Unlike the brakes or the TA, the main concern of the lube circuit is flow rather than pressure yet the indicator is based on pressure.
The 86 series tractors are designed with a flow of 5 gpm for the steering and the rest of the MCV. After the steering the priority is to the brakes and then the TA and then to the lube circuit. The brake lube circuit, differential lock, hydraulic seat, and clutch assist are teed into the brakes supply circuit. Other than the brake lube circuit which has a 1 gpm orifice at each brake cover fitting there is no flow drawn off unless one of the controlling valves is used since they are all of the closed center type. That leaves 3 gpm to supply the TA and TA lube circuit, the same as the earlier series tractors. The TA control spool valve is also of the closed center type leaving the 3 gpm less any leakage in the TA circuits available for lube.
If the lube circuit were to flow 1 1/2 gpm through the various areas that are lubed there would still be 1 1/2 gpm left. That flow would cause an increase in pressure in the lube circuit until the lube regulating valve opened to dump the excess flow to the reservoir. The lube pressure switch monitors this circuit. In this scenario there would still be 1/2 gpm excess to maintain pressure in the lube circuit if the brakes were being applied which would bleed off 1 gpm for brake application. Now if you drop the engine rpm to idle the pump may only put out 4 gpm. That still would leave 1/2 gpm excess in the lube circuit and the lube pressure would be maintained keeping the light off. If you now apply the brakes you will flow 1 gpm to the brake circuit and 2 gpm to the brake lube circuit leaving 1 gpm to maintain TA pressure and lube. Since the lube circuit flow 1 1/2 gpm the pressure in that circuit will drop off near zero but you will still have 1 gpm lube flow. That is why the book says it is not detrimental to the system if the light comes on as long as it goes out when pump flow once again can supply the entire 5 gpm. As long as the light goes out with increased rpm the lube circuit is still getting flow even though it is not enough to maintain pressure enough to turn out the light.
Another thing that enters the picture is that the after market rebuilders of TAs have made changes to the internal lube circuits of the TA assembly which in turn increases the flow demand. This may cause the light to come on in a tractor that did not have this happen before the TA was changed. You can also get 13 gpm pumps instead of the normal 12 gpm pump which may be enough difference to keep the light of at idle.
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