Printable Version of Topic
Click here to view this topic in its original format
914World.com _ 914World Garage _ Speedometer not working
Posted by: Noodles Aug 3 2012, 02:01 PM
So question for the experts! My speedometer don't regester mph's but my odemeter still seems to be turning. Was wondering if this is a common problem with 914s and if there was an easy fix. Like should I get a new angle drive or is it the speedo head itself that is bad? Any help would be much appreciated!
Posted by: The Cabinetmaker Aug 3 2012, 02:08 PM
Its in the head.
Posted by: ThePaintedMan Aug 3 2012, 02:28 PM
You can do what I did and find a "parts" speedo and try to salvage pieces from it. Make sure they're the same (ie. 120 mph, or 150 mph). Make sure to clean and grease the drive gears lightly. I've heard this is the cause of many broken speedos - the grease gets old, turns to glue and eventually something gives.
Posted by: Dave_Darling Aug 3 2012, 03:09 PM
If the odometer works, the signal (the rotation of the cable) is getting to the speedo. The odo and speedo both take a single input.
The odometer uses the spinning of the cable to turn a bunch of gears that spin the wheels that tell the miles covered. The cable also spins a magnet that is close to a cup-shaped piece of aluminum. This aluminum piece is moved by the magnetic pull of the magnet, which turns it against a spring. The faster the magnet spins, the more the twist against the spring.
It sounds like there is something disconnected inside the speedometer housing. Either the magnet from the cable, or the cup from the needle shaft, or the needle from its shaft, or the spring from the shaft. Once you get the speedo open, it should be straightforward (but possibly very fiddly) to find and fix what is wrong.
--DD
Posted by: struckn Aug 3 2012, 06:36 PM
Slightly OT, but unless you're running stock tire sizes the speedo's and od's aren't accurate. Mines off at least 5-10 MPH!
I bought a GPS and stuck it over the speedo to track actual MPH.
Posted by: mrholland2 Aug 3 2012, 09:18 PM
Can't be made of aluminum. Aluminum is non-ferrous which means that it is not influenced by magnets.
If the odometer works, the signal (the rotation of the cable) is getting to the speedo. The odo and speedo both take a single input.
The odometer uses the spinning of the cable to turn a bunch of gears that spin the wheels that tell the miles covered. The cable also spins a magnet that is close to a cup-shaped piece of aluminum. This aluminum piece is moved by the magnetic pull of the magnet, which turns it against a spring. The faster the magnet spins, the more the twist against the spring.
It sounds like there is something disconnected inside the speedometer housing. Either the magnet from the cable, or the cup from the needle shaft, or the needle from its shaft, or the spring from the shaft. Once you get the speedo open, it should be straightforward (but possibly very fiddly) to find and fix what is wrong.
--DD
Posted by: bandjoey Aug 3 2012, 11:41 PM
Many speedos for sale. Check the classifieds. 
Posted by: mrholland2 Aug 4 2012, 08:29 AM
And contrary to what the creepy fat guy at the public pool says, speedos are NOT one size fits all. 
Many speedos for sale. Check the classifieds.
Posted by: Noodles Aug 4 2012, 04:34 PM
Cool thanks for the informative replys, I'll tear into my speedo and see if I can just clean it out.
Posted by: bandjoey Aug 4 2012, 07:12 PM
Cool thanks for the informative replys, I'll tear into my speedo and see if I can just clean it out.
Ohhh Ouchhhh Geee Whizzzz, I didn't need to know....What's in the speedo?????
Are you in that much pain? And what about your speedometer?
Posted by: Dave_Darling Aug 4 2012, 08:44 PM
Can't be made of aluminum. Aluminum is non-ferrous which means that it is not influenced by magnets.
Not completely true.
http://en.wikipedia.org/wiki/Eddy_current
--DD
Posted by: Drums66 Aug 4 2012, 08:58 PM
....Question #1, to your knowledge has it ever worked? #2 if so did it just
happen??
Posted by: Noodles Aug 4 2012, 09:38 PM
....Question #1, to your knowledge has it ever worked? #2 if so did it just
happen??
I bought it this way so I'm going in clueless. Going to start on that tomorrow after I see if I can adjust my valves. First time doing this so really hope it goes smooth.
Posted by: slu234 Aug 10 2012, 12:43 PM
I had the same issue with my speedometer, speedometer worked but the odometer didn't. The usual suspect is that the main drive gear (pot metal) will slip on the axle that it is supposed to be pressed on. The cheap fix is to remove the pot metal gear and distort the axle hole in the gear with a few strikes with a punch to tighten it back up. Reinstall.
