2. Per the diagram I have, on later cars, the two WP wires go to W2 and a single black wire to W1. That black wire is a ground. There should be nothing else on that terminal!

I have studied the schematic of the OD system, and while I understand the principal behind using the throttle switch to prevent the OD unit from disengaging unless some throttle is applied, I can't say I fully understand how all that works.
Does anyone have a suggestion as to what is going on? I don't care if I use the throttle switch but I would like to understand why the OD used to work with the wires joined together, and now that situation causes a blown fuse.

The throttle switch and it's two connecting wires form what is called a "latching circuit" for the overdrive relay.

When you flip the OD switch "on", power flows to and through the operating coil(terminals W1 and W2) to ground. This energizes the coil causing the relay points to close, allowing current to flow to the gearbox switch and on to the OD solenoid. Since one leg of the throttle switch circuit(white/green wire) is connected to the same terminal(C2) as the solenoid circuit, current is also able to flow to the throttle switch. If throttle switch is closed(normal operation) current flows out the other leg(white/purple) back to the switch side of the relay operating coil. By doing this there are now 2 sources of power for the operating coil, the OD switch and the latching circuit(throttle switch). When the OD switch is turned "off", the latching circuit remains live, keeping overdrive engaged. Blip the throttle past the point where the throttle switch contacts "open" and the latching circuit goes dead. This de-energizes the relay operating coil causing the relay contacts to open, cutting off power to the OD solenoid AND the latching circuit, allowing OD to disengage. Simple, no?

One question regarding the previous wiring of your OD. When you joined the throttle switch wires together, that alone would have caused overdrive to disengage ONLY when the ignition was switch off. Are you sure you didn't also move one of the wires at the relay? I know I did the same throttle switch bypass decades ago and I also found it necessary to move the White/Green wire from C2 to C1. Combined, these two steps eliminated the latching circuit function, although the C2 to C1 move would have been sufficient to accomplish the same.

 

[Keoke]

Steve said:
Once the operating coil attracts the solenoid plunger, a set of contacts are opened that switch to a "holding coil" that is about 12 ohms. From Ohm's law, that reduces the continuous current to the solenoid to 1 amp.

One note of caution:
With a steady state current of only 1 amp protected by a standard fuse of 20 amps, that circuit can be damaged when faulted by the time interval required for a 20 amp fuse to blow..

 

[RAC68]

Hi All,

I must say that I thoroughly enjoyed this discussion, although it has made me quite insecure to realize how much of my past education has been left in the shadows of my mind.

A while back I installed a number of in-line fuses (easily hidden) for greater critical circuit protection and, in particular, installed a 10 Amp fuse on my OD. This fuse was placed on the white wire extending from the #3 terminal on the fuse block feeding the C1 terminal on the OD relay, that I understood, provides root power to the OD circuit. After reading the discussion, I am wondering if this fuse placement is providing the level of protection I had originally expected and, although all seems well and working properly, the 10 amp fuse has never been blown as a result of a high initial solenoid load.

1. Is this 10 amp fuse doing its job protecting my OD Circuit, including the solenoid?
2. If so, why is the 10 amp fuse not failing from a high initial solenoid actuation load?

Happy Holidays to all,
Ray(64BJ8P1)

 

[Keoke]

2. If so, why is the 10 amp fuse not failing from a high initial solenoid actuation load?

Pure Luck RAY Unless it happens to inadvertently be a slow blow type :congratulatory:

OR
IT was not connected in series with terminal C2 of the OD Relay