Oil pressure gauge trouble

[HDN]

I'm trying to figure out if my oil pressure gauge is shot.

The last drive during cooldown the gauge was reading under what idle pressure should be. Prior to that the gauge was reading properly with the truck parked and driving. I thought the truck was idling lower than usual, which could've had something to do with it, so I finally fixed the tach to see if that was the cause.

I fixed the tach and was getting zero pressure on the gauge. I shut the truck down ASAP and started troubleshooting:

1) Disconnect lead 36 at sender to check gauge operation. Gauge should be at MINIMUM.

Guess what? The gauge was reading at MAXIMUM.

2) Remove gauge cluster, connect negative multimeter lead to good frame or body ground and turn on electrical switch to check voltage at lead 27 at gauge with positive multimeter lead.

I get voltage at the gauge.

Did my oil pressure gauge die with prior symptoms of failure? Is there something else I should check? Will it work right if I reverse the wires? LOL

EDIT: I forgot to mention that when I touch the disconnected oil pressure sender to ground, the pressure gauge reads MINIMUM instead of MAXIMUM.

 

[ToddJK]

Sounds like the gauge took a . I don't think you'd get any readings at all from the sensor if it was bad or if the wire was shot.

 

[HDN]

I'm questioning whether the troubleshooting instructions in the M35A3 TM are correct now. I compared those instructions with the M998 HMMWV and the M939 5-ton:

• For the M939, if lead 36 is disconnected at the sender and the gauge needle deflects when lead 36 is grounded to the frame, the sender is faulty.
• For the M998 HMMWV, the sender is faulty if
  • the gauge reads 120+ PSI when lead 36 is disconnected from the sender
  • the gauge reads zero PSI when lead 36 is grounded to the frame

In this case the gauge is behaving exactly like it does with the M998, and I'm guessing the M939 as the M939 troubleshooting instructions actually don't call out needle position. That said, the M35A3 instructions are the same as the last troubleshooting instructions published for the M35A2. So I'm wondering what's actually correct now!

As if troubleshooting electrical issues wasn't bad enough

 

[HDN]

An update: It appears the oil gauge troubleshooting instructions for the M35A3 are incorrect. The gauge is working properly- I just had to follow the M998 HMMWV troubleshooting procedure to figure that out I'm not sure if this is just a thing with gauges going up to 120 psi. The same wording for the A3 is in the A2 instructions as well.

That said, the sender was giving me over 1 ohm resistance with engine shut down and cool. The manual says that means the sender is bad and needs to be replaced. A past TB says the same.

 

[ToddJK]

An update: It appears the oil gauge troubleshooting instructions for the M35A3 are incorrect. The gauge is working properly- I just had to follow the M998 HMMWV troubleshooting procedure to figure that out I'm not sure if this is just a thing with gauges going up to 120 psi. The same wording for the A3 is in the A2 instructions as well.

That said, the sender was giving me over 1 ohm resistance with engine shut down and cool. The manual says that means the sender is bad and needs to be replaced. A past TB says the same.

That sure is odd, but I know many of the deuces had the 60 psi gauge. If this is surely the case, perhaps there needs to be a pin on that specifically in case others have a similar issue.

 

[HDN]

That sure is odd, but I know many of the deuces had the 60 psi gauge. If this is surely the case, perhaps there needs to be a pin on that specifically in case others have a similar issue.

I kinda feel like the A3 is familiar enough to know a lot about how it works like the A2, but different enough for unfamiliar things like this to happen. That's probably because less than 6000 A3s of four flavors were built.

A new sender is on the way, then I'll try it again and see what happens. I'll probably throw the mechanical gauge on too to see what the pressure actually is. Either way, the old unit is STUCK ON THE MOTOR, so I need to work on that part of the replacement anyway before any more progress

 

[HDN]

An update: I solved the problem! And it was kinda silly how and I still have a sender on the way.

This post in the HMMWV board describes a condition where the nut inside the rubber Packard connection seal was loose and causing an intermittent connection through the sender. It turns out my oil pressure sender had the same problem! A 5/16" nut secures a plastic washer and the Packard connection seal in place on the sender body and also, as it seems, the electrical guts in the sender. This nut may back off the connector and loosen over time. Symptoms of this occurring include an inoperative oil pressure gauge and a loose Packard connection seal that can spin around in-place. Tighten it up with a 5/16" socket, though if you want a better grip on the thing the socket needs to be shaved down on the end to remove the inner taper. Thank you @SublimeMudTime for helping me fix this!

