• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Overdrive Delay??

Share this page

A

AlamedaMGB

Senior Member
Offline
Can anyone tell me about the electric overdrive unit? Sometimes when I shift into 4th gear, the overdrive doesn't engage. I hate to see my B rev at highway speeds as I commute to work.. I'm sure it's not an electical issue (I've isolated the gearshift mounted switch, and tried crossing the wires ) with the switch..

What else should I look at? Sometime if I shift back to neutral or 3rd, and then back to 4th, it engages.. How does the OD know to engage in 4th gear only? Is there a switch that I can check?

HELP?? I'm afraid to commute in my B until I resolve this..
 
Have you checked the oil level in the OD unit lately? Most likely it is low. Also, don't worry about revving the engine high on the highway. You can drive all day at 4500 RPM without worry. The B-Series engine is very rugged and won't be hurt by it.
 
[ QUOTE ]
and then back to 4th, it engages.. How does the OD know to engage in 4th gear only? Is there a switch that I can check?

[/ QUOTE ]
The most likely problem is the OD isolation switch. This is a small switch mounted on the transmission connected to the shifter mechanism that lets OD engage in certain gears only. I think that on the 68 to 76 models, the switch permits OD only in third or fourth gear. Somewhere around 77, this was changed to permit OD in fourth gear only.

I would check this switch first to see if it reliably makes contact. There are shim washers under the switch mount to provide for switch engagement adjustment.

The problem may be elsewhere, but this is a start. Since I'm not sure which of the several wiring variations you may have, you can find the schematic diagrams here:
https://www.mgb-stuff.org.uk/s_od.htm
D
 
Back
Top