MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Interesting Overdrive Gizmo

I ran across this eBay vendor who is selling an interesting optional add-on for overdrive transmissions. It is a little black box that disengages the overdrive whenever you shift, thus eliminating the problem of forgetting to turn it off -- then having it engage prematurely.

The vendor is not quite sure if the LH overdrive used on the late MGB (68-89) all-synchro gearbox is similar to the Triumph, or if the resistance of the solenoid is similar. Any thoughts? Seems like a neat idea.

eBay item 250118829570

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ih=015&sspagename=STRK%3AMEWA%3AIT&viewitem=&item=250118829570&rd=1&rd=1
Jim P

But then to shift into OD you'd have to flick the switch to OFF and then back ON, right>

Doesn't seem like that would be much advantage over just finding yourself in OD when you didn't mean to be and hitting the switch once to disengage.
Bill Spohn

If you read his description on the auction page..."We've all done it, an overtaking opportunity presents itself, change down, plant the throttle, and a second or so later...the overdrive kicks in!!"

Actually that happens to me quite often, in spite of the OD warning light I installed (hard to see in bright sunshine), the overdrive kicks in unintentionally and it IS annoying. I wonder if this thing will work on the LH type gearbox.
Jim P

Jim,

Sorry but that doesn't make sense - "change down, plant the throttle, and a second or so later...the overdrive kicks in!!"

If the OD was switched on and you change DOWN out of 3rd then the OD WON'T kick in.

If you were in 4th and change down to 3rd then the OD stays in.

So the guy obviously doesn't drive an OD fitted car!

Now if you change UP from 2nd to third with the OD switch left on then the change is like going from 2nd to 4th and will seriously impair acceleration but it takes no time to flick the switch off and regain direct 3rd.

Just learn to drive the car - always switch the OD off before changing down into second gear!
Chris at Octarine Services

Early TR6s have OD in second as well. This has been a popular gadget with some owners.
Ron
R. Algie

After 17 years I still don't remember to switch OD off when changing down to 2nd, so when changing back to 3rd it always clonks back in usually inappropriately, and there is another clonk when I *do* switch it back off. So I developed a circuit that doesn't affect changes between 3rd and 4th, i.e. if OD is on it stays on (unless you let the gear lever move out of the 3/4 plane) and if it is off it stays off. But when changing down from 3rd to 2nd, if OD is switched on, a relay operates which breaks the circuit, and lights a warning lamp. OD then stays off until you have moved the manual switch from on to off and back to on again. Details at http://www.mgb-stuff.org.uk/wn_gearsframe.htm and click on 'Overdrive Sequencer Relay', FWIW.
Paul Hunt 2

I just wish I HAD overdrve on my 71! I'd gladly face the challenge of trying to remember to turn it off...ha

Always jealous of those that have it, ..but saving up for one

Jeff
J Delk

my od is very strange
it works in second gear presumably has an inhibitor missing
also it is on the wiper stork just flick it towards you to operate but it wont work with the wipers on
have i got a faulty switch on my stork ?
or is this normal ?

thanks John

ps sorry if this seem like a dumb question its my first b
J LEARMONTH

Gentlemen- Try a wide dispersal white, bright, blinking LED mounted on the shift knob and on the dash above the tach/speedo. Or use color change blinking LED. I too sufferred the nag of forgetting to turn off the OD when downshifting, but rarely anymore with these LED in my face. I hooked one in series with the fuel pump so I could monitor a low tank, mounting it in the "seat belt" indicator rectangle. Then for good measure, one more for the auto fan, mounted in the other console rectangle. It's been fun! Vic
vem myers

John, The overdrive switch should work with or without the wipers on. Sounds like you have a faulty switch. Chris.
Chris82

Ok I will add what I have done with my car and this may be a little off track, but works as insurance for me.

I also have installed a light on dash that reminds me when the O/D is engaged. It is really nice to know that I not the only one who forgets to turn it off now and then.

However, I was concerned and wanted to be sure that I never "accidently" went into reverse with the O/D engaged. Therefore, I found a small solenoid switch that I mounted inside the fender wing. The solenoid is design for a wet, damp chemical environment so I figured it could stand up in the engine compartment of my car. The switch is in the normally closed position, meaning that power runs through it all the time. I ran the O/D power through this switch. Then I connected the power for this switch into my back up light circuit.

So when I put the car into reverse, if I have accidently left the O/D engaged, when the switch gets power from the backup lights it opens the solenoid circuit, cutting the power circuit to the O/D. It works excellent and is just insurance and only cost about $20 - a lot cheaper then repairing a damaged O/D!!

Robert
Robert Browning

Good insurance, but I believe the 3rd/4th gear switch keeps you out of trouble unless there is a double failure --- only one of them yours.


Dan Robinson

A CPO (clever previous owner) fitted an excellent setup on my Roadster that I wish I could replicate on my GT. It gets around all of the irritations above so I'll try to explain it.

There is a column stalk, on the opposite side to the indicator stalk. Tapping it down engages OD, tapping it up knocks it out, without your hands leaving the wheel.

