MC-8 just lost my low air warning buzzer....??

Jinxie · June 04, 2021, 08:01:40 PM

Hi
I own a 1977 MCI MC8. Aside from storing it over the winter, I haven't owned it for long. 2 weeks ago I started the bus and moved it in my yard. All was well. Power on, master switch on.....the buzzer for low air would buzz since 0 psi and would buzz til around 60 or so then shut off and the bus would continue to build air. The generator would start charging somewhere around 80-90 psi.
Just this past weekend I went to start it. Turned the switches on and it was quiet. No buzzing with the power on. The air tank showed 0 which is normal after it sits for a few days, but it should buzz. Then I start it....still no buzzing. I was able to verify the buzzer circuit works by grounding out the high temp sensors in the cylinder heads. Buzzer comes on. Also, the low air light in the dash doesn't come on.
Any suggestions? Does anyone know where the air system pressure sensor switch or sensor is for the buzzer system? Maybe the switch isn't grounding out. ? Maybe it is holding residual air thinking the system is charged up? I have the service and parts manual, but it isn't clear to me where the sensor is located. Or how the system works. All help is greatly appreciated!!

richard5933 · June 05, 2021, 06:04:33 AM

Can't help you locate the pressure switch, but hopefully the manual has that somewhere.

Once you find the switch, I think your thoughts about losing the ground connection are a great first step. While you're at it, scour the area for any other ground connections which may not be good.

From what I've seen, a large percentage of problems with the electrical system can be traced to bad ground connections. Sometimes they'll look good and even feel tight on initial inspection, only to find a layer of corrosion between the parts preventing a good connection.

ol713 · June 05, 2021, 04:33:25 PM

HI;

Look at the circuit closely. It will tell you where stuff is located. there is a flasher in the front junction box.

that is located in the sidewall next to the drivers seat. Also in your manual there is a layout of that box. It will

show you where the flasher is located inside the box. Chances are the flasher died. There are seperate flashers

different systems. My low air buzzer does not work also. I think the buzzer is just back ground noise and not

needed. The bus is not going any where until you get 90psi. Also the wiring diagram is oriented from front to back.

windtrader · June 05, 2021, 07:49:28 PM

Check the buzzer itself. It can be adjusted and probably has built up some corrosion

Jinxie · June 06, 2021, 07:40:50 AM

It's actually the PENN switch in the engine bay. I took it apart and it has corroded contacts....actually 1 is broken. So I will have to see if I can find another switch somewhere. Thanks for the suggestions!

richard5933 · June 06, 2021, 02:29:37 PM

Call Luke at US Coach in NJ. He might have one sitting on the shelf waiting for you.

windtrader · June 06, 2021, 06:09:06 PM

Quote from: Jinxie on June 06, 2021, 07:40:50 AM
It's actually the PENN switch in the engine bay. I took it apart and it has corroded contacts....actually 1 is broken. So I will have to see if I can find another switch somewhere. Thanks for the suggestions!

Nice detective work! What does the PENN switch look like and where in the bay is it located? I'll go check mine out too. lol

richard5933 · June 06, 2021, 06:14:45 PM

Penn Controls made lots of switches in the day, especially for pneumatic systems and HVAC systems. They were bought by Johnson Service Corp. which then became Johnson Controls.

I'm guessing that the are the ones who made the switch you've had problems with.

buswarrior · June 06, 2021, 06:29:16 PM

Quote from: windtrader on June 06, 2021, 06:09:06 PM
Nice detective work! What does the PENN switch look like and where in the bay is it located? I'll go check mine out too. lol

The Penn Air switch is a box mounted to the side of the junction box in the rear of an MC8. A couple inches square. IIRC, It is in the line of fire, when removing the alternator, wires coming from it can be crushed, if it gets away from you, down a makeshift ramp.

It activates the low air warning, and engages the hvac, once air pressure is built. Two for one!

Happy coaching!
Buswarrior

luvrbus · June 06, 2021, 06:47:50 PM

Just wire a preset 15 buck N/C low air pressure switch off any air line near the dash to the buzzer the low air pressure idiot light should still work it's on a different circuit with the switch under the dash it was on my MCI 8,doesn't the Penn control the charging relay for alternator too

windtrader · June 06, 2021, 06:54:57 PM

Good ideas guys. Like I mentioned earlier I'm sure my is the buzzer itself. I fiddled with it and it would buzz on and buzz off. Will take a multi-meter to just make sure that is the source of my problem first