From SSDiesel SupplyTM the 6.5 TD specialists, a brand new HD GM transfer pump/oil pressure switch/sending unit, aka: OPS Switch. This switch fits all 6.5 Turbo-charged engines in H1 Hummers thru 2001.
This is the switch that commonly fails on these engines. This switch is dual purpose. It not only is a sending unit for your oil pressure gauge, but it also turns your lift/transfer pump on and off. When it fails, your lift pump will not run. The truck may be difficult to start, but will run. You will notice a hesitation on hard acceleration-most common symptom.
This switch is located in the valley between the cylinders, screwed into the block. Easily changed in minutes.
How the system works. Basically, fuel is pressure fed to the injection pump by the transfer pump. It is a small electric in-line pump located on the frame rail under the truck. Normal psi to the injection pump is 3-5 psi. Fuel is normally in constant circulation from the transfer pump, thru the filter, thru the injection pump-returned to the tank along with fuel from the injectors. When the transfer pump fails (usually due to a bad oil pressure switch) the injection pump is made to draw fuel from the tank under negative pressure. This results in air pockets being formed and under high fuel demand periods such as hard acceleration/towing, for not enough fuel to flow resulting in hesitation. Fuel flow via the transfer pump also cools the injection pump. Running long periods without the transfer pump operation, will result in premature injection pump failures.
This switch is a HD GM part designed to solve the problem of frequent switch failures.
To check your transfer pump operation, install a test gauge (ssd-242) on the end of the water drain line hose, coming off the tee-handled valve located near your thermostat housing. Open the tee-handle valve. At idle thru full throttle, it should read 3-5 psi. If not, check for power at the pump. If power and no pressure, the pump is bad. If no power at the pump, this switch is bad. If no gauge is available, open the water drain valve (tee-handle) while idleing. If a steady flow of fuel is noted, it is probably ok. If no flow or the truck stalls (due to negative pressure) check at the pump as advised earlier. We also offer a permanent gauge kit ssd-215 to read your pressure at a glance.
Save your Injection Pump from early heat related failures. Check your supply pressure often!