Hi, got an ex milk tank FM13 8 legger with rear steer and hookloader body fitted. Whether its coincidence or not used it for the first time to pick up a load, only 6t including the body and air gauge failed. Firstly the lift axle didn’t go down so either it was not heavy enough or a fault lies there if its weight sensing to send axle down? Anyway loading went ok. As soon as I went to drive away one of the top air gauge showed zero, bottom one is fine. Checked and drained all tanks plenty of air present at high pressure. So my thoughts are that the air pressure caused by the back end going down whilst loading maybe wrecked a sensor? Does this sound plausible? The gauge did flicker a few times and briefly returned to showing in the green before dropping again to zero. Not with the vehicle now but am I correct in thinking the sensors will be located in the air tanks? If not please let me know where to look. I am guessing that I can either swap the sensors over or the wires to see if it the sensor gone down. Any help and ideas would be gratefully appreciated.
Not certain about the FM but in the FH the sensors are under the OS corner panel. Open the grille and look behind, hopefully they will be there. Remove the corner panel to get to them. There is an issue with genuine Volvo sensors they fail after a couple of months, I use non genuine at present. If the sensors are not there let me know and I will ask. If it’s air gauge 2 it can be a dashboard issue. When showing zero give the dash a smack above the gauge on the top. If it comes back on it more than likely a faulty dash. These can be repaired ok.
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They are really weird. I had a suspension fault and it was lifting up and going down on its own, we changed a headlight bulb and the problem was fixed. 
Thank you, I did eventually find the info that they should be behind that panel, next plan to swap leads over to check if its the sensor, fortunately dont have any jobs for it that cant wait a few days! Interesting that the cheaper pattern part may be more reliable.
borednow:
Thank you, I did eventually find the info that they should be behind that panel, next plan to swap leads over to check if its the sensor, fortunately dont have any jobs for it that cant wait a few days! Interesting that the cheaper pattern part may be more reliable.
Yes, swap the plugs to the sensors over, and see if the fault goes to the other gauge. If it does, replace the sensor, if it doesn’t its probably a gauge fault on the dash, but these can be sent away and repaired quite successfully, no need for a new dashboard unless you like big bills. I am normally a big advocate of genuine parts for reliability warranty and backup, but as said previously Volvo have had issues with their sensors and the non genuine ones last better than the genuine.
As for the lift axle, is the amber light for the axle flashing on the dash, showing steady or not on at all? Not wanting to tell you how to ■■■■ eggs, but the switch is toggled UP for axle up, and in the central position for normal running. It will only drop automatically when the drive axle approaches fully laden, but beware as this could mean the front axles are overloaded. So laded put it in the central position. The bottom sprung position is to dump the air from the lift axle suspension bags, but not lift the axle up. This transfers some weight onto the drive to aid traction but the lift axle stays down. The light on the dash shows a steady amber light.
Hope this helps