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Mobile Scanner - Severe static when air park brakes are released but clears up when air park brakes are engaged

L

ligonw

Member
Joined
Apr 7, 2021
Messages
7
I'm running into a frustrating issue with a Motorola PM400 installed in a 2019 Freightliner M2 106. The radio is mainly used to monitor county emergency services (I'm a volunteer firefighter), but I'm getting heavy static during received transmissions that makes them nearly unintelligible.

Here’s the strange part:
The static only occurs when the truck's air parking brake is disengaged. As soon as I re-engage the air brake (i.e., put the truck in "park"), the static immediately clears up. Pressing the brake pedal alone doesn't seem to affect it.

Radio power is sourced from a +12V distribution block fed directly by a ~4' heavy gauge cable to the batteries. The ground is to the chassis near that block.

Has anyone seen anything like this before? Any tips on how to go about troubleshooting it? I'm wondering if it's a grounding or interference issue, but the air brake connection has me stumped.

We have a 2014 Freightliner M2 106 with the same model radio, same programming, and same power and ground sources that doesn't have this issue.

Thanks in advance for any guidance.
 
KevinC

KevinC

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ligonw said:
I'm running into a frustrating issue with a Motorola PM400 installed in a 2019 Freightliner M2 106. The radio is mainly used to monitor county emergency services (I'm a volunteer firefighter), but I'm getting heavy static during received transmissions that makes them nearly unintelligible.

Here’s the strange part:
The static only occurs when the truck's air parking brake is disengaged. As soon as I re-engage the air brake (i.e., put the truck in "park"), the static immediately clears up. Pressing the brake pedal alone doesn't seem to affect it.

Radio power is sourced from a +12V distribution block fed directly by a ~4' heavy gauge cable to the batteries. The ground is to the chassis near that block.

Has anyone seen anything like this before? Any tips on how to go about troubleshooting it? I'm wondering if it's a grounding or interference issue, but the air brake connection has me stumped.

We have a 2014 Freightliner M2 106 with the same model radio, same programming, and same power and ground sources that doesn't have this issue.

Thanks in advance for any guidance.
Click to expand...
Does it happen with the antenna disconnected?
 
L

ligonw

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Joined
Apr 7, 2021
Messages
7
I have also determined that with the truck off (no systems on and engine off) but radio on, the position of the air park brakes does not have an effect and the transmissions come through clear.

Im leaning towards it being an issue with power or ground but I am not sure how to go about diagnosing what exactly is the problem.
 
dave3825

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ligonw said:
Im leaning towards it being an issue with power or ground but I am not sure how to go about diagnosing what exactly is the problem.
Click to expand...

Disconnect from the source and run directly to battery. That would rule out bad ground. I was having issues with a PCM and found, though not the problem, that the body ground was not making a solid connection to electrical ground. So anything connected to body ground was not getting a solid connection.

A Power probe (or equivalent) connected straight to battery can diagnose any resistance in positive or ground circuits.

 
ramal121

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So I had a ford van once that had poor AM reception until I stepped on the brake and suddenly it all cleared up. Never tried to fix it but did get me thinking. Either the pedal switch or the power or ground to the bulb was floating until it needed to be used. If there was noise radiating from this circuit then pressing the brake either grounded something or provided a load that killed what was being spewed out.

Sounds like if you disconnect the antenna the noise goes away it is indeed coming down the antenna. I would first heed the above and make sure your install (grounds and such) is top notch. If there is no relief in that you may have to find the parking brake switch and its associated wiring and start screwing with that. It will connect to an air pot somewhere on the hose. Then whatever it takes, ferrites, bypass caps, aluminum foil just to see if you can get any change at all.
 
Last edited:
L

ligonw

Member
Joined
Apr 7, 2021
Messages
7
I will have to mess with it more but I did find a relay that was chattering when the key is on and the park brake is released. That's part of a third-party system to the truck. It must be getting some low-level voltage, not enough to close the coil. When I remove this relay the transmission seem to clear up.

I never noticed it before because you would normally never have the hood up with the park brake released.
 
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