I think PP has a good tech article on the 911 and 914 speedo problems.
The good fix is to replace the main drive gear (the above mentioned pot metal gear) with a replacement that you can find at PP for about $25.
Dave is right, messing with the speedometers/odometers is kinda "fiddly". The PP tech article at least gives you some guidance on how to open it up and give some good direction.
Good luck.
oh, btw.......link to the article http://www.pelicanparts.com/techarticles/Mult_gauge_repair/mult_gauge_repair.htm
One watchout..I have read that you should not reset the trip odometer while the car is moving.
Brian
Posted by: Dave_Darling Aug 10 2012, 05:04 PM
Brian, you've got it the wrong way 'round. If the odometer doesn't work but the speedometer does, then it's the pot-metal gear slipping.
The OP's problem is that the odometer runs, but the speedo needle doesn't read. That's also something wrong inside the speedo, but it doesn't have anything to do with the odometer gears.
--DD
Posted by: scott_in_nh Aug 11 2012, 06:41 AM
related, but different:
my speedo is about 7% optimistic, but the odometer is pretty accurate (I have 195/65/15's on it).
Is there a way to adjust the tension on the spring?
Posted by: r_towle Aug 11 2012, 09:26 AM
First time I have heard of it breaking in this way and not the other.
I have fixed plenty of Odometer gears...never had this issue.
Question of the big brains.
When you get the speedo re-calibrated for different size tires, what do they do exactly?
rich
Posted by: slu234 Aug 13 2012, 08:51 AM
Brian, you've got it the wrong way 'round. If the odometer doesn't work but the speedometer does, then it's the pot-metal gear slipping.
The OP's problem is that the odometer runs, but the speedo needle doesn't read. That's also something wrong inside the speedo, but it doesn't have anything to do with the odometer gears.
--DD
woops!!! sorry for the mixup.
Posted by: 914_teener Aug 14 2012, 10:40 AM
First time I have heard of it breaking in this way and not the other.
I have fixed plenty of Odometer gears...never had this issue.
Question of the big brains.
When you get the speedo re-calibrated for different size tires, what do they do exactly?
rich
Rich,
I think they de-mag the metal wheel on the drive "head" to make the need armature less "attractive". There is some mention on the Bird board of it.
The housing is aluminum and guards the internals from any grease or junk getting in the housing when it spins is what DD already knows.
Just dug into mine to fix the Odo.
Posted by: bbrock May 16 2021, 08:20 PM
Reviving this thread to see if anyone has successfully repaired this problem. I just got back form putting the second mile on my car near the end of my restoration. This time I went fast enough to see that my speedometer is not working but the odometer is (hence, the 2 miles on it). Angle drive and cable are new (well, they have 2 miles on them now) and obviously working. Whats weird is that I tested the speedo with a drill about 3 years ago when I fixed the odo and it worked. I'm guessing I buggered something in reassembly so will be digging in to the gauge again. Thought I take a stab in the dark to see if anyone has gained new wisdom over the last 9 years.
Posted by: 914forme May 16 2021, 08:42 PM
Way Dave explained the function is correct. It does not take much to stop the needle from moving. The magnet does provide a huge pull and can be stopped rather easily.
Unfortunately you’ll have to break it apart. And check it out! Basics are just like the combo gauge, The needle should move with out resistance, and the small spring should return it to zero.
It hard and neat little device.
Check your odo gears for signs of cracking. They become brittle with age. Odiogears sells replacements you have to modify them tho get proper fit. Like setting the pinion depth on a differential.
Posted by: bbrock May 16 2021, 09:02 PM
Way Dave explained the function is correct. It does not take much to stop the needle from moving. The magnet does provide a huge pull and can be stopped rather easily.
Unfortunately you’ll have to break it apart. And check it out! Basics are just like the combo gauge, The needle should move with out resistance, and the small spring should return it to zero.
It hard and neat little device.
Check your odo gears for signs of cracking. They become brittle with age. Odiogears sells replacements you have to modify them tho get proper fit. Like setting the pinion depth on a differential.
Great! Sounds like I'm just looking for whatever is hanging the needle then. I gave the odo gears a good inspection when I was in there before and other than the loose pot metal gear, they looked good. I also didn't fully roll the bezel back onto the backside "just in case" I needed to get back in there, so popping the bezel won't be too difficult.
Powered by Invision Power Board (http://www.invisionboard.com)
© Invision Power Services (http://www.invisionpower.com)