7 replies to this topic

[megla_mini]

i was given by a mate of mine a RSM (Rev Speed Meter)Clicky . This requires an electric input for speed and revs, revs is clearly very easy but as far as i am aware the speedometer cable is mechanical in mini's. I thought the obvious thing to do would be to change my gauges altogether thus replacing the old mechanical gauges with new Electric ones. I was looking at the smith range called telelmetrix. Clicky.

So i was under the impressin this was gonna be easy but it doesn't seem that way. I need to fit a sensor to determin speed. I was hoping it would be a matter of plug and play. I emailed speedy cables and asked what i would need to connect these gauges to a 1993 mini 1275cc single hif38 carb. The reply is below.

"You will need a sensor to drive the speedometer, I would recommend that we
make a short speedo cable for you with a sensor fitted to the end of the
cable the cost of this is £40.00 for the sender and £12.00 for then cable."

Didnt really understand, i wanted a list of things so all i had to do is then order that list of things to fit all my new toys but no. So what will i need to fit electric gauges. Im so confused right now. any advice would be great. never done gauges before so please dumb down any answers.

Thanks

Edited by megla_mini, 20 June 2007 - 03:57 PM.

[Dan]

Well you need what CAI (Speedy) said you need, a speed sensor. You are right that the Mini (along with the vast majority of cars in the world) has a mechanically driven speedo. An electronic speedo won't simply be plug and play because there is nothing to plug it into. The speedo in the car, be it mechanical or electronic is simply a speedo head. It needs something to count to be able to determine the speed. There is nothing in the Mini gearbox that is providing an electronic signal which the speedo can count, so you need to fit something to do it. There are sensors available that will screw directly onto the mechanical speedo output on the gearbox but these may not be compatible with the gauge you have chosen. In this case the best idea may well be what CAI have advised. This will use the short cable they are providing to adapt the sensor onto the gearbox output. The output currently turns a cable which is connected into the mechanical speedo, the speedo uses the rotational speed of the cable to drag the needle up the gauge magnetically. You will be using this output instead to turn a small sensor that CAI will supply which will count the turns and provide an electronic signal to the new gauge.

Edited by Dan, 20 June 2007 - 04:35 PM.

[dklawson]

Forgive the following long reply.

You are correct that the Mini's speedometer is cable driven. The electronic gauges from CAI should use a 2- or 3-wire sending unit. Speedy Cables' suggestion to make an adapter cable for such a sending unit doesn't surprise me.

The Mini retained an odd thread on the gearbox for the speedometer connection, it is M19x1. This was used on a lot of British cars from at least the 1950s if not even earlier. Most electronic speedometer sending units I've seen use a smaller, coarse thread, the most common being 5/8 UNC male. Speedy Cables is probably suggesting making a short cable with the M19x1 connector on one end and the 5/8 UNC connector on the other so you can use an off the shelf sending unit.

You may have another option but it involves trips to a junk/breakers yard. Certain late 1980s Jaguars started using electronic speedometers. If you can determine which Jags to look for you may be able to buy as used sending unit from one of those. It will have the right threads to attach to the Mini gear box.

As for programming the speedometer, nothing will be plug-n-play as there's no way for a gauge manufacturer to know what vehicle the gauge will be installed in. I haven't seen the programmable Smiths (CAI) speedometers but I have worked with VDO units... presumably Smiths will be similar. The VDO gauges allow you to set your calibration one of three ways. For the average home user/installer, the easiest method is to take the car to a section of road with measured miles. You enter the gauge programming mode that is set up for driving a measured distance and at the end of the measured length you press the appropriate "stop" button. While you drive the gauge counts the pulses from the sending unit. When you finish the drive the gauge knows how many pulses it received for driving a known distance and it calculates all the scale factors for the analog needle and the odometer. It's very easy.

There is another option for you if you can't find the Jag sending unit or if you decide to retain your original gauges. As I mentioned, most of the programmable/electronic gauges are set up to accept inputs from 2- and 3-wire sending units. These sending units are typically Hall Effect sensors. They can also be inductive proximity sensors. You CAN go to eBay and buy a surplus inductive proximity sensor, bond a steel target on the inner pot joint, and fabricate a mounting bracket for the proximity sensor to look for the target. (Each time the driveshaft goes around the target on the pot joint goes past the proximity sensor and send a pulse to the electronic speedometer. ) That will allow you to leave your existing mechanical gauges in place and run a separate electrical signal to the new digital instrument in your first link above. Were I in your position this is probably the route I'd take as it involves the fewest number of changes and the least outlay of money. Mind you... that digital unit in your first link represents a sizable outlay of money to start with.

Edited by dklawson, 20 June 2007 - 04:40 PM.

[Silicon Skum]

Take a look at the way a bike computer works (have search, there are afew posts on here). Basically all it is, is a magnet and a coil of wire - the magnet is connected to some moving part that corisponds to vehicle speed such as the wheel hub, axle etc. (any part that turns at the same speed as the vehicle is moving forrward). As the magnet turns, it pasess across the coil of wire (sensor) and generates a small voltage in the coil, each time is pasess (so produces a pulse signal for each revolution of the wheels).

