jntwps Posted March 10, 2023 Share Posted March 10, 2023 Hi, I'm trying to replace the low air warning buzzer on a 1977 Mack. I haven't worked on it before and the wiring under the dash is suspect. I tested the old buzzer and confirmed it didn't work. Bought a replacement, tested, and it works. The only thing I can do once it's in the truck though is run constant power from the battery to the buzzer. What am I missing? How does this get wired so it shuts off once the air is at an adequate level? This is the style buzzer it had and was purchased to replace it Thanks in advance for helping Quote Link to comment Share on other sites More sharing options...
fjh Posted March 10, 2023 Share Posted March 10, 2023 15 minutes ago, jntwps said: Hi, I'm trying to replace the low air warning buzzer on a 1977 Mack. I haven't worked on it before and the wiring under the dash is suspect. I tested the old buzzer and confirmed it didn't work. Bought a replacement, tested, and it works. The only thing I can do once it's in the truck though is run constant power from the battery to the buzzer. What am I missing? How does this get wired so it shuts off once the air is at an adequate level? This is the style buzzer it had and was purchased to replace it Thanks in advance for helping run power from the accessory relay under the dash! OR better still to one of the breakers on the breaker bank under the heater or glove box on a switched circuit ! 1 Quote Link to comment Share on other sites More sharing options...
jntwps Posted March 10, 2023 Author Share Posted March 10, 2023 Would either of these be what you’re talking about? Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted March 10, 2023 Share Posted March 10, 2023 You need to run the power through an air switch that is screwed into the air supply. The air switch needs to be 'Normally Closed' with a crack pressure around 85 PSI. Once it hits 85, the switch will go to 'Open' then the buzzer will shut off.. jojo 1 Quote Link to comment Share on other sites More sharing options...
jntwps Posted March 13, 2023 Author Share Posted March 13, 2023 Thank you, I think I found the switch. But it’s giving constant power after the pressure drops down. It cuts off at 85, but when the key is out of the ignition and it drops below (even with the low air light off), it starts buzzing Quote Link to comment Share on other sites More sharing options...
jntwps Posted March 13, 2023 Author Share Posted March 13, 2023 (edited) This picture below; the bottom switch runs only ground, the top switch (has a white sticker on it) runs power to the low air light. When I use jumpers and connect it to the buzzer, the light stays on constantly, as does the buzzer. This picture below shows only the bottom switch. It is ground. When it's connected to the buzzer and has a power source connected to the buzzer, the buzzer shuts off at 85. When it drops below 85, it buzzes even with the key off/ out of the ignition This is a switch i found online. It's a similar style, but I'm wondering if I need to get it since it looks like it has a positive and negative connector on it, whereas the one on the truck only has a negative connection. I'm sure it's something real simple that I'm overlooking but I can't quite get it working correctly. Edited March 13, 2023 by jntwps Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted March 13, 2023 Share Posted March 13, 2023 That's the brake light switch. They are hot all the time.. can you find an air supply that is before the tractor protection valve to install the low air buzzer. ? Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted March 13, 2023 Share Posted March 13, 2023 the low air buzzer switch should NOT be powered by live wire; power source should be from acc relay side which is NO power with key OFF. 1 Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted March 13, 2023 Share Posted March 13, 2023 the low air buzzer has one wire for purposes of connecting ground circuit ; picture of on line switch would be used in a system having isolated ground circuits. always willing to learn , but would question the teacher as it being a brake light switch 😱. the brass tab lower right insulated . if it were ground side and top power side ;; sparks fly. unless lower right insulated enough to carry power circuit ;; THEN it could be a NEW style brake light switch:::: to which I stand corrected and have to stay for detention after school and write on the blackboard 100 times ::I will not question the teacher!!! been there done that toooo 😁 Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted March 13, 2023 Share Posted March 13, 2023 All I know is on R models, (can't tell you how far back it goes) the brake light switch is on the firewall, and hot all the time with no insulation. Yes, I have hit it when removing the rear valve cover.. with all the different switches now-a-days,,. There are options.. the single pole grounding type switch is a great idea for the low air buzzer.. as far as it being a new style switch..... What year is the first year of NEW stuff? I know sometimes I forget how far back some things go, and then I comment... 1 Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted March 13, 2023 Share Posted March 13, 2023 I'm glad you laughed Mech... The switch looks like a brake light switch, but not as new looking as the ones I have replaced in recent years.. I guess he can unplugg a wire to see if the brake lights go out.. Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted March 13, 2023 Share Posted March 13, 2023 definitely the brake light switch would be live all the time . us old folk are use to the round two pole live at all times switches . can't remember too well; wasn't the kysor low water/ high temp system using a two pole switch having isolated ground circuit ?? Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted March 13, 2023 Share Posted March 13, 2023 Not sure about Keyser switch. I don't do truck repair like I used too.. sometimes if I don't see it here and there, I forget it.. I'm too young to be senile. Oh well.. I can't spell either.. 1 Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted March 13, 2023 Share Posted March 13, 2023 enlarge the first picture ,I can see the top switch having a white sticker. I also noticed a pink wire not looking factory in picture. some ones been rewiring . Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted March 13, 2023 Share Posted March 13, 2023 i blame the years of brake kleen (the good stuff) , asbestos clutch dust breathing for some of my senility. not as much old age ..now let not go off topic 1 Quote Link to comment Share on other sites More sharing options...
jntwps Posted March 13, 2023 Author Share Posted March 13, 2023 GOT IT! Thanks everyone for the replies. It was an aftermarket mess under the dash and I had a hard time finding the right places to connect, but eventually found what was mentioned. Works like a charm and is very annoying and loud when it's supposed to be, and silent when it's supposed to stop 1 1 Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted March 13, 2023 Share Posted March 13, 2023 Awesome.... "J". Jojo Quote Link to comment Share on other sites More sharing options...
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