Low Air Warning Light Comes on at 50 psi (not 60psi)

[Fixer25]

Originally Posted by Skip426

Ok , the pdf I attached should be the correct for the majority of the wiring .
The engine section is for a Cummins , so that's no good .

The Freightliner owners manual I received with my coach has the dash info for your dash , but not for mine .

Info on the air gauge says the ; green needle is for the front axle brake system and the red is for the rear .
You mentioned that the needles drop at a different rate during your test .
Do both needles drop to 50 psi before the low air alarm or does one needle get to 50 and the other hit 60 and the alarm is triggered . Issue could be that one of the switches is bad and the alarm is set off by the other . Knowing which color of needle and the corresponding air line color could help identifying the defective part.
Here's a photo of my foot valve showing the brake light switches and the two colors of air supply lines . If there are four switches on your foot valve , then two are brake light switches and two are the low air pressure switches .
Second photo is my dash gauge package that Freightliner started using Jan 1999.

Thank you! I don't think the owners manual came with my coach, too bad they don't just make a PDF of the originals available for download. I have a generic freightliner manual but it doesn't list things like the air gauge needles.

Thank you for the photo. It looks like you only have one sensor. I just crawled under there and I have what looks like 4 sensors on the foot valve. In this photo, is this the brake sensor or the air pressure sensor?

To answer your question, the warning buzzer comes on at 50psi when the first one gets there or they both get there. The green tends to lag behind but it depends on how I press the brake pedal. Sometimes the green catches up to the orange.

OK, one more possible clue, when doing the air brake test, when the first needle hits 60 psi the parking brake light comes on for a half of a second then goes out. Could that help diagnosis this issue? Again, when pressurizing, the buzzer turns off correctly when at 60psi. It's just the depresserizing that it doesn't go off at 60psi.

[Skip426]

Ok, part of the wiring changes for the 1999 chassis was the elimination of those pressure switches , from the foot valve ; my coach has only brake light switches on the foot valve.

I'll look over the schematics to see if I can figure out an easy way of testing the light/buzzer system on you 98 chassis.

I'm thinking the two extra switches on the foot valve are for the buzzer only and not the light but I need to be sure.

[Fixer25]

Originally Posted by Skip426

Ok, part of the wiring changes for the 1999 chassis was the elimination of those pressure switches , from the foot valve ; my coach has only brake light switches on the foot valve.

I'll look over the schematics to see if I can figure out an easy way of testing the light/buzzer system on you 98 chassis.

I'm thinking the two extra switches on the foot valve are for the buzzer only and not the light but I need to be sure.

Thanks! Maybe I missed it, what is the part in your photo that I circled in red?

I have two of those parts (you have one). Plus, two other sensors on the other side (they are different). From the schematic you attached (and the yellow area you highlighted), it shows 4 switches, 2 low air pressure switches, and a park brake switch, and a park brake pressure switch.

I also have a short circuit that is killing my engine batteries so I have to charge them back up before I can run more tests. I don't believe this short is related to the air gauge issue, but you never know.

Thank you so much!

[Skip426]

Originally Posted by Fixer25

Thanks! Maybe I missed it, what is the part in your photo that I circled in red?

That's one of the two brake light switches , the other is above hidden in the shadows , but you can see the wires heading towards it.
Here's a photo from a different angle .

I had an issue , with cruise control no-op when roads were wet , there was enough dirt build up between the terminals , that when the dirt got wet, it passed trace amounts of voltage and the engine ECM thought the brakes were being applied and shut the cruise control down.
Two brake light switches is a redundant set up ; so if you loose air pressure to one half of the brake system , the ECM will still get a signal to shut down cruise control .
Nothing worse that trying to stop the coach with only 1/2 the brakes working , and the engine going WOT, ( Wide Open Throttle) to try and maintain speed .

[Skip426]

Originally Posted by Fixer25

I also have a short circuit that is killing my engine batteries so I have to charge them back up before I can run more tests. I don't believe this short is related to the air gauge issue, but you never know.

Just a little correction in terms ; excuse me for being picky .

In 12 volt electrical ; short circuits ; blow fuses , burn up wiring and other nasty things .

What you have is a unexplained power draw or " phantom draw " on the chassis batteries .

Now , I have to ask , is the coach plugged into shore power ?
If you charge the chassis batteries , how long does the draw take to bring the voltage down ?
Have you had both batteries load tested ?

[Fixer25]

Originally Posted by Skip426

That's one of the two brake light switches , the other is above hidden in the shadows , but you can see the wires heading towards it.
Here's a photo from a different angle .

I had an issue , with cruise control no-op when roads were wet , there was enough dirt build up between the terminals , that when the dirt got wet, it passed trace amounts of voltage and the engine ECM thought the brakes were being applied and shut the cruise control down.
Two brake light switches is a redundant set up ; so if you loose air pressure to one half of the brake system , the ECM will still get a signal to shut down cruise control .
Nothing worse that trying to stop the coach with only 1/2 the brakes working , and the engine going WOT, ( Wide Open Throttle) to try and maintain speed .

Yikes! Did you fix it by unplugging the harness, cleaning it and reattaching? (I'm assuming the harness next to the brake sensors).

Thanks for confirming the brake light switches. Here are two photos of my setup. I see the same (2) brake light sensors (someone painted one red). On the other side are two other sensors, what are these?

[Fixer25]

Originally Posted by Skip426

Just a little correction in terms ; excuse me for being picky .

In 12 volt electrical ; short circuits ; blow fuses , burn up wiring and other nasty things .

