• Hi Guest!
    If you appreciate British Car Forum and our 25 years of supporting British car enthusiasts with technical and anicdotal information, collected from our thousands of great members, please support us with a low-cost subscription. You can become a supporting member for less than the dues of most car clubs.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

MGB MGB Mk I O/D Question

Share this page

W

waveandwen

Senior Member
Country flag
Offline
In Classifieds I posted WTB O/D vaccum switch for a '67 O/D conversion project. No replies. I assume this a pretty hard to find part.
Question: How important is this item in the O/D system. They eliminated it on the MK II's. Can I install O/D grbx without it ? Will I be putting a rare MKI O/D grbx at risk of damage ?

Any one have one ?
Thanks in advance, Dave.
 
No risk of damage. The vacuum switch is a basic speed switch to prevent operation of the od in lower gears. The only damage that my occur is if you reverse with the overdrive switch in the "on" mode.
 
Sorry, I forgot to add that the later od units are fitted with a switch on the gearbox that prevents using the od in first and second gear. This switch operates like the reverse lite switch, and looks the same. /ubbthreads/images/graemlins/thirsty.gif
 
Grantura,
Thanks for the info. My understanding (usually incorrect) is that the gearbox switch kept O/D operation to only the top two gears and the O/D vacuum switch kept the gearbox in O/D after the dash switch was returned to "Normal" until engine rpm were lower enough (less mainifold vacuum) to allow the shift to normal ,lower, gearing without damage.

My concern is that without the vacuum switch the O/D could be turned off at the dash initiating a shift to the lower gear at any engine speed.

Reason for concern: this car is being prepared for my soon to be of driving age son. I probably (maybe not) could keep the use of the dash switch to within useable parameters. I am worried about an experienced driver throwing the switch when it should not be.

Thoughts ?
Dave.
 
While turning the switch off with throttle closed will jerk the car a bit, the OD's are pretty strong & I doubt if any bad thing will happen while he is "learning not to do that". Especially if you warn him not to.

Vacuum is more proportional to load than to engine speed.

Normally, the vacuum switch would prevent down shift under heavy coasting loads (high vacuum). As soon as a bit of throttle was applied, the down shift would occur. AH's did much the same thing with a throttle position switch.

Try the car yourself with no vacuum switch, then decide how much warning is needed.
Very nice BN4.
D
 
Dave, Thanks for the kind words re: the Healey. Yours too, I might add.
I was planning on doing just as you describe. Install it and see how it works without. Just wanted some validation that its not going to blow it up. Replacement of these is nearly impossible and rebuilding is a cost I'ld rather not think about.

The MGB is in an amazingly stock condition and as long as I'm changing to O/D I'ld to have all the components it would have come with. These vacuum switches seem harder to find than the early O/D gearbox. I'll keep looking, but do the retrofit anyway.

Thanks.
 
Dave,
I was just giving you a general idea...you seem to get it. You can always fit a lite to let your son know that the od is on. I have several cars that are fitted with od, I simply press the clutch pedal about half way before throwing the switch...I think that this makes it easier on the od clutch. I have had my B since I was 14 1/2 years of age, and have never had any od issues. Good luck! /ubbthreads/images/graemlins/driving.gif
 
Back
Top