How does one bench test 79 Speedo?

[TucmanXJ]

I've read the threads on lubing Speedometer innards, checking gears, using silicone spray etc.

My F model unit registers slower speeds than actual. Cable is intact & lubed, as is the drive.

My question is perhaps more basic ...
How does one "bench test" a speedo (before tearing into the case)?

thanks, Rob

[TopCatGr58]

Hey Rob,

I haven't checked to see what the turn ratio is of the Wheel speedo drive gear?? But if you have the speedo cable disconnected, you can turn the wheel and see how many RPM's the inner drive spins per each wheel rotation.

Then it's a matter of some simple math, determining how many revs the front wheel would turn for certain speed, and calculate how many rpms the inner speedo cable drive would spin, and then get a drill, and connect it, hopefully you can find a drill with preset rpms, and calculate what SPEED the speedo should read at that drill RPM, and then SEE if it actually does!? HTH?
T.C.

[red bandit]

I have used a #6 nail, cut the head off, chuck it in my 3/8 drill that is rated at 2500 rpm, put the point of nail in the speedo and it runs the speedo at 65 mph which seems to be 5 mph fast.

[TADracer]

I can vouch for what red bandit says. 2500 rpm should give you in the neighborhood of 60 mph.

Even if you open the case I am not sure there is any adjustment. The speedo works by two cups that turn one inside of the other at an angle. One has a magnet and is turned by the drive cable. The other, hooked to the needle, is drug around by the magnetic cup spinning in close proximity. So there is not any actual mechanical connection between the needle and the drive cable. Only the odometer has a direct geared connection.

Having said this, if the small shaft that holds the needle is not free to turn in its small bushing, you will get a slower than normal reading.

[ccogswel]

Well the Speedo is really calibrated with magnetism. But you can adjust it yourself another way. If you tear it apart look right under where the needle is. You will see a wound up spring attached to a round piece that resembles a pacman like shape. All you have to do is twist it counter clockwise or clockwise. Just go the opposite the way the spring is wound. When you see it you will know what I'm talking about. Just remember a little twist (the size of a hair) will do. You may want to mark it before you move it so you will have a reference mark.

Good Luck!

[planedick]

Calibrating

Almost all speedo's have the same ratio. 2000 rpm = 60 mph I have never seen one yet that differed. Back in the day, had a friend that did speedo calibration for the local PD and that was the way it was then and because of the mechanical method that's used still to this day, I believe it's still the same.

[yamahansolo]

what do you do when your speedo is sticky and is slow to accelerate and then when you get on the highway you punch it and the damn needle whips around and breaks off... mine did it 3 days ago... so i had to put the 85mph one back on. i do have a couple extra tach though with needles. i assume i'll have to pry all the edges loose to get the face plate off?

[skids]

Buy a new cable also.

Originally posted by yamahansolo
what do you do when your speedo is sticky and is slow to accelerate and then when you get on the highway you punch it and the damn needle whips around and breaks off... mine did it 3 days ago... so i had to put the 85mph one back on. i do have a couple extra tach though with needles. i assume i'll have to pry all the edges loose to get the face plate off?

[TopCatGr58]

Hey Chris,

Those needles are very fragile, and more so with age and sun exposure. If you'll check out the MODS section, electrical, you'll see the tip about replacing the faceplate on the tach I had to repair, I also repaired/replaced the needle with plastic from a model kit. If you can get one of the needles off of your other guages and put it on the speedo, then you'll be lucky. Sounds like you'll also need to do like Skids says and replace cable, and possibly even lube the innards as has been recommended in previous posts.
T.C.

[ccogswel]

Well to be honest with you all it's been a year or so since I had been inside of a speedo or even a tach. But from what I can remember you should be able to get to that little plate with that spring with out having to remove the needle. But like I said it's been so long since I had been inside one that I can't remember what kind of room is inside one. But with some patience one should be able to adjust it.

As for the breaking needle that yamahansolo had happen I don't think it's a bad cable. I believe you have trash inside. Now what I mean by trash is spider webs and dead bugs. I had a tach like that. Rev up slow and stop at two grand. Turned out to be a old spider webs nest. Well with the speedo most everyone knows how they work. But for those who hadn't been in one I'll give a short explanation. You have your cable that fits in the back of the speedo. Well that cables spins a shaft with a magnetic weight shaped like a rectangle. That magnetic rectangle spins inside of a drum like bell. That bell spins from the magnetic piece and moves the needle giving you your speed. It's really simple. And with breaking needles if you have any trash inside around the bell when the magnetic piece rotates and it all lodges up and locks that bell up with that magnetic piece and spins that needle around and breaks a needle. The only reason the needle breaks is most speedo's has a little peg at the end of it's travel. When the needle hits the peg it snaps.

Hope this helps someone.

[TucmanXJ]

Speedo receiving drive socket worn

Found the trouble - Speedo's upper receiving / driven socket has been worn out by the rotating cable end giving slow / false indication.

Andreas is sending a used 240 kmh speedo unit.

thanks all!

[yamahansolo]

i will have to agree with ccogswel.

as i stated before the cable worked just fine on the 85 mph speedo so why would it need replacing. after the needle broke i put the 85 mph speedo back on and it works just fine. doesn't drag or anything. it works like new. also i inspected the sqaureend and it's good. i had also lubed it up before putting on the 160 mph speedo.

the only problem is if i pull the needle off it might throw it out of wack. that happened with another bike of mine. i pulled the speedo needle off for some reason and i could never get it back on in the right spot.

[ccogswel]

Hey yamahasolo,

If you look at your speedo that broke. Does it have a peg for your needle to rest at zero. If it doesn't the stop is under the plate. If that's the case you can pull the needle off and put it back on. All you have to do is guess where the needle tip was pointing at. It could have been a little below or above or right on the begining mark.

Now if your speedo does have that peg you can place a mark somewhere around the bell to help place the needle back into position. Just raise the needle to a speed like 10 or 20 and mark. Then when you want to put the needle back on just go back to that mark.

Or if you are going to throw it away and it's a 160 mph speedo I'll be more then glad to take it.
I'll pay the shipping.

[yamahansolo]

i am going to fix it... it has the peg at zero. but the speedo is stuck at about 20 mph... i am having problems prying off the black metal part that is wrapped around the housing. i didn't want to damage it but it looks like it'll get all bent up by removing it...

[DiverRay]

If you take a small screw driver and use it, you may have more luck. You will probably have to go around about three times to pry the bezel away to clear the case. Putting it back on is another story. I would shape a bit of wood to fit the curve, and use that with a soft hammer to get it back on.