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Old 08-10-2022, 12:35 PM   #1
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Low Air Warning Light Comes on at 50 psi (not 60psi)

Can anyone give me guidance on what the problem might be? During an air brake test (key in on position, engine off, wheels chocked, parking brake off) and pumping the brakes, the needles (green and orange - primary and secondary tanks) go down at different rates. The low air warning light does not come on until 50 psi or so (when the first needle hits about 50). It is supposed to come on at or above 60 psi when doing this test. Everything else works for the air brake test (pressure holds, parking brake pops at 40 psi, etc).

I called frieghtliner and the best answer I got so far was "the alarm going off at 50psi means the alarm is working" The issue is that the alarm needs to come on at 60psi.

I am in PA. When I was going for my class B test, the tester pointed out that the warning light didn't come on when it was supposed to during the air brake test.

Any clues for me? Thank you!
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Old 08-10-2022, 01:41 PM   #2
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Quote:
Originally Posted by Fixer25 View Post
Can anyone give me guidance on what the problem might be? During an air brake test (key in on position, engine off, wheels chocked, parking brake off) and pumping the brakes, the needles (green and orange - primary and secondary tanks) go down at different rates. The low air warning light does not come on until 50 psi or so (when the first needle hits about 50). It is supposed to come on at or above 60 psi when doing this test. Everything else works for the air brake test (pressure holds, parking brake pops at 40 psi, etc).

I called frieghtliner and the best answer I got so far was "the alarm going off at 50psi means the alarm is working" The issue is that the alarm needs to come on at 60psi.

I am in PA. When I was going for my class B test, the tester pointed out that the warning light didn't come on when it was supposed to during the air brake test.

Any clues for me? Thank you!
There are two possible issues. The gauge may be inaccurate. Not as likely, but possible. If you can locate the air charging port, usually near the compressor, test with a known accurate gauge. The charging port is what a tow truck uses to air up when towing. If not equipped, air tanks can be accessed by removing drain valve or spitter and install gauge. Once in service tank and then emergency tank. Compare gauges to verify.

More likely, the pressure switch/s that activate the light/buzzer. May be located near the compressor or near inlets to tanks. Follow the line from compressor, they will be very near it, 2 wire thread in switches. Be sure to dump air before removing one. If you short the 2 wires together with ignition on, buzzer will sound. Good way to verify you found right switch.
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Old 08-10-2022, 09:27 PM   #3
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Welcome to iRV2.

Air system is part of the chassis as are the gauges can you add info about your coach's chassis manufacturer and year to your post for the best possible answers .
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Old 08-11-2022, 10:10 AM   #4
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Originally Posted by code2e View Post
There are two possible issues. The gauge may be inaccurate. Not as likely, but possible. If you can locate the air charging port, usually near the compressor, test with a known accurate gauge. The charging port is what a tow truck uses to air up when towing. If not equipped, air tanks can be accessed by removing drain valve or spitter and install gauge. Once in service tank and then emergency tank. Compare gauges to verify.

More likely, the pressure switch/s that activate the light/buzzer. May be located near the compressor or near inlets to tanks. Follow the line from compressor, they will be very near it, 2 wire thread in switches. Be sure to dump air before removing one. If you short the 2 wires together with ignition on, buzzer will sound. Good way to verify you found right switch.
Thank you! Much appreciated. It seems to have the correct amount of air reading on the gauges but that's a good idea on how to check it. The coach has an air chuck but I don't know if I have a gauge (but I can track one down).

It's strange because the warning light goes off at 60psi when building pressure. But when loosing pressure (air brake test) it doesn't go off until around 50 psi. I should also mention that the needles aren't in sync below abour 90 psi while doing the air brake test. The orange (secondary) drops faster but when it gets closer to 50-60psi, the green eventually catches up. And one more clue, when the orange needle (the lowest in this test) crosses the 60 psi mark, I see the "Park" light flash on the dashboard. Not sure how that is connected to the parking brake circuitry or even if its something to note.

I'll take a look near the tanks and see what I can find. Thanks!

My coach is a 1998 Newmar Dutch Star, Freightliner Chassis, cat 3126 engine.
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Old 08-11-2022, 10:11 AM   #5
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Originally Posted by Skip426 View Post
Welcome to iRV2.

Air system is part of the chassis as are the gauges can you add info about your coach's chassis manufacturer and year to your post for the best possible answers .
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!
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Old 08-11-2022, 10:39 AM   #6
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Ok , Fixer25 , I don't have any hands on experience on the 1998 Freightliner chassis , but I have picked up some info over the years .

I do know that Freightliner chassis underwent a major wiring and gauge change Jan. 4th 1999, and my chassis was built in Feb. 1999.

