oil pressure switch

[WLucas]

Can anyone give me the part number for OPS, 1994 6.5TD? The last switch I changed was an after market and it only lasted 3,000 miles.
Thanks,
Bill Lucas

[HowieE]

Hi Bill
I don't have the part number but if you are changing OPS that often consider the OPS mode to extend the life of the new switch. Look at my web site, in my signature, for a diagram of how to wire it.

[WLucas]

Thanks Howie,
I guess I will try your fix on the oil pressure switch. I haven't had very good results with after market switches.
Missed you at the rendezvous.
Thanks,
Bill

[catmandoo]

i think i'm dealing with junk switchs right now.my 94 sub was running pretty good but could smell somewhat unburnt fuel so checked inj's and a couple were down to 1650 lbs so changed em .on the drivers side i just bent the lines out of the way and changed em but the pass side i took the lines off ,i needed to change the cover gasket ,anyway while in there i had a hiccup on occasion so decided to change the ops well when i went to start it would almost start,so let it set awhile and try again and***ut about start,so let it set then try again and it starts and instantly dies,and hasn't attempted to fire since,and i have no pump now,you mention a relay is this something you add or does it use one to begin with?i need to trace the power ,where does it come from?i've traced the harness from the pump and it runs up to the junction box on the firewall,but from there i have no idea.can i rig the pump to run all the time,with the key on??the new ops shows 40 lbs cranking,so at least that part of it's working,but i still think it's in there somewhere.

[HowieE]

The orange wire at the OPS is 12 volts to the OPS switch. The Gray wire is the output to the fuel pump but will have 12 volts on it during the glow plug cycle. During the glow plug cycle this voltage is supplied from the oil relay and run the fuel pump during this cycle. Once the truck starts and oil pressure is above 4 lbs the oil relay will drop out of the circuit and the gray wire should have 12 volts as the OPS has now been closed and becomes the source for the fuel pump. The reason for this OPS side of the OPS is a safty to shut the fuel pump off if the engine shuts down during an accident. If you don't have voltage on the Gray wire after cranking the switch in the OPS may have failed.
A test would be to jumper the Orange and Gray wires at the switch and the fuel pump should run whenever the ignition is on.
If you do find the switch is bad and replace it consider the OPS mode as shown on my web site to prolong the life of the new switch.

[catmandoo]

well i figure the chances of getting 2 bad one's in a row is pretty slim,but it worked before i changed the inj's,i changed the ops before i did them so it has to be in there somewhere!i'll check that tomorrow ,just got out of the hot tub where i did some good thinking on the subject have a couple of other things to check over also,thanks for the info.

[catmandoo]

howie,i pulled the harness off the ops and checked the orange wire and had power(with no key in the switch)is it supposed to be like this.anyway jumped org and gray and nothing,so went underneath and checked harness at the pump and i had power(damned if the pump picked the greatest time to die,right when i do the inj's,were the diesel demons working overtime or what.anyway went out and got a pump and put it on and hooked everything back up.well with glows cycling,i had no power to the pump!had to jump the ops harness and let pump run til i could get it started and now it starts ok and after i shut off, the pump runs for about 2 seconds and quits.so i guess thats working o.k. i still wonder about the no power during g-plug cycle and power to the ops with key off???

[HowieE]

I have a set of 96 schematics and forget about the older trucks sometimes in my comments.
Depending on the year of your truck. 94-95 use start to enigerize the pump as newer truck use on. The Orange wire is battery power on at all times. The pump will run after shutdown till the oil pressure drops below the set point, about 4 lbs.

[catmandoo]

thats the way it works,i thought there was discussion on here awhile back about priming the system after a filter change and some said they could run the pump with the engine off in gear with the key on,but some couldn't