Bouncing MPH?

[Mark Maras]

 An easy way to find the gear ratio is, Put the rear of the car on stands, Mark a tire and the drive line with chalk or anything. Trans. in neut., e-brake off, slowly turn the tire exactly one revolution & count the drive shaft revolutions. In the case of a 3.34:1 the drive shaft will turn just a tiny bit past 3 1/2 turns.

[FastWoman]

I had the problem with my son's Ford Taurus.  Yes, I know it's not a Z, but...  It turns out the Taurus has a really crappy build of speedo, and as the mechanism wears, the spinning magnets start to contact the copper cup very lightly, causing the jumping, but only at certain speeds.  The solution in his case was to replace the speedometer.  However, because I'm a hopeless gearhead who hates substituting junkyard parts that are worn exactly the same as the part I'm replacing, I disassembled his speedo, ground a tiny bit off of the copper cup where it was making contact, and reassembled.  Problem solved.

[djwarner]

Early Datsun speedometers that start a 20 mph were notorious for bouncing at low speeds. This can be caused by a lack of lubrication, a kink in the cable, or binding where the square cable end fits into the speedo.

 

Mine had the typical bounce until I had the dash out for its leather cover. When I re-installed it, I took time to exercise the cable both rotationally and axially to make sure it was free. Then I made sure the cable was straight and in line before engaging it into the speedo. Lo and behold, the bouncing disappeared and hasn't returned.

 

My guess is that the cable had been hanging up on the receiver in the speedo before it was fully seated, causing binding in the cable jacket. A little lithium grease on the square cable before inserting it might help prevent this. 

[Captain Obvious]

 An easy way to find the gear ratio is, Put the rear of the car on stands, Mark a tire and the drive line with chalk or anything. Trans. in neut., e-brake off, slowly turn the tire exactly one revolution & count the drive shaft revolutions. In the case of a 3.34:1 the drive shaft will turn just a tiny bit past 3 1/2 turns.

 

If you've got both wheels off the ground, this might not work. I think you have to insure that only one wheel can spin or it doubles the ratio because of the differential function inside the gear case. You'll need to block the other wheel to prevent it from spinning in order to use this method.

 

Either jack up just one side or use a wheel chock or something to block the other side wheel from turning?

[Mark Maras]

Thanks Captain, I forgot about that. In the past I have blocked one wheel. I always had to jack up both sides so I could see my reference point on the drive shaft.

[EuroDat]

I think it could be wear in the outer cable, knicks or sharp bends in the cable. This can cause the inner cable to briefly bind and as a result you see the speedo jump went it releases. The long cable tends to act like a spring.
You noted a behaviour change with the new cable. Thats why I think this could be your problem. Lubricating the cable and trying to keep in as straight as possible with gentle bends will help.

 

Diff Ratio:
The 240Z had a 3.364:1 and the 280Z had the 3.54:1 R200. If you are still using the original diff you will need a yellow pinion drive for the speedo. If its black you have the one for the 280Z with the 3.54 ratio.

This is not causing your bounce problem. It will mean the speedo won't give the correct speed.

 

Chas

[Captain Obvious]

In the past I have blocked one wheel. I always had to jack up both sides so I could see my reference point on the drive shaft.

 

Whew. Good! I was hoping I was remembering that right. 

 

All I remember was that I was having a heck of a time measuring the ratio on a diff out of the car. Nothing was making any sense until I locked one of the axle flanges in place.

[Zed Head]

You can move the left and right sides independently as long as they both move the same amount when you're done.  In other words, if one wheel gets ahead of another, you can turn them independently until they match up again.  Because it's all gears and rotating stuff.  I put marks on all three parts and just move the two and watch the center one.

 

And, because it's all gears and stuff, the propeller shaft moves 1/2 as far when you lock one wheel.  3.36 becomes 1.68, etc.  Just like when driving, and the outside wheel moves twice as far as the inner for the same number of propeller shaft revolutions.  And I did confirm this on one of my garage diffs just because the whole spider gear rotating carrier thing is hard to visualize. 

Edited March 15, 2015 by Zed Head

[jalexquijano]

I think it could be wear in the outer cable, knicks or sharp bends in the cable. This can cause the inner cable to briefly bind and as a result you see the speedo jump went it releases. The long cable tends to act like a spring.

You noted a behaviour change with the new cable. Thats why I think this could be your problem. Lubricating the cable and trying to keep in as straight as possible with gentle bends will help.

 

Diff Ratio:

The 240Z had a 3.364:1 and the 280Z had the 3.54:1 R200. If you are still using the original diff you will need a yellow pinion drive for the speedo. If its black you have the one for the 280Z with the 3.54 ratio.

This is not causing your bounce problem. It will mean the speedo won't give the correct speed.

 

Chas

Okay. So what could then be the cause of the needle bouncing from 0 to 40 mph and then going steady? Sincé my car has a 280z transmission i can order the black pinion drive but will it solve the problem? The speedometer cable i installed is Brand new so i guess no lubrication is needed. Please advise.

Edited March 16, 2015 by jalexquijano

[EuroDat]

Okay. So what could then be the cause of the needle bouncing from 0 to 40 mph and then going steady? Sincé my car has a 280z transmission i can order the black pinion drive but will it solve the problem? The speedometer cable i installed is Brand new so i guess no lubrication is needed. Please advise.

The bouncing problem can be caused by several issues. One common issue is the outer cable. Its just as old as the inner cable and wears too although a lot less then the inner cable. If you fitted the new cable without extra lubrication it could be too dry. You don't want it packed full of grease, but to dry can make it stick and jump.

I cleaned my outer cable by pulling lint free "nylon" string/rope through it. The string was soaked in brake cleaner. I repeated that until all the old dryed up grease was gone. After that I used a similar string covered in white lithium grease to relube the outer cable. Then lightly greased the inner cable and fitted it all back together.

Another probable cause is wear in the speedo itself. Fast women mentioned it in her post. If thats the case, you might need a replacement. Don't try to lubricate it by spraying anything in the drive end. You could make the situation a lot worse. It works with a small air gap between discs and any kind of fluid in there will send it to maximum speed.

Pay special attention when mounting it to the speedo and the pinion adapter at the gearbox end. Make sure its lined up correctly. Poor alignment can cause your problem. Try to avoid sharp bends.

Back to the pinion.

The pinion in mounted in the transmission, but it is not related to which transmission you are using. It depends on your diff. The transmission came out of a 280Z which uses a diff ratio of 3.54:1.

If you haven't changed the diff it should still be a 3.364:1. The yellow pinion gear would be the correct pinion for that situation. It won't cure you bouncing speedo, but it will give you a more realistic speed.

Chas

[jalexquijano]

Well im back with this issue! My speedometer is bouncing between o to 10 mph now! I can swap it with another speedometer i purchased on ebay to see if the problem is sorted out! Any other recommedations to solve this silly bouncing. The pinion gear is also Brand new as it was replaced this year.