noisy/ knocking compressor?

[hklvette]

All,

I have a 931A2 that I'm getting up to top mechanical shape, and have a problem that is a bit confusing. The truck builds air pressure fine, and except for one of the tank valves leaking, holds air great. The air governor cuts out at 125psi and cuts back in at ~95psi after being adjusted (more on that later). When the air pressure gets up to around 100-105psi, the compressor develops a "knocking" sound, getting significantly louder than it usually is at lower pressures or compared to any other truck compressor I've heard in the same pressure range. When I first received the truck, the governor was set way too high, cutting out at 150psi and back in at 120-125. Could this have damaged something inside the compressor itself? Any thoughts on what to look at first before throwing parts at it? I've done a cursory look through the -23 manuals, but see nothing about "noisy compressor", only ones that don't build air pressure. Any help before I start taking it apart would be appreciated.

Thanks!

 

[davo727]

Both of mine occasionally do that knocking thing for half a minute or so and then quiet down. Im new to this gig so don't know if its a problem or not.

 

[WillWagner]

First try turning the steeringwheel all the way left or right and see if the noise goes away. If it does, you COULD have a worn drive spline on the p/s pump or an air comp out of time, but, What you describe is absolutly normal.

 

[hklvette]

I'll workout the power steering this afternoon and report back. Doesn't sound like a spline skipping, but won't rule it out yet. Maybe a better way to describe the sounds is a really nasty case of piston slap, if you're familiar with what that sounds like.

 

[Julian]

One type of noise that only occurs at high pressure is a rattling of the drive gears. When the engine is at idle and the pump is pumping at high pressure, when the pump piston goes over TDC, the pressure in the head space of the pump will try to drive the pump piston back down. This is in effect reversing the torque on the pump input shaft- the pressure on the pump piston will try and drive the pump faster than the engine is turning. This takes up any backlash in the drive gears between the pump and the crankshaft. If there is much gear backlash, you will hear the gear teeth knock when the torque on the pump shaft reverses and tries to speed the pump up faster than the engine is going. For that brief period in the stroke while the pump head pressure is high, the motor is holding the pump back instead of driving it. This does not happen at low air pressure, high rpm or when the pump is not pumping. You can hear this if the knocking disappears when the compressor stops pumping. Other things could cause knocking, but this is certainly one of them, and is not a big cause of concern. I have heard new motors do it.

 

[hklvette]

One type of noise that only occurs at high pressure is a rattling of the drive gears. When the engine is at idle and the pump is pumping at high pressure, when the pump piston goes over TDC, the pressure in the head space of the pump will try to drive the pump piston back down. This is in effect reversing the torque on the pump input shaft- the pressure on the pump piston will try and drive the pump faster than the engine is turning. This takes up any backlash in the drive gears between the pump and the crankshaft. If there is much gear backlash, you will hear the gear teeth knock when the torque on the pump shaft reverses and tries to speed the pump up faster than the engine is going. For that brief period in the stroke while the pump head pressure is high, the motor is holding the pump back instead of driving it. This does not happen at low air pressure, high rpm or when the pump is not pumping. You can hear this if the knocking disappears when the compressor stops pumping. Other things could cause knocking, but this is certainly one of them, and is not a big cause of concern. I have heard new motors do it.

I suspect that you may be right about this being the cause Julian. I tried to get it to make the same noise today and it was there, but not as loud as normal. I didn't let the engine warm all the way up, so once everything expands the gear lash could increase a little, causing the sound. In any case, I'm glad that I don't need to take the pump apart for it.