The pulse signal is compared to time (a clock signal, think of a ticking clock - one tick per second) Vs the diamiter of the wheels, and the number of revolutions in that space of time. This gives the exact speed the vehicle is traveling at. This is a slightly over-simplified version of how the speedo works, but good enough.

What you need to add to make your clocks works is a pulsed signal like the bike computer uses. There are a couple of different ways this can be done, but usually it's a simple thing like a magnet and a sensor (either a coil of wire, or a "Hall effect" transistor") attached to the drive train of the car (either directly on a drive shaft or indirectly using the speedo drive), or it's a optical sensor (basicaly a light, a light sensor and round disc with a slot cut into it, has the same effect as the magnet and coil).

It may be possible to build a simple sensor that will work with your speedo yourself, but I think you may be better off getting the speedo cable made up for you, and having it fitted by a garage if you are not sure about fitting it yourself. The speedo cable they are gong to make for you will convert the mechanical speedo cable into an electrical signal for use with the speed, so in a sense, this will be a "plug and play" fitment, you just need to swap the existing speedo cable for the new one.

Hope this helps, was not really too sure whith what part you were having dificulty with.

SS

[Purple Tom]

The others have covered it pretty well, but here's my (admiteddly pretty simple) take on it.

Have a look at my first post on this page of my project thread to start with.

I chose the Smiths Telemetrix range of gauges for my Vaux-Clubby. The speedo sender is really easy to wire up - basically there's a live and an earth to the sender unit, and also the pulse wire for the speedo - so three wires in total to get the thing working, plus illumination and power feeds to the speedo itself.

With the three magnets mine is pretty accurate (to within 1mph of GPS confirmed speed) and doesn't lose accuracy the faster you go.

Hope that helps a bit.

[dklawson]

A couple of minor points...

The bike speedos are not sophisticated enough to have a current producing sensor. The magnet (attached to the rotating component) closes a simple reed-switch as the magnet goes by it. The speedo is looking for and counting switch closures and the speedo is providing some small, voltage across the switch leads. It works well but you have some issues with wire capacitance and balancing the on/off times. That's why some are not suitable for use on motorcycles and cars.

The 3-wire devices are in effect behaving as transistor (switches) as mentioned above. This is true whether they are Hall Effect (looking for a magnet) or inductive (looking for a metal 'steel' target). You will find plenty of cylindrical inductive proximity switches on eBay. I use a ton of inductive proximity switches at work and were you over here I'd just send you one in the mail. They come in a variety of sizes and sensing distances. I'd look for some that have 8mm or 12mm diameter threaded bodies and use them. Look for ones with long sensing distances. The threaded bodies will allow you to make simple "L" brackets with through holes for mounting. As mentioned above, three wired devices are easy to wire up. The typical industrial color code is Blue wire to ground/earth, Brown wire to switched power, the black wire is the sensor output. Your programmable gauge will certainly have instructions telling you how to connect three wire sensors.

EDIT: I stand corrected. The online manual I found makes no mention of how to support 3-wire sensors. If you don't already have the manual, see:
http://www.apexi-usa...ntation/199.pdf

What the manufacturer does show/mention on pages 6-9 of this PDF is that the speedo taps ("T"s ) directly into the speed pulse input for a "modern" car with digital instrumentation. You should be able to wire the output wire from a 3-wire sensor directly into the speed sensor input of the Apexi speedo.

90% of that PDF manual is devoted to hooking up the Apexi unit to the ECU of modern cars.

Edited by dklawson, 20 June 2007 - 07:12 PM.

[dklawson]

Rather than edit my last edited post....

I went back to the Apexi web site and they have a second manual for the instrument your friend gave you.
http://www.apexi-usa...ntation/199.pdf
They specifically say in their instruction manual:
"Do not use this unit in conjunction with with any vehicle that is not listed in the Vehicle Setting Tables" And there are MANY pages of Japanese vehicles to choose from.

It turns out that this Apexi unit taps into the ECU and uses data from it so all you're supposed to need to do is tweak minor changes in its scale values by altering the "tire size" you're running. It doesn't appear to have any of the programming features that I mentioned the VDO speedos have.

In short, I'd see if you can make this Apexi unit work on the bench using a 12v power supply and an inductive proximity switch before you contemplate ANY major expenditure for new speedo sending units or the CAI speedometer. You may not be able to get the unit to work with a 3-wire sensor AND even if you do you may not be able to get it to read accurately on the Mini since they don't make it easy to program the pulses per mile.

[megla_mini]

Thanks for evey ones replies. I have just got in from work so have just read them all. I hadn't thought about the calibration on the RSM, good point. might have a bit of a play see what happens. Thanks for explaining how it might all work though some times it just takes a bit of cave man talk and every thing becomes clear. lol. Thanks