Now my oil pressure reads ~45 PSI at cold idle and ~35 PSI at warm idle. I'm tempted to try the new sender just to see if the readings are any different. My old sender is a little rusty on the bottom, and it seems that the upper threads rusted to the screw-in location, so it might be worth replacing anyway. The nut on the bottom of my sender for screwing it on is 7/8".

If the oil pressure sender is stuck on your M35A3:

1. Unbolt horn from the fender and move out of the way to make more workspace. Also remove the side fold-down panel.
2. Put a 22mm wrench on the nut under the sender and whack at it with a heavy hammer to loosen. I used the 22mm wrench instead of a 7/8" wrench since the metric wrench fits tighter and allows for better impact transfer from the hammer, through the wrench, and thus into the nut to bust it loose.

My findings in summary:

• The 120 PSI oil pressure gauge will show MAXIMUM when disconnected at the sender and MINIMUM when connected at the sender with the electrical accessory switch turned ON. This is opposite of what the M35A3 maintenance TM says, which appears to assume a 60 PSI oil pressure gauge like on the M35A2. It seems to be actually copy-pasted from the last M35A2 maintenance TM.
• The oil pressure sender may be repairable by checking whether the nut within the Packard connection seal is loose. Tighten it with a 5/16" socket and test it on the bench. A working sender should read <1 Ohm at zero PSI with the multimeter's positive lead on the Packard connection and the negative lead on the sender's threads where it screws into the motor.

 

[ToddJK]

An update: I solved the problem! And it was kinda silly how and I still have a sender on the way.

This post in the HMMWV board describes a condition where the nut inside the rubber Packard connection seal was loose and causing an intermittent connection through the sender. It turns out my oil pressure sender had the same problem! A 5/16" nut secures a plastic washer and the Packard connection seal in place on the sender body and also, as it seems, the electrical guts in the sender. This nut may back off the connector and loosen over time. Symptoms of this occurring include an inoperative oil pressure gauge and a loose Packard connection seal that can spin around in-place. Tighten it up with a 5/16" socket, though if you want a better grip on the thing the socket needs to be shaved down on the end to remove the inner taper. Thank you @SublimeMudTime for helping me fix this!

Now my oil pressure reads ~45 PSI at cold idle and ~35 PSI at warm idle. I'm tempted to try the new sender just to see if the readings are any different. My old sender is a little rusty on the bottom, and it seems that the upper threads rusted to the screw-in location, so it might be worth replacing anyway. The nut on the bottom of my sender for screwing it on is 7/8".

If the oil pressure sender is stuck on your M35A3:

1. Unbolt horn from the fender and move out of the way to make more workspace. Also remove the side fold-down panel.
2. Put a 22mm wrench on the nut under the sender and whack at it with a heavy hammer to loosen. I used the 22mm wrench instead of a 7/8" wrench since the metric wrench fits tighter and allows for better impact transfer from the hammer, through the wrench, and thus into the nut to bust it loose.

My findings in summary:

• The 120 PSI oil pressure gauge will show MAXIMUM when disconnected at the sender and MINIMUM when connected at the sender with the electrical accessory switch turned ON. This is opposite of what the M35A3 maintenance TM says, which appears to assume a 60 PSI oil pressure gauge like on the M35A2. It seems to be actually copy-pasted from the last M35A2 maintenance TM.
• The oil pressure sender may be repairable by checking whether the nut within the Packard connection seal is loose. Tighten it with a 5/16" socket and test it on the bench. A working sender should read <1 Ohm at zero PSI with the multimeter's positive lead on the Packard connection and the negative lead on the sender's threads where it screws into the motor.

Glad you figured it out. It's funny how a problem can be the darnest thing, lol.

 

[HDN]

I put everything back together and took the truck for a test run. I have oil pressure and it's within spec! It seems that up to this point I wasn't getting accurate oil pressure readings. Previously the oil pressure gauge would read 60-75 PSI heading down the road while still somewhat warming the motor up, then get down to between 35 and 50 PSI after warmup and driving around in locked 4th gear and 40 MPH (according to speedometer). Now I'm reading about 45-50 PSI while doing 40 MPH and a little under 30 PSI to 35 PSI hot idle.

On an unrelated note, I learned that there's really no difference in sound volume in the cab after removing most of the rubber floor mats to remove the toe board and loosen the mid driveline tunnel. They might keep the cab warmer in the winter, but I'm not driving the truck when it's that cold out anyway. I guess the A3 muffler is doing most of the... muffling