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MG MGB Technical - O/D Speedo cable.. Bigger head??

Hello. I think this is my first post.
Asked this somewhere else, but no one knows.
I put an Overdrive tranny in my 74.5 gt. The Speedo cable from it has a bigger head on it where it attatches to the back of the speedometre. WHY? and how can I connect it? I know about the different type Speedo's for the O/D too, but I will live with the discrepancy for a while.
Thanks
Paul
PAUL

Paul,

There's no difference between O/D and non-O/D speedos. There are 1000 tpm and 1280 tpm (turns per mile) speedos. My 73 MGBs have 1280 speedos and my 74.5 GT has a 1000 speedo. Look at the bottom of the speedo face - you'll see, in small print:

"Made in the UK SN-5230/13 1000"

There's not normally a difference in cables. In fact, for non-overdrive MGBs, a nice solution to breaking expensive transmission angle drives is to use a 72" overdrive speedo cable - long enough to make a gentle curve into the tranny without the angle drive. So the cables are interchangeable. I did this on my '73 convertible for several years before installing O/D.

Your next question is what year MGB is your O/D tranny from? If it has a dipstick, it will require a 1280 speedo. So, get a 1280 speedo from a chrome bumper parts car - circa 1968-1973. If you want to preserve the odometer reading on your car, contact a speedo shop to see if yours can be converted, or if you can get a conversion angle drive. Look at the rubber gaitor on the clutch release fork under the car. Does the gaitor have an oblong shape or a square shape? If it's square, you need a 1000 speedo. Does your car shift into O/D in third gear? If not, it's a VERY late overdrive - perhaps 1979 or 1980.

I'm not sure about the larger head on the speedo cable, because it might be that the redesigned speedos in 77 and later cars used a larger cable head. Maybe somebody else can answer this. Also, there were perhaps two years circa 1970 where they used an angle drive on the speedo, and it's possible that the angle drive used a large cable head. Look at a Moss catalog and see if there are different speedo cables listed for either early or late MGBs. Yes, you will note there is a separate listing for O/D cables - but the only difference is length of the cable.

Something else I don't know about would concern the other cars Leyland built with the 1800 engine and overdrive. Is it possible that your O/D tranny came out of another Leyland product, thus explaining the different cable head? If that's the only problem, just get a new 74.5 O/D cable.

' Hope this helps. Allen

Allen

Thanks for the great information, Allen. I took the motor and the blue lable O/D Tranny out of a 1977 MGB. This cable was attatched to the Speedo, with no angle irons anywhere. It is a 72" as well. It has the overdrive switch in the shifter, which is where I left it when I installed it. I can't see the Speedo now since I am at work, but I will look when I get home tonight.
The cable that was on it was dammaged a bit anyway while transferring it, so a new one will have to be purchased anyway. I will get this one with the propper sized head of course.
Perhaps you could answer another question? The wiring that comes off of the transmission; there is a seperate branch that two wires plug into a small white box, that looks like two tubes go into it, perhaps vacuum? and a black, I guess, grounding wire as well. The rest I attatched to the existing wiring from the old transmission. What hooks into this "Box" and where does the black wire go?
Thanks again
Paul
Paul

Paul,

OK, if the O/D tranny is from a '77 B, I believe your 1000 tpm speedo will be correct. They restyled the dashboard in '77 and I suspect the new style speedo had a larger fitting for the cable. Basically, all you need is a 72" cable.

The little box you speak of was on the driver's side in the back corner of your engine bay, right? Near the brake master cylinder. It's called a TCSA (transmission controlled spark advance). The tubes are to be in line with the vacuum advance line from intake to the distributor. It has no function except to dissable the vacuum advance unless you are in 4th gear, and guess what? It's there only to please the US EPA. I don't think I've ever seen one that was actually hooked up! It has no function except to make your engine run a little worse in any gear in which you want to accelerate.

If you choose to make your O/D switch more original looking for a '74.5, note that all MGBs from about '68 through ' 76 came with the overdrive switch - whether or not the car had overdrive. It's the windshield wiper stalk - if you pull it toward the steering wheel, or push it away, you will see how it works. On the passenger side of the engine bay in the back corner where a bout a million other wires and connectors come together, you should find a yellow one that isn't connected to anything. That's the O/D switch wire. Which ever switch you use, make sure it's fused. I can't remember if the 74.5 is fused or not, but the 77s were not, and a short in this circuit could therefore result in a Nasty Situation.

Not too many of us 74.5 GT guys around. What color is yours? I bought mine about 1-1/2 years ago and confess that my ultimate goal is to install a Rover V8 - since this particular model is the ideal platform on which to copy the factory V8. In the meantime, it is my daily driver - and though I have an O/D tranny for it, I haven't bothered in view of my long range plans for the car. I think the B/GT is the neatest, most useful, best looking car MG ever built.

Cheers,
Allen
Allen

Well, Allen, Thankyou for the info. I was going crazy with that stupid thing. There are enough hoses and wires under that hood, three less is good.
I had checked my wiper stalk several weeks ago, and I don't think this option was even given on this car, but, yes, I would eventually like to put it in the stalk, I will look for the yellow wire tonight when I get home.
The car was bought 3 years ago in NY state, brought to Toronto, given a cheep paint job, motor stripped for valves and rings, and left to die for three years in a mechanic's lot by my work. It was home to a huge racoon when I rescued it for $800.00.
The sills, rails, rockers all need replacing, but the rest is quite good. I can get the parts from my mechanic up here for a great price, but I can't find anyone that would take on the job, at least with not wanting to do a frame up, which I really don't have the money. I also have a '78 TR7, (only 1200 made) which I finished last year. I love that car.
I bought the motor, and O/D tranny two weeks ago for $900.00. The motor, which I just got fully installed and running on Thursday runs like a dream. I didn't even have to do anything with the HS4 carbs, (yet). My mechanic told me he could get me $2200 for the Tranny the day I picked it up.
I will send a Pic of the car to you if you want. It was origionally Aconite, but was painted red. I would like to get it back to Aconite. I think it is quite sharp.
Thanks again

Paul
Paul

Paul,

We've got lots more to share and I have some pictures to send you. I'm going over to e-mail to do that.

Allen
Allen

This thread was discussed on 19/06/2004

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