The smart bit is that it's sprung so that it centres itself, moving back to a sort of neutral position after you tap it. This means that when you change into second and overdrive drops out as you move across the gate, it stays out when you change back up, avoiding that thump as OD kicks in unexpectedly. The stalk sits in the middle so you don't have to move it to an "off" setting before using it again. There's a dash light to tell you when OD engaged, as the neutral position of the stalk doesn't show you either way.

If any clever current owners can make a guess at the circuit I'd be interested and will try to confirm it if I can.
Steve Postins

John Learmouth - I'd be very suspicious of this, the lock-out switch on the gearbox could have been bypassed to provide OD in lower forward gears, but means if you leave it engaged when driving in reverse you could destroy the OD. Does sound like (additionally) a dodgy switch as Chris says if it doesn't work at all with the wipers on.

Steve - sounds a bit like mine in function, although yours would seem to have a self-latching relay i.e. using the make contact with a non-latching switch, whereas mine uses the original latching switch and has a non-latching relay. There could be several ways of achieving that, so I can't see much point in providing a possible way, as it may be different to what actually happens. I assume you have a car with the wipers on a dash switch so the left-hand stalk (sp!) is an addition. It all depends on whether the additional stalk has make, break or a mixture of contacts. If simply make contacts then the effect could be achieved with electronics. If a mixture of make and break then just using the contacts, a relay, and no electronics. The possibilities are endless ... as Eddie Izzard has said.
Paul Hunt 2

Thanks Paul. I'm pretty sure it's done with relays as there are a few of them under the bonnet that I don't recognise and I pray never go wrong. That's my main incentive for trying to understand the wiring. It certainly makes for a good drive and if I had the time I'd fit similar to my other car.
Steve Postins

Steve

The circuitry will depend on the specific characteristics of the switch. The "stock" OD switch is (from the wiring diagrams) apparently an SPST (single pole singe throw) switch that is either ON or OFF. It sounds like you have a spring centered switch that toggles to a momentary ON position when pulled up and (momentary?) OFF when pushed down.

I would love to get any details of this switch to see if I can replicate it for my B. Any chance that you can take the "cowl" off of the switch assembly and take a picture or two of the mechanism? The real “key” is the mechanism that centers the stalk between the ON and OFF positions and the type and number of contacts on the switch.

Depending on the switch configuration your circuitry might look similar to that found at http://www.the12volt.com/installbay/forum_posts.asp?TID=51294 scroll down to the Latching Relay circuit diagrams.

If your stalk switch is DPDT with momentary ON contacts when pulled and momentary OFF contacts when pushed (spring centered) adding a relay that is energized from a connection between the Transmission Interlock Switch and the Overdrive Solenoid in between the relay terminal 87 and the junction to the OFF switch should give the behavior that you describe for your roadster.

FWIW

Larry
72BGT
Larry Hallanger

You could also have a simple switch with make contacts both sides, and a relay in series with a resistor connected to ground. The 'on' switch operates the relay which has a make contact in parallel with the manual 'on' switch to latch the relay and send voltage to the OD. The 'off' switch sends 12v to the junction of the relay and resistor, so 'shorting out' the relay which releases. You would probably also need a diode in series with the relay to prevent 'off' current flowing backwards through the relay and OD, so keeping the relay operated.

As I say the possibilities are endless, but this is about the simplest I came up with while pondering the question on a 450 mile drive for the New Forest Run this weekend. It primarily depends on the switch configuration and then the imagination of the designer. But if you say it has 'relays' i.e. plural then it could be much more complicated than that, but still only achieve the same end!
Paul Hunt 2

Paul

I discovered that Hella announced a bistable (latching) relay early last year. The description from Thomasnet.com (news.thomasnet.com/fullstory/481317) is "With current-carrying capacity of 30 A max at ambient temperature of 185°F, Bistable Plug-In Relay minimizes amount of electrical power drained by vehicle equipment that runs in standby mode. Relay's blade terminal, which is normally designated as break-contact, acts as 3rd common-coil connection. To change from one switching position to another, voltage pulse of few milliseconds is sufficient to reset relay." Sure looks like it is a good fit for the OD control challange. However, I cannot find any technical details or schematic.

Apparently this is being offered to the vehicle manufacturers. Anybody know anything about how this is wired and where it might be available?

Larry
Larry Hallanger

Larry

It sounds interesting, but I'm not sure that it will work for this application. It would work with a centre-sprung switch with on-off-on terminals, but it would need a pulse from somewhere when you changed to a gear lower than third.
Dave O'Neill 2

Sorry for the delay but I've finally found a few minutes to investigate the switch. It has three wires onto it (yellow/stripe), top bottom and centre. I'll post a picture of it and the many relays.


Steve Postins

relays


Steve Postins

OK, so the switch is a single-pole, double-throw, non-latching, and it sends an earth on one wire or the other depending on which way the switch moves.

The middle relay looks like it is the starter relay and if so can be ignored.

The left-hand relay seems to have purple and purple/black wires, which are the colours for the horns! No MGB had a horn relay from the factory, so this could be another PO mod, or simply random colours used for part of the OD circuit.

The right-hand relay *could* have a couple of yellow/green wires going to it, but several other wires as well. If it is a relay we'd need to know what the terminal numbering was, and as I have used an old relay can to hold my electronic OD circuit it may not even have a relay inside it!
Paul Hunt 2

This thread was discussed between 30/05/2007 and 11/06/2007

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.