I have had several issues over the years with Scorpio Speedometers. The usual issue with the odometer gear falling apart, and the more annoying issue where the needle either gets stuck at a slow speed, or at another seemingly random speed. The only way to fix these is to tap the dash pad and get it to release.
If your speedometer is working, but the odometer is not, then 90% of the time it is a bad odometer gear. Here is a website with a procedure for replacing the odometer gear: http://www.odometergears.com/documentat ... ctions.pdf
Here is a vendor that sells the odometer gear. There are two of them and I recommend replacing them as a pair: The kit includes C11 & C12. $39 per set + shipping. Website: http://www.transmissioncenter.net/speed ... ____va.htm
About half way down the page.
I have a speedometer that works perfectly, but the odometer is dead. I checked and replaced the odometer gear even though the original wasn't broken. I checked for binding and even advanced it manually. Everything seemed to work, but it would not advance. Up until now the only way I had of testing it was to install it in a Scorpio and drive around. Not an ideal situation.
Parts Needed: Ebay or equivalent
(1) NE555 Duty Cycle and Frequency Adjustable Module Square Wave Pulse generator PCB
(1) 12V 2A Power Supply AC 100-240V To DC Adapter Plug For 3528 5050 Strip LED
(1) 10 PCS OF A 3 PIN , FEMALE TO FEMALE , 2.54 MM , DUPONT WIRE JUMPER
(1) Connector to the speedometer or equivalent so you can interface with it. You can make your own jumpers if you don't have a spare connector to use.
My total cost by purchasing on Ebay was $20.23. Shipping included.
Step 1: Define what colors you are using for wiring scheme. You will need (-), (+), and output signal from NE555 (VSS signal).
Step 2: Clip off end of the 3 conductor wire connector, strip them, and connect the (+) and (-) to the appropriate power tap on the 12v power supply and use the connector on the other end to plug into the NE555 board. It is clearly labeled. Make sure you have the correct orientation/polarity or you could fry the NE555
Step 3: Get another piece of 3 conductor wire and clip both connectors off of it. Define your (+) and (-) with this section and connect one end to the appropriate (+) and (-) tap on the power supply and then the other end will go to the speedometer. The speedometer connector has options for 5 conductors, but only 3 are used. The top one is (-), the bottom left one is (+), and the bottom right one is the (VSS signal)
Step 4: Connect the two sections of wire that you are using for the output from the NE555 together. I soldered them together and then taped them off. If you are using the connector to the speedometer, then connect the other end of the NE555 output to the (VSS signal) input on the speedometer.
Step 5: If you plug the speedometer connector to the speedometer and then plug in the 12v power supply, you should be in business. By adjusting the left potentiometer you can adjust the frequency of the square wave generated. (When I plugged mine in the speedometer initially went to around 50 mph)
Step 6: This set up cannot verify the accuracy of the speedometer since you don't know what the output frequency is at any given time, but it will allow you to verify if the step motor is moving for the odometer. If it is and the odometer isn't advancing, then the odometer gear is the culprit. In my case, the stepper motor isn't moving at all, so the fault is on the board somewhere, not a bad gear.
Here are pictures of the various parts and the final assembly:
NE555 Board:
Diagram of the board: NOTE: I had to move the jumper to the 1KHZ - 10KHZ position before I could get a reading on the speedometer. I also only adjusted the frequency, not the duty cycle.
Finished product:
In operation:
Hope this can help people diagnose faulty speedometers. If anyone has a question, please ask, and I will do my best to answer!
