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MG MGB Technical - Overdrive roulette

Here is the world record for poorly worded and vague question:
My new overdrive tranny is installed in the '77 MGB, replacing a 4 speed. I remember reading an archive posting about overdrive units that said not to actually drive the car unless I was sure the ??? was installed properly. If I do drive it with this incorrect instalation it will damage/distroy the OD unit. Does anyone know what that caution was about? Should I just roll the dice and see what happens? Any comments appreciated.
Bob Duke
RW Duke

Bob,

The most critical thing is not to have the overdrive operate in reverse. If you do it will break the one way clutch inside the overdrive and will require a rebuild.
How do you test?
You cannot test by listening to the solenoid because, unlike the J Type, with the MGB overdrive once it makes the first click it will not return to its original position until it is operated with the car running.
Use a portable voltmeter (Multimeter) to check when 12V is applied to the solenoid.
With the ignition on, put into each gear in turn and operate the overdrive switch.
There should be 12V at the solenoid while the lever is in 3rd and top. No voltage at the solenoid when in 1st and 2nd, but MOST IMPORTANTLY no voltage when the lever is in reverse.

Mick
M F Anderson

Bob. Mick is mostly correct. The part you are referring to is the overdrive isolation switch. This is what must be functioning if you are to get power to the solenoid as Mick describes. However, on the US cars, perhaps all?, the later model cars, including yours had a "Transmission Controlled Spark Advance" or TCSA system. This means that on both non-OD cars and OD cars the overdrive isolation switch was used to provide electric current which opened a valve allowing the vacuum advance to function in 4th gear only. This means the system has been modified so that, on the later trannies, the OD works only on 4th gear instead of the 3rd and 4th as on earlier models.

There are two reasons this switch will not work. First, bad switch. Second, improper adjustment. This is something that is most easily tested with the transmission on the bench. The best thing to do now is to engage 4th gear, turn on the ignition and put the car into overdrive and see if you have voltage to the overdrive solenoid as Mick suggests. If not, you need to check for power input to the switch. If you have power input to the switch, but no power to the solenoid, it is either a bad connection or a bad switch. It is possible, barely, to lower the tranny for easier access to the switch. Les
Les Bengtson

For a complete test (particularly on the bench) check current is flowing in the circuit, to prove continuity through the solenoid as well as everything else. On my two cars the solenoid is about 180 ohms resistance giving about 70mA at battery voltage, or 80mA at running voltage. This can be done quite easily on dash and column mounted switches by connecting the ammeter across the manual switch connections, which must be in the off position, of course.
Paul Hunt

With the ignition on and in top gear.
M F Anderson

In all gears, to make sure it only flows in overdrive gears.
Paul Hunt

This thread was discussed between 30/07/2003 and 01/08/2003

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