Erratic speedo at Idle

[j_ds_au]

The dakota digital GPS speed sender only has a ground for its own power supply: http://www.dakotadigital.com/pdf/GPS-50-2.pdf
Grounding B6 showed no effect when I tested it.

Grounding is fine if all grounds are at the same potential, but they never are exactly the same. If B5 and B6 are a differential input (which I don't know, without the PCM schematic), then connecting B6 directly to the GPS ground terminal could be beneficial.

Joe.

[h4x0r]

Back to the drawing board

Tried a shielded B5 wire and tried to ground the B6 to the VSS but im still getting the same problem. I wonder if its the dakota digtal VSS. Might see if I can figure out how to use a pull up/down resistor

[antus]

Get a 10k resistor and connect one end to the signal wire, and the other end to the power supply for the sensor or ground. Be careful with voltages, I dont know what that sensor is running at. If its 5v, its probably not a good idea to pull it up to 12v. Someone with more experience than me can probably describe the gotchas better.

[delcowizzid]

The sensor is a 2 wire reluctor

[j_ds_au]

Another idea ... try a 10k (or a bit lower) resistor between B6 (no other connection) and B5/GPS.

Joe.

[h4x0r]

THANKS GUYS!

Got it working . Basically the schematic for the 424 says that VSS wires B5 & B6 are powered less than 0.5V so I decided to try what I think is called a "pull down resistor". I started with a 10k ohm resister on the B5 signal wire to ground but no results. Then a 5k ohm, then a 1k ohm, then a 500 ohm. All I had left laying around was a 50 ohm resistor and BINGO the speed stopped bouncing around at idle. You could hear the quality of the engine idle smooth out also!!! I took it for a test drive with multiple stops and the VSS worked perfectly! The 50 ohm resistor was a little warm to touch so not sure if I'm breaking any laws of physics here but hopefully this is the long term solution.

I'm using the Dakota Digital GPS-50-2 which converts GPS speed to square or sine wave VSS.

[j_ds_au]

Oh, dear. You've got a near short-circuit on your GPS output. It won't last for long.

You might try a series resistor instead, or if that's not good enough, a potential divider.

Where did you find the schematic for the '424? That would certainly help.

Joe.

[h4x0r]

Oh, dear. You've got a near short-circuit on your GPS output. It won't last for long.

You might try a series resistor instead, or if that's not good enough, a potential divider.

Where did you find the schematic for the '424? That would certainly help.

Joe.

OK so this is not a "pull down resistor"?

[j_ds_au]

Oh, dear. You've got a near short-circuit on your GPS output. It won't last for long.

You might try a series resistor instead, or if that's not good enough, a potential divider.

Where did you find the schematic for the '424? That would certainly help.

Joe.

OK so this is not a "pull down resistor"?

It is, but it's way too severe. You're going to blow the output of your GPS.

If as you've seen, a reasonable pull-down resistor value doesn't do the trick, then you need something other than (or as well as) a pull-down resistor. You might try my previous suggestion, or try a series resistor, with or without a pull-down resistor (in a potential-divider configuration).

Joe.

[h4x0r]

It is, but it's way too severe. You're going to blow the output of your GPS.

If as you've seen, a reasonable pull-down resistor value doesn't do the trick, then you need something other than (or as well as) a pull-down resistor. You might try my previous suggestion, or try a series resistor, with or without a pull-down resistor (in a potential-divider configuration).

Joe.

I tried running a resistor in series from 1 ohm, 10, 20, 50, 100, 500, 1000, 5000 & 10000 ohms but it did nothing.

My VSS output is a sine wave with 0.01 volts and is 0volts with the pull down resistor setup. I have changed to your suggestion which also works. Why do you think adding this second resistor will protect my VSS long term versus without it?