Scorpio Tachometer and Speedometer Diagnosis Tool
Scorpio Tachometer and Speedometer Diagnosis Tool
Last year I created a testing rig for generating a signal for the Scorpio speedometer to aide in diagnosing issues with it outside of the vehicle.
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.
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.
Last edited by FocusSVT on Thu Jan 28, 2016 9:16 pm, edited 7 times in total.
Darren Needham
Andover, Ks.
Andover, Ks.
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Re: Scorpio Tachometer and Speedometer Diagnosis Tool
Thanks this is great!!
89 RS Scorpio for sale
http://www.cardomain.com/ride/3939013/1 ... ur-scorpio
http://www.cardomain.com/ride/3939013/1 ... ur-scorpio
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Re: Scorpio Tachometer and Speedometer Diagnosis Tool
Impressive; thank you for your time & diligence!
A man meets another addict and says "oh yeah, I have a Merkur too!" and the other man says "no way!!!!! ............how many?"
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Re: Scorpio Tachometer and Speedometer Diagnosis Tool
An option for those comfortable with soldering is to replace the original open frame potentiometer with a replacement that is fully sealed and then re-calibrate.
There was a guy who used to do this on the old IMON email list.
There was a guy who used to do this on the old IMON email list.
Specialization is for Insects
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Re: Scorpio Tachometer and Speedometer Diagnosis Tool
Deeper details for those that don't speak potentiometer
Jeff
Re: Scorpio Tachometer and Speedometer Diagnosis Tool
It is a variable value resister that is part of the tachometer circuit board. It is used to calibrate the tachometer. The unit used for this application isn't sealed so it can have oxidation build up causing erratic or non-operating tachometer. One of the steps in my procedure is to use electronic cleaner and to rotate the "wiper" on the potentiometer several times to help remove any oxidation and hopefully restore consistent and accurate tachometer operation.blugg wrote:Deeper details for those that don't speak potentiometer
Darren Needham
Andover, Ks.
Andover, Ks.
Re: Scorpio Tachometer and Speedometer Diagnosis Tool
This is cool!
But is it a tach fix, or tach calibration?
What I am experiencing with mine is, the tach responds soooo sloooowww compared to what the engine is actually doing.
But is it a tach fix, or tach calibration?
What I am experiencing with mine is, the tach responds soooo sloooowww compared to what the engine is actually doing.
Re: Scorpio Tachometer and Speedometer Diagnosis Tool
If the problem is the potentiometer, then this procedure can help fix it. By cleaning the carbon build up on the internals of the potentiometer it will eliminate the problem most likely. The test rig should allow you to generate a calibrated signal to see what the tachometer is reading. From there hopefully it can be fixed. Jeff Herson might be a good resource for a used tach also.my8950 wrote:This is cool!
But is it a tach fix, or tach calibration?
What I am experiencing with mine is, the tach responds soooo sloooowww compared to what the engine is actually doing.
Darren Needham
Andover, Ks.
Andover, Ks.
Re: Scorpio Tachometer and Speedometer Diagnosis Tool
If you decide to sell repaired ones in good working condition count me in .
89 RS Scorpio for sale
http://www.cardomain.com/ride/3939013/1 ... ur-scorpio
http://www.cardomain.com/ride/3939013/1 ... ur-scorpio
Re: Scorpio Tachometer and Speedometer Diagnosis Tool
do you have a closer picture of the wiring test assembly you made?
'89 Merkur XR4Ti Racecar-The Beast
'88 Merkur XR4Ti-The Build
'87 Merkur XR4Ti-The Beauty
Always looking for a '85 Merkur XR4TI (blue preferably) for my dad
'88 Merkur XR4Ti-The Build
'87 Merkur XR4Ti-The Beauty
Always looking for a '85 Merkur XR4TI (blue preferably) for my dad
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Re: Scorpio Tachometer and Speedometer Diagnosis Tool
Tachs can also be tested outside the car pretty easily- connect the hot and
cold wires from the battery and get the signal from the coil connector (green
wire).
This doesn't calibrate it, but if you have a tach and dwell meter, the tach can
be synched to it by adjusting the pot.
cold wires from the battery and get the signal from the coil connector (green
wire).
This doesn't calibrate it, but if you have a tach and dwell meter, the tach can
be synched to it by adjusting the pot.
Re: Scorpio Tachometer and Speedometer Diagnosis Tool
scorpioaddict wrote:Tachs can also be tested outside the car pretty easily- connect the hot and
cold wires from the battery and get the signal from the coil connector (green
wire).
This doesn't calibrate it, but if you have a tach and dwell meter, the tach can
be synched to it by adjusting the pot.
Nice, I had been thinking how I can compare my XR tach to something, but not what.
Duh, an external tach for comparison!
Re: Scorpio Tachometer and Speedometer Diagnosis Tool
I am having a bit of a problem getting a reliable reading on Hz and Duty Cycle. The Duty cycle ranges from 18-95% every time I check it. The freq is just as bad (2-450 Hz). I have power being applied throughout the tach so I know its getting power. Is that a characteristic of a bad potentiometer or just a crappy user?
'89 Merkur XR4Ti Racecar-The Beast
'88 Merkur XR4Ti-The Build
'87 Merkur XR4Ti-The Beauty
Always looking for a '85 Merkur XR4TI (blue preferably) for my dad
'88 Merkur XR4Ti-The Build
'87 Merkur XR4Ti-The Beauty
Always looking for a '85 Merkur XR4TI (blue preferably) for my dad