Erratic speedo at Idle

[vlad01]

Well now the path to ground is 100R which is not as close to the short that 50R was. I would try the same layout but with something like 1K resistors instead and see if that works, it would be much better for the GPS if it did.

[j_ds_au]

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?

The single series resistor would only be effective if it's significantly greater than the ECU input resistance, so your results suggest the latter is relatively high, at least in the k-ohm range. So a single series resistor might still work, if it's high enough. If a high value is needed though, it would need to be located near the ECU terminal to minimize noise pick-up.

Anyway, adding the second resistor allows you to attenuate the GPS signal (which is what has worked so far) without overloading the GPS output to do so. For this though, you need to increase the resistor from the GPS by up to 100-fold. At the moment (both resistors at 50 ohms), you only have a 1:2 attenuation courtesy of this potential divider, with the majority of your attenuation still due to overloading of the GPS output.

I still get the feeling this solution (attenuating the GPS output) is a kludge. Did you try connecting the GPS output directly to B5 and adding a resistor (say 1k or 10k) between B5 and B6 (with no other connection to B6)?

Joe.

[h4x0r]

Thanks for the help This setup works!!!!

B5 "Vehicle Speed Sensor Signal High" (measures about 0.01 volts)
B6 "Vehicle Speed Sensor Signal Low" (measures about 0.05 volts)

I connected both wires to the VSS output and the fluctuating stopped. Just as a side test I put a series resistor between B6 but it was only stable below 50ohms. Anything above 50 ohms and it became more erratic.

Is this setup ok?

[j_ds_au]

Thanks for the help This setup works!!!!

B5 "Vehicle Speed Sensor Signal High" (measures about 0.01 volts)
B6 "Vehicle Speed Sensor Signal Low" (measures about 0.05 volts)

I connected both wires to the VSS output and the fluctuating stopped. Just as a side test I put a series resistor between B6 but it was only stable below 50ohms. Anything above 50 ohms and it became more erratic.

Is this setup ok?

No.

Your GPS output is supposed to swing between about -6V and +6V. The fact that it measures near 0V means you've found yet another way to short it out.

Good luck.

Joe.

[h4x0r]

Your GPS output is supposed to swing between about -6V and +6V. The fact that it measures near 0V means you've found yet another way to short it out.

Joe.

I tested the VSS output voltage with nothing connected, it reads:

0.01v @ 0kph
0.005v @ 96kph

I tested B6 output pin from the ecu with nothing connected and it reads 0.03v

[j_ds_au]

Your GPS output is supposed to swing between about -6V and +6V. The fact that it measures near 0V means you've found yet another way to short it out.

Joe.

I tested the VSS output voltage with nothing connected, it reads:

0.01v @ 0kph
0.005v @ 96kph

I tested B6 output pin from the ecu with nothing connected and it reads 0.03v

Well, that's not consistent with "The 50 ohm resistor was a little warm to touch". Maybe you've blown up your GPS output already.

With the ECU unpowered, you should measure the resistance between B6 and ground. That will tell you if it's a dedicated ground or if it's one half of a differential input.

Joe.