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Speedometer linkage?

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Warpman89

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Since I got my 73 midget back from the shop, I have been driving it a little. However, it seems the speedometer is off and reads lower than I am actually going. I have 165r13 tires, and when the speedometer says I'm going 50 mph, my rpm is around 3500. Does this sound acurate? I'm not sure what my rear end ratio is, but I guess it could be higher than stock? Is something loose in my speedo linkage or cable? Help!
 
Since your speedometer is working and you don't report its needle is jumping or bouncing, the problem is not likely in the cable.

If possible, take the car to a section of road with measured miles. Alternatively, if you have a GPS, take the car somewhere where you can drive relatively straight for a distance and record your initial mileage both on the GPS and the car's odometer. Drive a reasonable distance and compare the mile marker distance (or GPS distance) to the car's odometer and compare the two readings.

If the mileage on the odometer matches the actual distance you drove, the speed indication problem is the calibration of the speedo itself. (The odometer is gear driven off the speedo cable while the speedo pointer is not). An accurate odo and inaccurate speedo can be addressed by a DIYer by googling for and downloading the 27 page PDF by Anthony Rhodes (Repairing Jaeger and Smiths Speedometers). If you don't want to tackle this job yourself, the main places that work on Smiths gauges are Nisonger, Palo Alto Speedometer, and APT (gaugeguys.com).

IF the odometer does not match the distance you drove, you've got a gearing mismatch somewhere.
 
My quick calculation, assuming a 3.9 differential, says that you should be going about 62.5 mph at 3500 rpm in fourth gear on 165-80/13 tires.

Same assumption, at 50 mph your tach should read about 2800 rpm.
 
Or you can do what I did and picked up a Sigma 1200 bicycle speedo for < $25.00 and I now have a digital Speedometer that is extremely accurate. The Sigma 1600's are backlit for nightime driving I believe. Mounted on top of dash to center windshield post bracket. Magnet J-B Welder to rear brake drum. Little magnetic reed switch and some wiring and away you go. Just did the same thing on Colleen's Mercedes 190E, Speedo went out in the dash and shop wanted more than $600 to replace whole instument panel. For $25.00 and some fabrication time for a bracket to hold the sensor all was fixed.

Google Lotus 7 and Bicycle and there is an article on installing in a Lotus7 clone. Speedo BTW reads to 180 mph.
 
It would be prudent to verify that your tach is working correctly, and to determine the gear ratio in the diff.

Then, you could determine if the speedo is faulty and how to correct it.
 
Wow great idea on the Sigma, here is their latest the BC 1606L DTS Wireless Computer for 49.95. No wires to run and is backlit to boot! Too cool.

H-SIG-BC1606L.jpg


You can get it here, I am!
https://www.thstore.com/thstore/ProductInfo2_2water.asp?ID=H-SIG-BC1606L&GroupID=392

Thanks for the info!!
Ðan
 
Regarding the Sigma....
You really don't want wireless. I know it's tempting... but you don't. The signal has to travel from somewhere under the car to the receiver IN the car. Stick with the less expensive wired version so you don't have to worry about transmitter batteries and line-of-sight transmission of the signal.

Also, don't epoxy the magnet onto the drum. Put it on the driveshaft where it won't be subject to heat AND where the wires you run won't be subject to the amount of travel (flexing = fatigue) that the suspension components see.

I have the BC-800 (older model Sigma) on my Mini and have a BC-1600 upgrade waiting for installation when I finally get around to it. Yes, the BC-1600 is backlit... but it is only temporary (like for 3 seconds after you push a button). You shouldn't plan on using that backlight regularly as it draws power off the 3V coin cell that powers the bike computer. You'll need an external light source for night driving.

So... why would I upgrade from the BC-800 to the BC-1600? The BC-1600 has an eeprom to store all its settings when you change the battery (no-reprogramming).

Consider the Sigma for an accurate additional speedometer. However, it's still nice to have the working and (somewhat accurate) Smiths speedometer, particularly for night driving.
 
I like the GPS option. Can't wait to run with the Garmen and see what is actually going on with speeds. There is a link that can do the calculations of speeds and such for you. I will send you the link when I get home.
 
