Cummins N14 Celect Plus - 14L - 1995 ish - cold idling rpm problem

Thank you - it belongs to a friend who is not computer/internet savvy - I will be there today looking at sensors - I am guessing its going to be a faulty temp or pressure or even throttle sensor somewhere - will feedback when I know more. Cheers for input.

 

Update - from cold...

Start engine and it idles at 750ish and stays there while things warm up and air pressure buzzer stops after reaching enough psi in air tanks.

Soon after air pressure buzzer stops - the rear air pressure relief valve midway down the back of the chassis - blows or releases as it has got to it's relief pressure.

As soon as that pressure relief blows - which is a small tank and the relief valve is located at the bottom of that tank - the rpms starts to rise - like clockwork to 1000 ish and stays there.

Pump brakes to bring down air pressure and buzzer comes on - and after a minute - the revs come down - but only once - it then climbs back up after pressure relief valve blows. You can only bring down pressure once for the revs to come down - after that - no matter what you do - it seems temperature sensor somewhere overides this pressure sensor and keeps it at 1000rpm.

So summing up - from cold - pressure relief or release is the trigger for rpm to rise from 750 to 1000 - yet when pressure is released - it will come down to 750 only once by pressure release - then after that - no releasing of air will bring it down.

Relief tank has a 2 wire sensor connected to nothing... wires come away but go nowhere - don;t know if thats anything.

Also disconnected foot pedal plug - no difference in rpm.

Hope that points someone to the problem to help me.

 

I had... a Chinese scanner that did not read the fault codes but read some data live so sent it back and have ordered a better one.

I have a laptop but to get Insite or other with connectors is arms and legs $$ territory !

I am aware to monitor what the data is saying at the time before and just after it starts to rev up.

I unplugged maybe 5 sensors yesterday with no response - the only response was when I uplugged the throttle pedal connector - it revved up maybe 5rpm - only just noticeable to the ear.

I am aware things can appear connector but are co-incidental - when you are diagnosing a weird fault - you think of everything.

atm - I am wondering... if the air pressure relief valve on the rear tank is relieving itself too low and the ecu is assuming the engine needs more rpm to produce more psi to compensate for the proper psi not being reached - even though there is no sensor at the relief valve tank connected to anything - there may well be a pressure sensor elsewhere that is detecting this pressure relief? or 'under pressure release' ?

This 'air release' trigger may be co-incidental however it appears to the the trigger - as soon as you hear that pressure release - up go the rpm immediately.

I have a question about the air release - it blows and stops - it is like one puff of steam - shall we say then stops. So if that is a relief ( or is it a relief and then a bypass switch? ) - the compressor does not appear to have an electric clutch - as at the back of the compressor appears to the the diesel pump - so that has to keep going - therefore if relief blows and stops - that means pressure keeps building elsewhere? so thats over pressurising the air system which would explain the rise in pressure being paralell to the rise in rpm?

Appreciate your/any input.

 

I am away from vehicle now - but am looking for an air pressure sensor and can't find any - however in a wiring diagram - have found an idle increase/decrease switch - that could operate - after system is up to pressure - or the brake switch has allowed it to operate - so am going to search the dash for this switch - I will paste it here if I can.

That Brake switch looks like a pressure switch - and when pressure allows - it allows the idle inc/dec to operate - so will find this switch next - it could be operating according to the preset idle speed - and we are blindly unaware of it's existence

PTO was engaged - yes - good point !!!! - but from memory - rpm raises without PTO engaged as well

2 areas to search now