M46 proper engage/disengage of OD!

[Boots]

Glad you got your OD problem sorted out...

For reference, there is a complete wiring diagram for the overdrive circuit, including the relay internals and pinout here: http://forums.turbobricks.com/showthread.php?t=202693.

I'm certainly going to save the drawings at the end of the post for any future diagnosing I may have to do...

Boots.

[RedArrow]

Wow, that`s really a great source! I saved them for later! Now I just have to learn how to use them Isn`t it easier to understand Egyptian hieroglyphs in Gizah (at night)? That last wiring diagram drawing scares me more than a triple flat tire at 90mph.

[Boots]

It's just what you get used to I suppose. (I'm an electronics engineer by trade so I have to read schematics like these all the time!) The last diagram is actually of the circuit internal to the relay itself and is a very belt and braces design to ensure that the OD doesn't drop in or out due to electrical noise from the car's other equipment. In a 'cleaner' electrical environment, you could do the same job with about 5 components rather than the 20 odd shown there.

What's nice about the earlier diagrams is that they show ALL of the connections, their locations on the vehicle and which legs of the circuit carry current under which conditions - very handy...

Summary of the relay operation is as follows - for anyone needing to test one:

Pin 15 - +12v from ignition switch via fuse.

Pin 31 - Permanent circuit ground. (Chassis).

Pin 85 - 4th gear switch connection (enable). Relay will only engage if this pin is connected to chassis(grounded). (If relay is engaged and this pin is disconnected from chassis, relay will disengage.)

Pin 86 - Trigger input. Applying 12v to this pin momentarily (normally done using the pushbutton switch on the gear lever) will engage or disengage the relay sequentially for each application of the 12v signal. (If pin 85 is grounded.)

Pin 87 - Output of relay. Supplies 12v when relay is engaged. In normal use, supplies the current to light the bulb in the '5' light on dash and to engage the solenoid on the OD.

Interestingly, all of this is only needed to allow the use of a push-button rather than an on-off type switch to select or de-select overdrive.

If a slide switch or similar were used, none of the fancy circuitry, or even a relay would be necessary! It would just supply current to the lamp and OD solenoid in the 'on' position and not when 'off'.

Boots.