I have several gauges on my 1995 Dakota 5.2 not working or not working correctly. I bought a used instrument cluster from EBAY. The odometer on the new cluster reads about 156,000 miles. My truck has 176,000 miles on it. Is there any way to get the correct mileage onto the new odometer? I don't want to swap the odometers because the old speedometer is not working correctly and it looks like the odometer and speedometer are one unit.

 

For a 1995, it's an electronic speedometer.

What you CAN do is take a 12V power supply, a function generator, and some jumper cords.

Power the speedometer up by using the 12V supply on PLUS and GND; connect the output of the function generator to GND and INPUT; and set the function generator to the proper frequency.

And here's where the books come in good - it's 8,000 pulses per mile.

So 8,000 pulses per minute is 60 miles an hour.

Or, to make the math easier to convert to frequency (which is per second), 7,200 pulses per hour is 54 miles an hour (check - 7200/8000 = 54/60 ) which would be 120 pulses per second, or 120 Hz.

You could crank in 240Hz for 108MPH; leave it for about ( 20,000 / 108 ) hours. That looks big, but it's actually under 200 hours, or less than 8 days.

And presto! You've brought it up where it needs to be.

Power supply - 12V 1A or more is plenty for the speedo; 12V 2.5A Adapter Power Supply Used Leader NU30-4120250-13 | MPJA.COM is 2.5A so plenty of reserve. I'd pick up a 5.5/2.1mm Power Jack-Screw Terminals | MPJA.COM to connect to it; but you can also cut the tip off and strip the ends. (I'd rather use the socket though.) For a function generator, check Variable Frequency/Width Pulse Generator | MPJA.COM . So about $17 plus shipping, and you're set to go. Well, add some jumpers, say Set of 5 - Copper Alligator Clip Leads - 20" | MPJA.COM .

Note: Those are not the only possible sources! I just find MPJA great at having such items to hand; and they help show off what I'm talking about.

On the back of the speedometer, you have 3 pins. If you take the speedometer into your hand, and turn it so the top's at the top but the needle and face are in your hand, looking at the back, according to page 8E-9. GND is on your right, 12V in the middle, and VSS input on the left. The function generator AND the power source both need to be connected to GND; the 12V to 12V in the middle (and to the function generator power in); and the FUNCTION output of the generator (or SIGNAL) connected to the VSS input to the speedometer.

Suggestion: Start at a lower speed, say, 30Hz, and make sure the speedo is working before you crank up 240Hz to it. 30 will be about 14MPH or so approx (didn't do the math, just estimating there).

RwP

 

I took the speedometer/odometer out of the instrument cluster. The circuit board on the back comes off with two screws. The indicator needle pops off. The gauge comes off the back of the face with four screws. The odometer number wheel is held in by a pin, running through the center. Turns out you can gently pop the pin out on one side and slide the number wheel out. I did that to both instrument clusters and swapped just the old number wheel into the new speedometer. I had to re-zero the indicator needle. It has a clock spring that keeps tension on it.

Now I have a tach that works and a speedometer that only reads 5 mph too fast (I need different Speedo gear to fix that). I’ll see if the fuel gauge works any better than the old one, when the fuel level gets low. All the other gauges and indicator lights work fine.

 

Are you sure you got the needle back on correctly? If it's 5MPH off, it's an incorrectly placed needle; if it's "off 2.5MPH at 30 and 5 MPH at 60" then it's the speedo gear.

RwP

 

I’ll check on the needle placement. I need another driver to match speeds with. I know if it is off 5 mph off at all speeds it is the needle. If it is off proportionately it is the gear. I checked it against a speed monitoring sign at 40 mph just to see what it read. The sign said I was going 35 but my speedometer said I was going 40. I’ll do the real test when I get someone to drive with me at multiple speeds.

 

One other test - find some nice level interstate.

With a friend riding, have them start a stop watch when you pass a mile marker at an indicated 60MPH.

Run 2 or 3 miles.

See how many seconds it takes.

That'll tell you how far off it is - if it's dead nuts on, it'll be 60 seconds per mile.

Take the number of seconds it takes, put it under 60 * the number of miles you drove, and that'll give you a percentage off (or more accurately, percentage ACCURATE. The difference from 1.00 is the percentage OFF.)

(If you need to reset the pointer, the function generator and power supply method lets you dial in almost any speed - take the miles per hour, multiply by 8000, divide by seconds per hour, or 3600, and that gives you the frequency to set the generator to.)

RwP

 

Another way to check speed is to use a Garmon if you know someone that has one. Just saying.
Oz

 

Most cell phones have a GPS app as well.

 

Yes.

And my Samsung Galaxy S10 has me going 55 - 75 MPH immediately back to back ... the GPS transponder sucks in a lot of cell phones out there.

The Garmin is a better idea.

But my method has the advantage of being easy math (well, overall easy math). And not dependent on some accountant not cost reducing the GPS transponder in your $1,100 cell phone.

RwP