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My overdrive

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Well, I finally got my tranny/J-type overdrive unit back from the vendor. He got it right this time! Everything works well in the shifting, what a treat. Now, the speedo is WAy off. When I am doing about 70 (if my math is correct) the speedo says 55. Bummer. John, at Quantum (the vendor) says the correct components are in the rebuild, the dog-leg is new, and the longer cable is new. What gives? Think I will send the speedo off to Nisonger (someone tore the mileage counter off) for a rebuild. Too, the new switch for the overdrive that installed on the column fell apart. They are all made in Taiwan and are crap. All of them are reversed from what they should be, in other words, up is off. So, I modified the new one to where up is on and recurved the lever so when I put the blinker on I don't go into overdrive. Oh for some REAL Lucas parts. Have ya'll all noticed that everthing made aftermarket is generally inferior to OE?
So, any comments on the speedometer?
Bill
 
Bill--What is your "math" based on? Engine rpm? If so, don't forget to account for the 22% reduction the O/D provides. Is your MPH indication off with the O/D engaged and disengaged?

The O/D switch on my '72 is the same as yours--UP is OFF. I'm not sure how they left the factory though. It would seem safer to have DOWN for the OFF position since you could accidently flick it up (ON) when messing with the turn signal switch. Vice versa (UP is OFF) and you could accidently over-rev the engine.

Rick
 
Dr. Bill, congrats on your Jtype.

I assume you received a new angle drive (fits into the gearbox end) and new speedo cable. If not, that's what you need. If you did not install a different angle drive for the Jtype, the speedo won't function correctly.

BTW, my column OD switch is also off in the UP position. I've finally gotten used to it and actually like it now.

Let us know how it goes.
 
The speedo drive on the transmission is geared for a particular axle ratio. It has no bearing on which transmission is used. Only the axle ratio. You must have the correct angle drive ratio in the transmission for the axle ratio & tire diameter being used. Your transmission guy may not have matched the drive ratio to your particular axle ratio.

You can check by marking off a distance of 52' 9 1/2". Disconnect the cable at the back of the speedo. Push the car over the marked distance & exactly, to a quarter turn, count the number of turns that the cable end makes. Eg. maybe 12.25 turns. Multiply the number of turns by 100. This gives the number of cable turns per mile. This number should match a number marked on the speedo face. Something like 1225, 1350, 920 or what ever. If there is a serious difference you need a different angle drive ratio or to have the speedo recalibrated. Changing the angle drive ratio requires taking the transmission apart again. You will need this same turns per mile info to recalibrate the speedo. There are other ways to check. A good GPS is very accurate to determine actual speed. Speedo shops can make a ratio adapter that goes inline with the cable to make corrections.

As was mentioned, calculations involving the tach are only as accurate as the tach, your tire measurments, & assumed axle ratio.
D
 
I took apart two cars and both overdrive switches were UP is Off. That is the way it went back on my TR4A. /ubbthreads/images/graemlins/driving.gif
 
Dave,
Thanks for the info. I will do all these measurements with the car and send the gauge to Nisonger to re-gear the Speedo. A heluva lot simpler than taking the tranny back out! Does it ever end?

Bill

Oh, and guys, page four of the Bentley book says,"Move the lever up to engage the overdrive and down to release it."
Boy, talk about a stickler for originality, I modified the Taiwanese overdrive lever to work like the original.
 
Dr B, doesnt make sense, if your speedo worked before it should work now correctly. 3.7 diff in all usa cars. Can you borrow another speedo and hook it up and let it hang underneath and check against that. Did you change tires, magnet in speedo could be shot. I am sure your O/D man knows what he is doing, if thats is his biz.
 
Box,
No clue. The speedo worked fine before the overdrive install. I did change the cable to the longer variety.
It didn't work properly with either the first unit he sent or the second. Again, my plan is to pop the speedo, do the push-the-car-over-a-measured-distance thing and send it to Nisonger. I did comment to Quantum that the speedo was off the first time, soooo, I know he took note of that when he sent another unit. Too, is the overdrive install gonna affect the speedo anyway? Whatever, guess it could be coincidentally shot. One NEVER finishes working on these little jewels.
Thanks for the concern.

ps, I did change the tires, slightly smaller, but that was before the OD install, and it affected the registered speed just a hair.

Bill
 
What year is your TR6? Does it have the service counter mounted on the bulkhead, that the speedo cables are connected to. If so, try bypassing it. I said this in my first post of a few minutes ago, but forgot that you have to submit it umpteen times for it to go thru
 
The car is a 73, no counter box. Too, the reset thigamajig is broken off. I will get Nisonger to just rebuild it. I rebuilt the tach myself but never got around to the speedo.

Bill
 
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