I use a GPS for speed checking. I've got an older model, but it still gives a nice speed reading and odometer. I'll be installing some sort of hidden mount for mine into the Tunebug.

I've used my GPS for years as an accurate speedo on motorcycles -- they often have around a 10% error (read too high), though the odometers are usually very close. Stupid lawsuits. /bcforum/images/%%GRAEMLIN_URL%%/smile.gif
 
I've got solutions and recommendations. Installing on the driveshaft was tried by David Lieb and speedo cannot count that many revolutions. Already been tried and proven that it does not work. Use J-B Weld to attach magnet to the drum not one of those 5 minute epoxies. Try and clean the rust off first.

I updated the Magnetic Reed Switch used to activate the the Speedo. Google Hamlin 59125-020. Little Mini Reed Switch in a flat 1/4" thick x 1.24" long x 3/4" wide package including mounting lugs. Good for several million contact closures. $2.95 including similiar sized magnetfrom a local electronic surplus house. Radio Shack did not have anything like that in a suitable package.

I took a simple piece of 1/2 flat bar mending brace the I bolted to the bottom of the spring hanger on the rear wheel. There is a nice solid bolt within easy access to attach to. J-B Welded the Reed Switch to the bracket, I think I added a litle right angle corner brace or two to position the Reed Switch at a close angle to the drum.

As far as cords and vibration, I used a piece of telephone line mounting cord. Designed to be flexible and move around and size of wire is very close to that on the speedo anyway.

Since I was already using a phone mounting cord and it had modular plug on the end of it, I ran that through the tranny tunnel and ended with the phone cord under the dash. I connected another phone cord with modular jack on it(the other end of the phone cord I already had that I cut off to attach to the Reed Switch). Now I put an female to female modualar cord to modular cord connector in there and stuffed up under the dash.

Now for trouble shooting on the reed switch or positioning of the magnet, I can disconnect before it goes into the speedo and with my VOM connected can see if reed switch is opening and closing as the drum/wheel turns. Switch needs to be close enough but not too close or speeds will be inaccurate. Other issue is that axles do move around in and out as you go around corners. So proper positioning of the sensor, just a little trial and error will ensure that speedo stays working on really sharp directional changes. During an Autocross David Lieb noticed as axle moves in and out seppdo would stop reading properly. I pretty much resolved by using the flat magnet that came with the Hamlin Switch. When Speedo does stop working it comes back on a second or so later as axle moves back into normal position.

Wireless - I did research on that and range is only 24" according to the manual. That won't work for us.

Re. Lighting at night. Was in Autozone over the weekend and they sell little mini blue LED Directional lights that run on 12VDC. One setup featured 2, I'd only use 1 LED that mounted in a directional housing fastened with double sided tape. Was like 1/2 wide / 1/2 high. Little bitty thing that could stick on top of dash where I've got my SIGMA 1200 mounted, could be rotated and aimed, and provide light at night. In fact this could sit in the curve of the padded dash, be switched to only work when ignition was on and provide light at night. Lord know Spridget instruments are not the greatest and even after updating, repainting inside of cans white, seeing the speedo and tach at night is not easy. That's a real simple solution and you don't need to worry about night time lighting.

Hope this helps.
 
I've had one of these things on my Land Rover for years. I put the magnet on the wheel with a wheel weight clip that I melted out of the weight. then a bracket for the reed switch mounted on the front axle. (solid axle) I will say it was close enough for a wireless, but I used an old coiled telephone wire (the one to the handset) and soldered it directly onto the contacts of the head. It had permanent memory so I made a voltage divider and powered it from the ignition switch. It is absolutely foolproof, except it is limited to either 99mph or 99kmh, neither one of which will my Land Rover achieve.
 
See Donn, you did the same thing. Rumaged around in the garage or the basement, made it out of junk you had laying around. All built out of spare parts other than the Speedo head and the reed switch I used. Now that I've built two of these and both work as designed I realize this is a pretty simple application.
 
