M1088A1 Starting Issue

[Ronmar]

I believe matthew is correct, 15PSi low oil switch(S27) is on a little manifold on the drivers side of the block a little above the oil pan just forward of the starter...

That provides the ground path to energize K11 and the dash low oil idiot light. When it hits 15PSi it opens the circuit to ground, puts out the dash light and de-energizes K11 which shifts control power from the start switches to the alternator.

 

[BeetTrucker]

Is this what you are referring to?

 

[BeetTrucker]

This is the one that I got.

 

[MatthewWBailey]

Is this what you are referring to?View attachment 933992

I have a 2004 A1 with '99 dated 3126 and I have the same One-Wire oil sender. The big angled one is the sender for the psi gauge. The one below it is the switch which activates the idiot light and K11. Trouble is, Cat is sourcing the 3 wire sender which seems to be off later models and C7's. I'm not sure how to wire in a 3wire sender to the one wire trucks. That's a @Ronmar or @GeneralDisorder question or other masters on here.

BUT, it's the pressure switch that needs replacing if you're not getting K11. Unless, as I my case, it's not fully connected. Check the wire connector and make sure it's fully plugged in and not corroded. Those plugs are tricky.
In this photo, the device in the lower right is the psi switch. It's connector is not in frame but it's a pesky little bugger to get snapped together properly.

 

[Ronmar]

That silver one is the gauge sender. The switch Is in the background of your pic(black plastic body against the block, harness toward the rear). Also seen lower right of Matthews pic, against the block, painted yellow.

that parts picture you added of the 3 wire oil pressure sensor, looks like it may be an oil pressure sensor for the HEUI system(found up on the HEUI pump?

the part that will cause no idiot light/no crank is a simple 15PSI oil switch. Its not a cat part, the ones used on the LMTV were made by Nason. If you unplug yours and jump across the two pins in the truck side cable you should get a low oil idiot light in the dash when you turn on the ignition…

can be replaced with a common 15PSI low oil pressure sw. The Nason has 2 leads, one connects to ground and the other runs up into the dash wiring. A standard single terminal sw will work, you just need to get the correct wire of the two (one that feeds up to the cab) attached to the single sw terminal and it will ground thru the switch body to the block like most auto applications do… the wire you want is the one that lights the oil light when grounded(ign sw on)…

 

[BeetTrucker]

The one I’m pointing to in the first picture? The second picture is the plug coming off of it. Is there a replacement that you know of that is plug and play? I replaced the silver oil pressure sending unit above it with no luck.

That silver one is the gauge sender. The switch Is in the background of your pic(black plastic body against the block, harness toward the rear). Also seen lower right of Matthews pic, against the block, painted yellow.

that parts picture you added of the 3 wire oil pressure sensor, looks like it may be an oil pressure sensor for the HEUI system(found up on the HEUI pump?

the part that will cause no idiot light/no crank is a simple 15PSI oil switch. Its not a cat part, the ones used on the LMTV were made by Nason. If you unplug yours and jump across the two pins in the truck side cable you should get a low oil idiot light in the dash when you turn on the ignition…

can be replaced with a common 15PSI low oil pressure sw. The Nason has 2 leads, one connects to ground and the other runs up into the dash wiring. A standard single terminal sw will work, you just need to get the correct wire of the two (one that feeds up to the cab) attached to the single sw terminal and it will ground thru the switch body to the block like most auto applications do… the wire you want is the one that lights the oil light when grounded(ign sw on)…

 

[Ronmar]

Yep that's it. Plug and play? other than the exact same nason switch provided by one of our resident suppliers, no...

There is nothing special about it, it is a normally closed 15PSI low oil pressure switch(completes a circuit until it sees >15PSI). The 2 wires in the truck side plug go to ground and the dash respectively and that is all it does, provide a circuit to ground when the oil pressure is below 15 PSI. These are typically one wire switches with the ground path provided by the switch screwed into the engine block(just about every engine ever created). Its 2 wire config is what makes it unique...

have you confirmed it is actually bad, and not some other wiring issue? on the truck side connector plug for that switch, use a wire to connect between the two pins. Turn on the ignition switch and you should see a low oil light in the dash and if the trans is powered and in neutral, be able to crank the truck. That will confirm the rest of the circuit is OK and that it is indeed a oil pressure switch issue.

You can replace it with a 1/8" NPT single terminal 15PSi low oil pressure switch, probably on the shelf at just about any auto parts store or online(search ebay/amazon, common item). Install the new switch in its 1/8" port, Cut the pigtail connector off right at the old switch and strip the ends of the 2 wires.

Like the above test, ignition on, connect one of the wires to the new switch terminal. if the oil light in the dash lights, that's your wire, install a terminal lug on it and attach it to the new switch connection terminal. if it doesnt light, try the other wire. once you find the correct wire, tape over or cut off the unused wire...

 

[BeetTrucker]

Yep that's it. Plug and play? other than the exact same nason switch provided by one of our resident suppliers, no...

There is nothing special about it, it is a normally closed 15PSI low oil pressure switch(completes a circuit until it sees >15PSI). The 2 wires in the truck side plug go to ground and the dash respectively and that is all it does, provide a circuit to ground when the oil pressure is below 15 PSI. These are typically one wire switches with the ground path provided by the switch screwed into the engine block(just about every engine ever created). Its 2 wire config is what makes it unique...

have you confirmed it is actually bad, and not some other wiring issue? on the truck side connector plug for that switch, use a wire to connect between the two pins. Turn on the ignition switch and you should see a low oil light in the dash and if the trans is powered and in neutral, be able to crank the truck. That will confirm the rest of the circuit is OK and that it is indeed a oil pressure switch issue.

You can replace it with a 1/8" NPT single terminal 15PSi low oil pressure switch, probably on the shelf at just about any auto parts store or online(search ebay/amazon, common item). Install the new switch in its 1/8" port, Cut the pigtail connector off right at the old switch and strip the ends of the 2 wires.

Like the above test, ignition on, connect one of the wires to the new switch terminal. if the oil light in the dash lights, that's your wire, install a terminal lug on it and attach it to the new switch connection terminal. if it doesnt light, try the other wire. once you find the correct wire, tape over or cut off the unused wire...

Yes, i verified by connecting both pins on truck side and the oil pressure dummy light came on. I found and ordered one. Hoping to finally put this issue to rest. Will keep you all posted.

 

[BeetTrucker]

On a different note, I wasn’t getting a prime at the primer assembly and I believe it was your YouTube video Ronmar that helped me resolve that issue by clearing debris from the flap inside the assembly. I was able to get a prime in 18 pumps. But now it is running extremely rough and valves nailing. Could this be air in the system from that flapper not seating properly or am I dealing with a different issue?

 

[Ronmar]

That flapper only effects primer function, it opens when running to allow full flow/bypass of that primer bulb.

you may have an air leak sucking air into the system. You need to hook up a primary fuel pressure gauge to test pri pressure. I show where in my fuel sys overview vid and i show what an air leak looks like on a primary gauge in another vid.

 

[BeetTrucker]

I will check them out, thanks!