viewtopic.php?f=29&t=28760&hilit=scorpio+speedometer
I had some time and needed to look into a defective tachometer on the Scorpio, so I adapted the rig for generating a signal for this as well.
This should also work on an XR tachometer since electrically they are the same.
This picture is of the back of the tachometer with the inputs labeled. Top is the (s)ignal tap, middle is the (+) tap, and the bottom is the (-) tap.
The rig consists of a DC power brick from Radioshack that is adaptable from 3 volts to 12 volts @ 1 amp.
You will need one of these: NE555 Signal Generator Adjustable
Amazon Link: http://www.amazon.com/5V-15VDC-Frequenc ... +generator
Here is a diagram of the circuit board:
You will need (2) 3-pin 20 cm female to female 2.54 mm wires with DuPont connectors. You will connect these to the power supply, the signal generator, and test leads. You will also need the Radio Shack power adapter tip that goes from 2-pin male to bare wire so you can connect it to the dupont connector to power the signal generator.
http://www.ebay.com/itm/10-PCS-OF-A-3-P ... xyUI1TIfIZ
Here is a picture of what the wire test assembly looks like:
I will try to add some closer up pictures of the wire test assembly soon.
You will need a multimeter or other device that will allow you to view frequency and duty cycle of the pulse signal generator. I used this Craftsman multimeter that I got on Ebay for a very reasonable price.
There are three test leads as stated above. They connect to the back of the tachometer as pictured below. I labeled the signal lead as to not cause damage by crossing up that lead with a power lead.
Connect the positive lead of the multimeter to the signal tap, and the ground lead to the ground tap on the tachometer.
The power brick should be set to 6 volts. The tachometer is driven by 5 volts. You could probably run the power brick on the 4.5 volt setting, but I ran it on the 6 volt setting instead. Plug in the power brick. Turn on the multimeter and select duty cycle. Adjust the duty cycle to ~50%. Within a couple of % is fine.
Switch the multimeter to frequency and adjust the signal generator to ~200 Hz.
If the tachometer is working correctly this should make it display a 4000 rpm reading.
If it doesn't read 4000 rpm, there is a potentiometer in the tachometer that probably needs cleaned and adjusted. Sorry, this picture isn't the best. The potentiometer is sticking up from the circuit board with some white plastic melted onto its side to keep the wiper from moving.
Unplug the power to the testing rig. Use some electronic contact cleaner, spray some into the potentiometer. Use a small screwdriver and turn the adjustment back and forth several times. This, in addition to the cleaner, should remove the oxidation that has built up on the wiper of the potentiometer.
Plug the testing rig power back in and adjust the potentiometer until the tachometer reads 4000 rpm. If you continue to have problems with it, the potentiometer may need replaced. Digikey.com should have a compatible replacement.
I did not go into the removal of the instrument cluster. This is a whole procedure unto itself. The biggest caution I have is the green and red connectors that connect to the back of the instrument cluster are very brittle. They will break if forced out. Be careful.
I will detail how to add a small adapter so you can use this same rig for the speedometer. Look for that soon.
I got some helpful info on the tachometer from here: http://www.dinoplex.org/tachoconversion/
The tachometer guts are manufactured by Veglia.
The cost of the test rig, not counting the multimeter, was under $35.
I hope this helps someone. I will finish up the speedometer portion ASAP.