Ok, Wireless is tempting /bcforum/images/%%GRAEMLIN_URL%%/smile.gif but you are right hard wire would be more reliable. This is somthing I am definantly going to research more on. Thanks for the great tips Jim and Donn and all of you! The GSP idea is somthing I have used on boats and cars and motorcycles alike. Is a great way to see if you are off or how fst you are going, and the newer GPS's with WAAS are even more accurate. I use a Garmin Nuvi 660 and love it.
Good info one and all.
Ðan
 
Jim, it sounds like you ran into some of the gremlins I ran into when I started using the BC-800. I also replaced the Sigma reed switch with an industrial part to be better sealed and to minimize the problems with vibration. Vibration also required that I make a "beefier" support bracket for the reed switch. You can see what I ended up with for the Mini in this JPG.
https://home.mindspring.com/~purlawson/pictures/SigmaBracket01.jpg
I extended the wires from the Sigma to the pickup using light gauge 3-conductor cable and like you I put a plug on it to make it easier to disconnect when necessary.

You've obviously made the glued on magnet work with your brake drums but I'd still be concerned about the affect of heat and the elements. Is there any possibility that you could run the wire through the brake back-plate and mount the switch inside the brake drum assembly with the magnet on the wheel flange instead? Just curious. I understand what you've got is working.

The early Mini has the advantage that the inner end of the half-shaft is a great place to mount the magnet as seen in the picture. I had not considered the driveshaft RPM on a Spridget relative to the wheel speed. You're certainly right, the Sigma would not pick up the driveshaft RPM. I corresponded with Sigma tech support about this last year and they report that the BC-1600 will not accept input pulses faster than 29 Hz. It's likely that the Spridget driveshaft would be somewhere around 3 times that at max speed.

Do you happen to remember a part number for the LED lamp you purchased? I made my own for the BC-800 but would like to see a "more refined" off-the-shelf part.

Back on the topic of this thread, you can use measured miles or GPS as a reference to see whether the error your car's speedometer is showing is a calibration or gearing issue. I still recommend having a working Smiths speedometer even if you supplement it with the Sigma.
 
Sorry, I failed to mention that the speedo needle DOES jump and bounce a lot. I assue the Tach works fine. Sometimes the speedo seems accurate at higher RPM's, but other times at the same RPM's the speedo says 20 mph and the odometer says I have gone less distance than I know I have gone. Does this help you understand my problem? It has to be something with the linkage or cable
 
I think i'd make sure there were no kinks in the speedo cable, and then lube it. If that didn't cure the problem then try a new speedo cable. From your description, I doubt the problem is in the speedo itself. However, wait for another opinion because I've certainly been wrong before.
 
I do find that my Speedo Needle jumps around. Some days it is right on and other days I can get the zero to 90 swing. When you take the back onn of the speedo you can see that the only thing that keeps the speedo pegged is another magnet that goes round and round pulling a piece of steel and opposed by a vey fine spring. The magnet rotates and if thee is any glitch in either the cable of more likely dirt or rust or junk on the jewel bearing it will bounce all around.

Re. little mini LED Lamp. Got to the local auto parts chain store. Look in the do dads and auto accessory aisle. I found perhaps 6-8 different styles of LED lamps, typically blue or green. All terminated in a 12 VDC lighter plug. Light weight, thin, thin wire. There were all sorts of styles. The one that attracted my attention, tiny perhaps 3/4" wide by 1/2" high U shaped plastic mounting bracket that allowed lamp to be rotated and aimed. LAmps started from $6.95 to $12.95 at Autozone.
 
Thanks Jim. I'll look for those LEDs the next time I'm in Autozone or Advance. The light I made works but it would be nice to have a more refined looking piece. I find it interesting that you and I appear to have selected similar reed switches to replace the one supplied with the Sigma.

I agree about the cable and Trevor's comments. The bouncing needle can be caused by either the cable or a dirty needle bearing in the speedo. Start with the cable since it's easier to work with. There have been numerous threads about speedo cable problems and how to remedy them (both here and in the Triumph forum on this board).
 
Could be the fly wheel thingy is gummed up.the spedo on my BE had the same issue more miles traved Based on indicated speed.Cleaned the insides with a contact cleaner being careful to not get any on the face I have no idea if it would hold up to the cleaner. Then let it dry and sprayed it with some silcone lube spray.Again nothing on the face. replaced the dried out oring bezel seal. It showed a little low on speed at first 50 miles? I assume to much lube on the fly wheel thinggy but it works great and has for 3500 miles.
 
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