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MG MGB Technical - overdrive turns on then off

I have had this problem with my 1970 MGB for about 3 summers. I just havent had the time to fix it. Last year I changed the fluid in the transmission to make sure it wasnt a low fluid level.

The overdrive turns on and stays on for maybe a few seconds then it turns off. Sometimes it will go off and on.

Where do I start to diagnose this? I am thinking electrical issues?

Thanks
Matt
matt kaser

Start by connecting a test light or voltmeter to the lead that goes into the solenoid on the overdrive. With the overdrive switch and ignition key on run the gear shift thru all positions. The lamp should light when the trans is shifted into 3rd or 4th gear. If it doesn't remain on work your way back in the circut. Likely cause would be the trans lockout switch or the overdrive switch. Also check all the bullit connections in the circuit, there is at least one under the dash and one at the wire harness junction at the right corner of the firewall shelf under the bonnet.
John H

Matt. John is correct. The overdrive inhibit switch is located at the top, left side, of the transmission and it has some fiber washers used as shims, between the switch and the transmission. One shim too many, or the switch not being screwed into the transmission firmly, could cause the problem you describe.

This switch is difficult to get to when the transmission is in the vehicle. It can be done, however, if one lowers the transmission cross member. Being able to put the car on an elevated lift helps, significantly, when doing this. While you are at it, see if the cross member has been modified to allow the transmission to be easily attached to the cross member. Ollie Stephenson, who used to post here, had a website showing the modification. I have made this modification on two cars and will make it on all of my vehicles and I add the OD trannies to them. Makes the assembly an order of magnitude easier.

Les
Les Bengtson

Thanks guys for the response.

Is the lead for the solenoid the single wire coming out from the bottom of the unit or is it the wires that are located up top by the shifter?

What exactly does the inhibit switch do?
The light should light up and then go off? Or not light up at all?

Thanks
Matt
Matt Kaser

Matt. Take a look at a wiring diagram for your car. The power comes from the white wire circuit to the overdrive switch, then to the overdrive inhibitor switch, through the switch and down to the solenoid. A test light attached to the wire going from the inhibitor switch to the solenoid will tell you if the solenoid is getting power to it--i.e. the switches, wires and connectors are allowing the power to flow consistently. If you have good power, all the time, any problem would lie in the solenoid or the hydraulic system inside the overdrive. Any inconsistency in the flow of power would cause the overdrive to kick in and out of operation as the power flow is interrupted and resumed.

The inhibitor switch, on the gear box, is a "press off" switch which, when the transmission is not shifted into third and fourth gears, interrupts the flow of power to the overdrive, taking the transmission out of overdrive. (Side filler transmissions only have over drive on fourth gear. Earlier models on third and fourth.) Thus, if the overdrive switch is left in the "ON" position, there should be no power flowing in reverse, first, or second gears. Basically a method of preventing the over drive being left turned on and the transmission being shifted into reverse where damage to the unit could take place.

As to the light, it should only illuminate when there is power flowing--over drive switch in the "ON" position and transmission in third or fourth gear. If, with the over drive switch on, you have power (light illuminated) in first and second gear, the over drive inhibitor switch is incorrectly adjusted and needs to have additional shims installed so there is only power in third and fourth gears.

Les
Les Bengtson

It's easier, and more accurate, to put a meter in series with the circuit either at the manual switch or where the relevant wire in the main harness joins the relevant wire in the gearbox harness (yellow and yellow/red respectively for dash and column switches, white for gear-lever switches). If you use it on the 12v scale the OD won't operate, but when the meter shows 12v you know you have continuity right through the circuit including the solenoid winding, which using a voltmeter at the solenoid doesn't check. If you use it on the 1 amp scale the OD should operate normally, and indicate 800mA for an LH-type OD. D-type solenoids draw 17 amps to pull in and 2 amps to hold in, so dependiong on what type of meter you have it might be better to use volts for this type.

I think both reverse light and OD lockout switches (they are the same part) are press=on switches i.e. they are normally open and have to be operated to close. It is the brake light switch on the pedal box that is the press=off type.
Paul Hunt2

This thread was discussed between 05/05/2008 and 08/05/2008

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