MGB: Overdrive works great in 4th, but not at all in 3rd. Wuzzup?

I recently got a 1980 MGB and just took it for a test spin after redoing the brakes and fixing the non-working overdrive switch on the shifter. The overdrive now works great - kicks right in and kicks right out (my last overdrive took many seconds sometimes!).

But I found that the overdrive only works in 4th, not in 3rd. Doh! What's up with that?

Many people never use it in 3rd, and I'll bet a lot of people don't even know it's supposed to work in 3rd! I never use overdrive in 3rd, but still, I want it to work as it's supposed to.

Where should I look first? I assume it's an electrical problem. Does it make sense that an override switch would fail in such a way that it would work in 4th but not 3rd?

that's the way it was designed.

much earlier cars worked on 3 and 4, the later ones, and yours qualifies here, 4th only.

there may be a way to modify things, I don't know.

You only get o/d in 3rd in earlier models. Yours is correct.

Very late-model overdrives only work in 4th. I put a '79 unit in my '74 a few years back and it only worked in 4th.

I'm having a look through "Original MGB" to see when the change was made.

Here it is, pg 150:

"Rdst NA 428538 (Feb 77)
Optional extra overdrive now operates only on top gear; RHD cars continue with overdrive on third and top gears fitted as standard".

And.....as The Wiz and I discovered a few months ago....there were two different methods used to lock out 3rd gear. One easily defeated, the other not.

Some have a microswitch on the right front of the trans that the shaft closes only in fourth. That switch is wired in series with the inhibitor switch, so OD only works when both switches are closed. Taking the microswitch out of the circuit gives you OD in 3rd.

On others, the shaft has a different profile - and doesn't close the inhibitor switch in 3rd. Activating 3rd OD on those would require the replacement of that shaft.

I have no idea when the shaft change was made....

Doh! I shoulda known that! I coulda sworn that it worked in 3rd on a '78 MGB I had a few years ago.

Okay, thanks for the information and correction. Problem solved - there is no problem!

Thank you all very much!

Maybe your '78 had the switch mod done. I was told the trans I have came from a '79. If that's true (and I have no idea if that info is accurate), the shaft change must have come sometime in '79 or '80.

Simon Austin Wrote:

If your OD was set up to operate in 3 and 4 (up until reading the above posts, I thought the all worked in 3 and 4), it could be the lockout switch is out of adjustment. A quick was to check is to drive in 3rd w/ the OD "on" on push the sifter laterally away from you (i.e. towads the passenger seat). If the OD kicks in, thw OD switch needs to be adjusted.

You can get to the switch from the shifter access hole in the top of the tunnel, but you have to drop the back of the tranny a little to get enough room to get a wrench in there.

Good Luck

Paul

Search here using "All Dates" and you will find one or two message threads that describe a modification that will provide OD in 3rd as well. I believe you have to weld or braze an ear or high spot onto a lobe so that it engages the switch. I bought a black label OD gearbox to put in my 69 and discovered what had to be done. I sold it to someone else and found a blue label instead. Look for threads with "mgbjockey" and "Overdrive". The messages are from early 2007.

Oh! Found it! Link below. Make sure you find the message in the thread from Max71. It's most of the way down on the first of the two pages.

http://www.mgexperience.net/phorum/read.php?1,524274,524274#msg-524274

"Some have a microswitch on the right front of the trans that the shaft closes only in fourth. That switch is wired in series with the inhibitor switch, so OD only works when both switches are closed. Taking the microswitch out of the circuit gives you OD in 3rd."

Joe - You solved a mistery about the micro-switch. I told another guy about a microswitch on a 77 I had and he didn't know anything about it.

Thanks,
Bob

I'm pretty sure that microswitch does double duty - and also functions to activate the TSCA solenoid, which only allowed spark advance in 4th gear....