What you have is a unexplained power draw or " phantom draw " on the chassis batteries .

Now , I have to ask , is the coach plugged into shore power ?
If you charge the chassis batteries , how long does the draw take to bring the voltage down ?
Have you had both batteries load tested ?

No problem. I am pretty sure shore power doesn't charge the chassis batteries. It does charge the coach batteries. But as an example, I ran the RV a bunch of times one day, then 8 days later the batteries were totally dead. I thought I had found the phantom draw in the hazard circuit (pulled fuse) but apparently that wasn't the only cause. I haven't load tested the batteries yet. They are only a few months old so I assumed my problem was in the RV. I am noticing a voltage drop on one of the batteries while it's sitting unconnected. I want to give this more investigation. Last night it went from 13.08v to 12.80v this afternoon. Maybe a bad battery?

[Ray,IN]

Originally Posted by Fixer25

Thank you! I'm eager to learn and help.

I entered my coach info in my bio, but maybe I didn't put it in the right place? Anyway, I have a 1998 Newmar Dutch Star (38'), Freightliner Chassis, cat 3126 engine.

I really appreciate the help on this!

To create "your signature", go to user CP on the left in the title bar, click, then go to Settings and Options in left column, click, then go to Edit Signature and enter your information. Be sure to click Save Signature at page bottom when finished.
Feel free to use mine or any other members signature as a model.

[Skip426]

Originally Posted by Fixer25

Yikes! Did you fix it by unplugging the harness, cleaning it and reattaching? (I'm assuming the harness next to the brake sensors).

Thanks for confirming the brake light switches. Here are two photos of my setup. I see the same (2) brake light sensors (someone painted one red). On the other side are two other sensors, what are these?

I didn't undo the wiring connections , just scrubbed between them with an old tooth brush and applied silicon non conductive grease .
The red paint is the factory corrosion protection , so I'd think the brake switch without the paint has been replaced at one time .
The other side of the foot valve the sensors are your 60 psi , warning switches .
Large green and red plastic tubes , with the small tubes on the same fitting are air supply from the tanks ( large ) and gauge supply ( small).
My air system doesn't have yellow , so I'm not sure on that one.

Did you try and get an air system diagram from Freightliner ?

[code2e]

Originally Posted by Fixer25

No problem. I am pretty sure shore power doesn't charge the chassis batteries. It does charge the coach batteries. But as an example, I ran the RV a bunch of times one day, then 8 days later the batteries were totally dead. I thought I had found the phantom draw in the hazard circuit (pulled fuse) but apparently that wasn't the only cause. I haven't load tested the batteries yet. They are only a few months old so I assumed my problem was in the RV. I am noticing a voltage drop on one of the batteries while it's sitting unconnected. I want to give this more investigation. Last night it went from 13.08v to 12.80v this afternoon. Maybe a bad battery?

Batteries take on a surface charge while charging and will measure higher voltage for a time after being unconnected. However 12.8 volts is still above 12.6 volts that a fully charged battery should read. If yours are not dropping under 12.6 volts, you may not have a problem.

Batteries stored unconnected will still lose some charge over time. So, several weeks voltage may drop to say 12.4. Still not a problem. When they drop to 12 or under, best to fully charge and load test.

[Skip426]

Originally Posted by Fixer25

No problem. I am pretty sure shore power doesn't charge the chassis batteries. It does charge the coach batteries. But as an example, I ran the RV a bunch of times one day, then 8 days later the batteries were totally dead. I thought I had found the phantom draw in the hazard circuit (pulled fuse) but apparently that wasn't the only cause. I haven't load tested the batteries yet. They are only a few months old so I assumed my problem was in the RV. I am noticing a voltage drop on one of the batteries while it's sitting unconnected. I want to give this more investigation. Last night it went from 13.08v to 12.80v this afternoon. Maybe a bad battery?

Over 13 volts would be a surface charge and even a battery in the best of condition wouldn't hold that long ; at rest 12 hours+ since charging with no load ( disconnected ) 12.7>12.8 is good .

Is you converter charging your coach batteries at a rate over 13.1 volts ?

If it isn't , converter output and coach battery condition have to be investigated .
If voltage is over 13.1 at the coach batteries and the BIRD system isn't connecting the battery banks to charge the chassis batteries , more testing of the BIRD system is in order.

[Skip426]

I guess I should have asked if the back wall of you power cord bay has these two items on it.

[Fixer25]

Originally Posted by Skip426

I didn't undo the wiring connections , just scrubbed between them with an old tooth brush and applied silicon non conductive grease .
The red paint is the factory corrosion protection , so I'd think the brake switch without the paint has been replaced at one time .
The other side of the foot valve the sensors are your 60 psi , warning switches .
Large green and red plastic tubes , with the small tubes on the same fitting are air supply from the tanks ( large ) and gauge supply ( small).
My air system doesn't have yellow , so I'm not sure on that one.

Did you try and get an air system diagram from Freightliner ?

Thanks. That's very helpful to know those are the 60 psi warning switches. So do you think I should focus my attention on the sensors there for this issue of the warning light not coming at 60psi on during depressurization? I assume they are the same device and correspond to the red/green hoses. So that, the first needle to get to 60 should set off the buzzer.

[Fixer25]

Originally Posted by Skip426

I guess I should have asked if the back wall of you power cord bay has these two items on it.

Thanks. Yes, I have a bird and the solenoid(s). Do you think the battery drain issue is connected to the no alarm at 60 psi problem? I wouldn't think so.