First a question does your gauge cluster look like the photo below ?
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Old 08-11-2022, 01:55 PM   #7
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Ok ; I've highlighted the sensors ( for the air gauge ) and the one for the park light ( park brake on warning light and warning buzzer have separate sensors ) in this pdf of 98 Freightliner wiring diagrams .
The diagram gives no real indication of the 60 psi sensor location ; BUT ; given they are shown in proximity of the brake light switches , they may be below the floor attached to the brake foot valve.

BTW : On switches and sensors : NC , refers to a switch Normally Closed ; NO is Normally Open .

The park brake warning light and buzzer sensors , if anything like my 99 are above the differential on the drivers side frame rail ( inside ) and look like the photo below.
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File Type: pdf 1998 Freightlier with ISB schematics 2.pdf (1.50 MB, 10 views)
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Old 08-11-2022, 02:04 PM   #8
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One of the two are broken, that would be the air pressure gauge(s) or the buzzer switch.

Replace the switch first.
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Old 08-11-2022, 04:30 PM   #9
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Something I don't have in my collection ; is an air system diagram for the 1998 F/L chassis ; so I cannot tell you for certain if your air gauge is manual ( air line directly to the gauge ) or electronic , ( air pressure sensor mounted somewhere in each 1/2 of the system ) but , given the wiring diagram doesn't show wiring to a sensor , I believe the gauge is manual .

It may be worth a phone call to the Freightliner free help line @ 1-800-385-4357 to see if they will send you an air system diagram . Have your chassis serial number handy when you call .

Note : recent reports from other members say that the help line has been less than helpful , sign of the times I guess.

EDIT: I should ask, did you fail the driving test because of this issue or is this just a repair you're undertaking because of the instructors comment ?
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Old 08-12-2022, 09:11 AM   #10
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Originally Posted by Skip426 View Post
Ok , Fixer25 , I don't have any hands on experience on the 1998 Freightliner chassis , but I have picked up some info over the years .

I do know that Freightliner chassis underwent a major wiring and gauge change Jan. 4th 1999, and my chassis was built in Feb. 1999.

First a question does your gauge cluster look like the photo below ?
Yes, mine looks exactly like that!
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Old 08-12-2022, 09:16 AM   #11
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Originally Posted by Skip426 View Post
Something I don't have in my collection ; is an air system diagram for the 1998 F/L chassis ; so I cannot tell you for certain if your air gauge is manual ( air line directly to the gauge ) or electronic , ( air pressure sensor mounted somewhere in each 1/2 of the system ) but , given the wiring diagram doesn't show wiring to a sensor , I believe the gauge is manual .

It may be worth a phone call to the Freightliner free help line @ 1-800-385-4357 to see if they will send you an air system diagram . Have your chassis serial number handy when you call .

Note : recent reports from other members say that the help line has been less than helpful , sign of the times I guess.

EDIT: I should ask, did you fail the driving test because of this issue or is this just a repair you're undertaking because of the instructors comment ?

I did call Freightliner and I have been unimpressed so far. The comments on this thread so far have given me much more actionable information. FL is still researching the issue and told me they will get back to me.

The tester said that they felt like the coach was unsafe because the warning light didn't come on sooner. He intimated other testers might be ok with it. It's kind of a pain to get over to the testing site so I wanted to make sure this gets taken care of so I can take the road test.

I am super busy this weekend but I hope to investigate more this coming week. Please keep any ideas coming. I am very appreciative!
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Old 08-12-2022, 09:20 AM   #12
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One of the two are broken, that would be the air pressure gauge(s) or the buzzer switch.

Replace the switch first.
Thanks! I will try to investigate them more. My initial guess would be pressure gauge because it works when pressurizing (light goes off at 60psi) but not when depressurizing (doesn't come on when dropping past 60psi).
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Old 08-12-2022, 09:23 AM   #13
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Originally Posted by Skip426 View Post
Ok ; I've highlighted the sensors ( for the air gauge ) and the one for the park light ( park brake on warning light and warning buzzer have separate sensors ) in this pdf of 98 Freightliner wiring diagrams .
The diagram gives no real indication of the 60 psi sensor location ; BUT ; given they are shown in proximity of the brake light switches , they may be below the floor attached to the brake foot valve.

BTW : On switches and sensors : NC , refers to a switch Normally Closed ; NO is Normally Open .

The park brake warning light and buzzer sensors , if anything like my 99 are above the differential on the drivers side frame rail ( inside ) and look like the photo below.
I have to say that you are awesome! Thanks for this VERY helpful information.
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Old 08-12-2022, 09:36 AM   #14
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Yes, mine looks exactly like that!
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.
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