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Hi all, I’ve got a problem on my hands. My low air buzzer is going off all the time in my 1990 Mack RB688RST nz model. I’ve replaced and now unplugged the new air pressure switches on the fire wall and the buzzer is still going off. The only way to stop it is by pulling the circuit breaker out or turn the key off. Any help would be much appreciated, photo below of the truck and the buzzer set up. Thanks again

Short somewhere...start pulling fuses/breakers til the light goes out to isolate the short.

Need to get the chief cook and bottle washer to have a look at your exhaust if you can't see it when starting So it sounds like your loosing fuel completely If it was mine I would pressurize the tank a bit, wind the pressure down on the shop air and have a really good look for leaks It maybe leaking were you can't see it If you block the tank vent off and pressurise it a bit, it should i theory hold air pressure Have a fiddle and report back I'm busy playing with a different type of horse power at the moment Paul

Will check it out tomorrow morning Tks again

lol, that’s rough on a fella. I guess I should mention the pump has some governor issues so it’s coming off and going to pump shop. I’m curious about turning it up or if that’s possible to match a 454 setup. Obviously will pull injectors and would change turbo if the pump was turned up that much.

Another way to look at it that may help: If you had a "normal" truck and wanted to see why the Cummins fuel solenoid wasn't working and what to check to see if there was power there with the key on, you would clip one end of the test light to either the frame or engine block and use the pointy end to the stud with the wire from the ignition switch, and if the light lit, you would know you had power to the solenoid. Nothing is different when the truck is positive ground, you hook the test light up exactly the same and the results you see are exactly those of a "normal" truck.

Tks

yes, just different terms. It refers to the the wire that is carrying the current when one side of the circuit is grounded to the frame or surrounding metal. Look at it this way: You could run two wires from the battery (positive and negative) to every device and the surrounding metal could be "neutral" neither + or -. Instead of doing that you tie one side (either positive or negative) to the metal and eliminate one of the two wires. Ships houses, many things use the two wire method so any metal you touch is not a part of the circuit. Low voltage vehicles they cut the wiring in 1/2 by using the body for one of the wires. It makes no difference from an operational stand point which polarity is tied to the chassie. Positive or negative, it makes no difference, Light bulbs, most motor, many gauges, will function without any changes. Volt meters and some designs of temp and fuel gauges, you have to have the + lead connect to "ground" and - lead goes to feed (hot, live, whatever term you use).

What’s the difference between hot and live,aren’t they the same?

I feel the same!! The only way I will skip Macungie is my health or $6 fuel..

And Tom I know you’re just blowing smoke, but this tread is for bullshitting… bob

Yes

Yes, more pictures the better! Thank you. .....Hippy

I have no real idea when the compressor went out it blow water out the recovery tank and truck ran hot cut off I clean the blow by filter which was full of oil moisture slug cleaned it out with carb cleaner and gas maybe that’s what was causing it to build up pressure to push the oil out of the little breather vent on the filter housing I’m not sure but I checked the filter again after it worked all day and no slug or moisture in the system now and no water in oil hopefully it continues to work til I can get some money together

Am I losing it ?? I looked back a page or few, and saw a different picture. Ive seen this one before... Still pretty good,..

Sorry your post got sidetracked there for a bit, I didn't realize I was steering it off course, that's a highly unusual occurrence here.

Ok, then anything that is ground, the frame, the steering column, etc will light the light. Anything that has "hot" power will not. Move the clip to the positive (ground side) and the light will light when the probe touches something that is "live". I think what is confusing you is you are so used to positive being "hot" and negative being "ground", but your system is the reverse.

Positive ground?

Geez, you're never satisfied, you're always looking for more power! You should just get a race car and be- oh, wait...never mind... 🤣

That looks like fantastic work, no doubt!

I am using the type pictured,and I had the clip on the negative post I have 2 12 volt batteries

And yes tom,,,,, i like the.chick man… bob

And Tom clearly the more braces you get on that shelter the stronger it will be

At that vintage polarity only matters with some gauges, alternator (but not generator) and any radio with transistors. What type of test light are you using? If you are using a non powered one like pictured, normally the end with the alligator clip gets hooked to ground (positive in your case) and the item being tested wire will supply the negative. Cummins solenoid doesn't care about polarity, it will work just fine on positive ground.

Well, I can’t see the smokestack from the driver seat, but it was definitely nothing coming out of the feed line before I started to change the lines. I wind up blowing air into the fuel tank and then breaking it loose right at the pump until it starts running out after the new lines waited a week and it still would not start, but I shut ether in it and that did it but clearly it’s still losing somewhere and the plunger leaks so bad that I don’t think it will work at all once I unwind the wingnut and pull it out. It’s just dripping with fuel.