Erratic speedo at Idle

h4x0r · Post by **h4x0r** » Sun Sep 15, 2019 6:47 pm

My 424 is showing a fluctuating and erratic speed anywhere from 0 to 40 kmh when stationary at idle even when disconnected from my dakota digital gps speed sender. When I'm moving it shows correct speed.

There are 2 speed signal wires:

B5 "Vehicle Speed Sensor Signal High"
B6 "Vehicle Speed Sensor Signal Low"

The problem still occurs when both wires are disconnected.

If I ground the Blue/White B5 "Vehicle Speed Sensor Signal High" wire it goes straight to 0 km/h.

What is causing this problem?

Post by **vlad01** » Sun Sep 15, 2019 8:06 pm

Probably RFI noise, Sounds like a simple pull-up/pull-down resistor should fix the issue.

h4x0r · Post by **h4x0r** » Mon Sep 16, 2019 3:29 pm

Thanks.

I tested it again this time by pulling the "B5 "Vehicle Speed Sensor Signal High" wire from the back of the ECU itself. Every time I disconnected the wire the fluctuating stops.

I assume I need to make a pull down resistor. Do I just add a resistor to the speed sensor wire and ground?

Post by **vlad01** » Mon Sep 16, 2019 7:30 pm

How do you have the wiring configured? Sounds like something isn't wired right. Even though a resistor should in theory patch the issue there must be a cause to it.

h4x0r · Post by **h4x0r** » Mon Sep 16, 2019 9:39 pm

This is my plug: download/file.php?id=15460&mode=view

I have 2 VSS wires:

B5 "Vehicle Speed Sensor Signal High" is connected to the output of my GPS Dakota Digital.
B6 "Vehicle Speed Sensor Signal Low" is not connected to anything.

Correct speed works while driving but when I stop at Idle I can get fluctuating speeds.

Post by **delcowizzid** » Mon Sep 16, 2019 11:03 pm

Plug an old gearbox reluctor sensor on the wires and see if it goes away at idle .You might just have issues cause there isn't a resistance over the wires so the wires turned into an antenna and picking up electrical noise

j_ds_au · Post by **j_ds_au** » Tue Sep 17, 2019 7:04 am

h4x0r wrote:This is my plug: download/file.php?id=15460&mode=view

I have 2 VSS wires:

B5 "Vehicle Speed Sensor Signal High" is connected to the output of my GPS Dakota Digital.
B6 "Vehicle Speed Sensor Signal Low" is not connected to anything.

Correct speed works while driving but when I stop at Idle I can get fluctuating speeds.

Maybe B6 needs to connect to the output ground of your GPS?

Joe.

h4x0r · Post by **h4x0r** » Tue Sep 17, 2019 4:01 pm

j_ds_au wrote: Maybe B6 needs to connect to the output ground of your GPS?

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.

I have done some DIY test shielding of the B5 wire with aluminum foil

Seems to be happening less.

Anyone know how to shield it properly or put a resistance on it to eliminate interference? Other online posts say its common for VSS to pickup interference from injector pulses etc.

h4x0r · Post by **h4x0r** » Tue Sep 17, 2019 4:03 pm

delcowizzid wrote:Plug an old gearbox reluctor sensor on the wires and see if it goes away at idle .You might just have issues cause there isn't a resistance over the wires so the wires turned into an antenna and picking up electrical noise

I think you might be right.

Any idea how to shield or fix the resistance?

Post by **delcowizzid** » Tue Sep 17, 2019 4:17 pm

You could get some shielded single core wire and earth the shield at one end its how a lot of tps signal wires are sheilded

Erratic speedo at Idle

Erratic speedo at Idle

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