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OD question

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Sorry Paul,
But damage to the overdrive caused by reversing, is not "Bunk". I only recommended to Janel that she checked before reversing "if the switch was stuck in the closed position". It does happen, have had experience of this twice in my career. Both were caused by double faults,
1: the solonoid drawing very high current.
2: the driver having the habit of shifting while the o/d was engaged.
First one was a Triumph 2000, solonoid was faulty, inhibitor switch contacts welded themselves closed, o/d rebuild under warrenty due to driver doing his upmost to reverse.
Second one, Healy 3000, solonoid was out of adjustment (early type Laycock o/d) solonoid pull in coil was not diss-connecting, same thing. Inhibitor switck contact welded together, o/d re-build at customers cost.
Both times the sprag clutch had been jammed so hard, it suffered damage.
Cheers Ric
 
Janel, I think i may have an extra speedo hanging around the barn. Do you want to borrow one if I do?
email me or PM me across both boards as I am cruising in here and there and will try to hit this more as now Steve S is here and I miss him. Cant remember if I will have email access or not in Stowe. I am heading there tomorrow AM.
 
My, what a lot of fuss over a routine issue.

The OD on the MG works just fine, thank you. The isolater switches do not often fail and if they do, the OD doesn't switch in. Easy to test one as you can put a light on the relay it goes to under the dash and move the gear lever in and out of gear.

More often a wiring issue than anything else, but easy to change if a switch does fail - remove the shifter cover on the tunnel and get access that way.

As for idiot lights to tell you if the OD is on or off...well, they are aptly named.

The OD is usually only useful on top gear although you can certainly get into a situation where it is easier to shift from 3rd to 3rd OD and back. I often run in 3rd OD on the MGC in traffic on the highway as it is easy to flick down to 3rd to pass when the opportunity presents, but that isn't an option on the much lower (higher numerical) geared MGB

The OD is very useful in racing - use them on both the TVR and MG Twincam, in conjunction with a high ratio (4.55 or 4.88) diff.
 
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Easy to test one as you can put a light on the relay it goes to under the dash and move the gear lever in and out of gear.


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I'm surprised it took 5 pages until this was suggested!

A $4 DVOM from Harbor Freight would work fine too. Connect one probe to a lug on the OD dash switch, the other into the lug previously feeding aforementioned switch. Put car in 1st/2nd/reverse and switch the OD on with dash switch. If you show 12V on the DVOM, the gearbox bit is toast.

It is possible to replace it in situ, if you have small, strong fingers, but is a major hassle.

As I recall, there are fiber washers that adjust the nose of the OD switch on the gearbox tower; make sure you reinstall them and test the continuity of the switch.
 
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Easy to test one as you can put a light on the relay it goes to under the dash and move the gear lever in and out of gear.

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Just for the heck of it, & because Janel doesn't know which relay, could you point us to the relay under the dash that controls the OD?
 
Hmmm, Ahhhh, Ohhhhh, sigh.
 
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Easy to test one as you can put a light on the relay it goes to under the dash and move the gear lever in and out of gear.

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Just for the heck of it, & because Janel doesn't know which relay, could you point us to the relay under the dash that controls the OD?

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Tony - we both know there is no relay under the dash; only a switch on the dash. Perhaps the original poster meant the switch that feeds the OD solenoid (which is half a relay). ??

/ubbthreads/images/graemlins/laugh.gif
 
/ubbthreads/images/graemlins/lol.gif /ubbthreads/images/graemlins/wink.gif I know....
 
Oh, the half relay, that's different. Straight ahead, take the second left.
 
& before you know it you're in Never-never-land!
 
You mean the Idiots didn't use a relay to energize the overdrive!?. I sure wouldn't want a 19 AMP transient going through a switch on the dash panel /ubbthreads/images/graemlins/rolleyes.gif. Add The relay, Add the Relay.----Keoke /ubbthreads/images/graemlins/yesnod.gif--- /ubbthreads/images/graemlins/grin.gif
 
Hay, these were built on the cheep remember.

But is good advise.
 
[ QUOTE ]
You mean the Idiots didn't use a relay to energize the overdrive!?. I sure wouldn't want a 19 AMP transient going through a switch on the dash panel /ubbthreads/images/graemlins/rolleyes.gif. Add The relay, Add the Relay.----Keoke /ubbthreads/images/graemlins/yesnod.gif--- /ubbthreads/images/graemlins/grin.gif

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That's why there are these problems. Most of the switches used by the Brits are inadequate for the amount of current they have to handle. That's why Dan Masters is selling his improved wiring harnesses. I'll be buying one for my TR3 resto. There's just too many circuits that need relays. /ubbthreads/images/graemlins/hammer.gif

/ubbthreads/images/graemlins/cheers.gif
 
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Hay, these were built on the cheep remember.

But is good advise.

[/ QUOTE ]

--------Relays is cheep too!.---Keoke--- /ubbthreads/images/graemlins/laugh.gif-- /ubbthreads/images/graemlins/laugh.gif
 
But I mean cheep, the word is they were concerned over every shilling, remember they were only 500 or so pounds when they came out.
 
I kind of hate to continue this thread but---
I have a '66 MGB (which DOES have an OD relay, per the manual) but which also has a vacuum sensor in the circuit. the relay and the vac sensor were DOA but now have cleaned points and work fine. My question is on the vacuum sensor- at what vacuum does it kick in/out? I can't find it in the manual. I set it arbitrarily at 15" but would like to know just what it should be. (you gotta do the little stuff when you are a temporary gimp /ubbthreads/images/graemlins/smile.gif )
Bill
 
As I am studying the manual I see that the vacuum switch is not an OD cutout (ie "passing gear") but is a "hold-in" switch to keep the OD from kicking OUT if the driver shuts the OD switch off during rapid decelleration, is that weird or what? The British engineer- gotta love him!!
Bill
 
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