nicklepickle

I have a Mack MB 400 1974 cab over model. The wiring is fairly simple, but even so I'm having trouble getting the thing started. I parked it 6 months ago, and when I did the motor on the two speed back axle burned out and everything on the instrument panel went dead. I didn't try to start it right away, and now, even though the battery and the alternator are fine, and the starter and solenoid are new, it isn't turning over. The first couple of times it chugged in a sluggish way and now it won't do anything. My feeling is that because everything on the dash won't work, and they are all, I assume, wired into the relay off the tumbler, that the relay is the problem. Does this sound right? Any advice would be helpful.

Everyone, thanks. Love this forum and all your quick responses. I'll get it taken care of shortly. Nick

i have a Mack MB 400 1974 single axel with 4 gears and what I think is just basically an Eaton two speed on the rear axel. While driving, the two speed gave out but the rest of the four gears continued working. the electrical pull switch in the cab started smoking and both it and the rear axel started smelling like sulpher. And most of the electrical components on the panel (the battery charge meter and the temp sensor) went out at the same time. Kind of strange. basically with everybody I've talked to they say I've most likely just burned out the motor on the two speed, and that I can still drive it with the remaining gears but I have to disconnect the electrical input to the motor. Does this sound right? And when I get this thing back into town, how difficult is it to repair, and really how expensive? I've never worked on this part of the truck before, so I feel kind of up in the air. Up until now everything has been working great. Any advise would be appreciated from you all. Nick

I'm having some trouble tracking down a part. It's a duel foot valve for the air brakes. The number on the valve itself is T-239649-C but it has no indication of the manufacturer. The truck is a '74 MB400. Is there a field repair or rebuild kit for these things? Thanks

Macks, you were right. It ended up being a bad fitting on the low pressure indicator way up in the front of the cab and a burned out o-ring in the brake release , so I couldn't hear it. Replaced it today, $50 for a piece of plastic, a spring, and a watch battery, and everything works fine. And i have a newly rebuilt compressor that I didn't need. thanks all.

Well, I ran shop air into the tank, and capped the line back into the compressor just to isolate it from the rest of the system. I found out i have a leak somewhere past the wet tank, but i can't figure out where. So unless anyone else has a better idea, I'm going to try and isolate each of the lines, pressurize them, and see which ones leak. by the way, there is air coming out of the compressor discharge, and I can feel, with the air filter removed that the compressor is sucking air in, so I'm pretty sure everything is fine with the compressor itself. However, if anyone knows off hand where the air pressure gauge reads from it would be helpful. Otherwise I'll trace it tomorrow.

Ever hear of running shop air back into the compressor itself through the discharge line, building it to 100 PSI, and listening for leaks as a way to discover a open discharge valve? I'm going to try both this and running air into the tank. At least then I'll know which end is which.

The in take is fine. It got cleaned and a new filter was put on when the thing was rebuilt. All the valves should be fine also.

So, i have a 1974 MB400 with a Scania engine. Last winter the head on my Tu-Flo 501 air compressor cracked. I had the compressor head replaced, then started it up. i build around 30 PSI but nothing more than that. Assuming it was the regulator, I took it off to check. It would build up to 60 PSI, and i could rev it beyond that, but it would always drop back down to 60. I then bypassed the air dryer and routed it directly into the tank. Still nothing more than 60 PSI. So, I had the compressor rebuilt, put it back in yesterday without the regulator and now it won't build any pressure at all. I can feel air coming through the output of the compressor, and can follow it all the way to the tank. There is a small leak in the parking brake release in the cab, but it doesn't seem big enough to be taking all the pressure away. Don't really know what to do now.