BB AARE Cummins 8.3 Oil Pressure Sensor

[miscrms]

My next debugging adventure is to try and get the oil pressure gauge working on my bus. It’s a ‘97 Blue Bird All American rear engine with a mechanical 12v Cummins 8.3L.

When I bought it the oil pressure gauges (dash and engine compartment) seemed to work intermittently, but were pegged high most of the time. It does go to zero with the engine off. Occasionally it will start working and show a reasonable pressure at idle. The dealer swapped the gauge, but that didn’t help so they ordered me a replacement sensor from Cummins. Swapping that out didn’t help either, though I did notice a lot of oil in the electrical connector which made me wonder if the old sensor was leaking.

Unfortunately I haven’t found a wiring diagram for the the sensor circuit yet. I found a service manual online for a ‘99, but that didn’t seem to have it. It did at least have a table of sensor resistance vs. pressure, which was 10 Ohms for 0psi and 184 Ohms for 150psi. Assuming mine should be similar, but not sure.

So far this is what I’ve been able to trace out:



Unfortunately that’s not a lot to go on [emoji6] I believe the blue connector on the front gauge is power for the illumination lamp. White on both seems to be ground. When running and pegged I read ~12.5V on one of the remaining screw lugs, and ~13.5V on the other. Both gauges read the same voltages. The lamp on the dash gauge does work when the headlights are on, and the blue wire connected to the spade connector also reads ~13.5v. So ground at that gauge at least seems to be good.

I’m assuming there is a problem in the wiring between the sensor and the gauges. Unfortunately the sensor wires are probably not original, and are both just unmarked black wire which quickly disappear into a harness. The wires from the dash gauge are nicely labeled, but end at the main chassis connector, and disappear into the harness. The rear gauge wires also quickly disappear into a harness. It’s nice everything is so tidy, but makes it a challenge to chase. I’m assuming I’ll have to start breaking open some of those harnesses to figure out where things go, and then start disconnecting and ohming things out to find the fault.

Rob


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[somewhereinusa]

First thing would be to hook up a mechanical gauge to see what the oil pressure actually is. Since you have arear gauge it should be quite simple to run a new wire from sender to rear gauge as a test. Use jumpers to eliminate all bus wiring.

[cadillackid]

also make sure they didnt send you an oil pressure switch.. if you can have the engine off, key ON, and your opil pressure reads 0. but reads pegged when you start the en gine, then it almost sounds like they sent you an oil pressure light switch.. or the connectors at the engine are reversed (if the engine has a light and a gauge)..

-Christopher

[miscrms]

Well Christopher, you might have got it. Looking up the part number (3926401) I'm seeing descriptions like oil pressure switch. Digging a little more, I'm seeing lots of posts indicating Cummins had issues with the oil pressure sensors on the 5.9/8.3, and recommended a retrofit with a switch. This included a harness change from a 3 wire to a 2 wire connector. That would explain why my sensor wiring didn't look original, I have the 2 wire connector. So even if I could find the long out of production sensor, it wouldn't connect in any more.

So it's likely that it's actually working more or less as intended, Cummins just turned the gauge into a glorified low oil pressure idiot light. Bummer.

I can probably test this by just shorting the two terminals of the connector together to see if it goes to ~0. When open / disconnected it reads high.

Rob

[cadillackid]

the wires are likely still there just a few inches back in the harness.. so you could peel back the sheath and likely find the 3rd wire.. someomne woiuldve just choppped off the original connector somewhere close to the end and spliced a new 2 wire in.. you could order a 3 wire sender with connector.. sometimes called a harness repair kit.. (at least thats what navistar calls connectors with pigtails, im guessing others are similar).
-Christopher

[miscrms]

Thanks, I’ll look into it. Could be trickier since I think it’s no longer offered by Cummins but it seems like there might be some aftermarket options.


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[TJones]

You should be able to replace both the sender and gauge with an after market kit and use the factory wiring.

[miscrms]

Possibly. I️ may even be able to use the built in gauges assuming I can figure out how the wiring was modified.

I’ve found documentation that both a ‘98-‘03 and a ‘10 BB AARE Cummins used a 150psi 10-180 Ohm sender. The 2010 manual calls out a VDO part number for the sender. Not 100% sure the ‘97 uses the same, as there was a big model change in the ‘98 time frame including switching to the ISC electronic 24v engine and a more digital dash. But it seems pretty likely the basic oil pressure sender/gauge setup was carried over from earlier designs, as the BB electronic dash controller apparently had to have a special bit to select an analog pressure sender input for oil pressure when using the Cummins engine.

150 psi 10-180 Ohm seems to be a common standard for oil pressure senders/gauges, so the chances of finding a generic replacement are probably pretty good.

Picked up a mechanical oil pressure gauge with a bunch of adapters, so hopefully tonight I can verify what the actual pressure is, and what thread the port takes. Then I need to open up the harness and see if I can figure out where the wiring was modified.

Rob


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[miscrms]

So, good news I think on the pressures and possibility of getting a universal replacement. Cummins was apparently kind enough to use 1/4” npt threads on the port, so I didn’t even need an adapter on the test gauge. Pressure started at about 70psi cold idle, was down to about 20psi warm but not fully up to temp. Takes a long time to get up to temp idling, didn’t have that long. At 2200 rpm the spec is 35-50psi, I read 65psi but again not fully warmed up. So seems pretty reasonable?

The less good news is the wiring is still a mystery. Putting a 1 Ohm resistor into the sensor connector had no effect on the gauges, so may still have a broken wire on top of the wiring changes made to swap from sensor to switch? Hopefully this weekend I can disconnect the rear gauges and play with them directly. Hoping the connections are just the basic bulb, +, ground and sensor to ground. Should be able to test by removing what I think should be the sensor wire (purple) and connecting various resistors to the ground pin.

Now that I know the thread, I may try to get a sensor here by the weekend as well to play with. Even if it doesn’t have the correct 6psi low warning contact just getting the gauges working would be an improvement.

Sorting out the existing wiring will likely be a pain, all the harness connections go behind the engine which are very hard to see/access on the 8.3 in the BB AARE.


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[somewhereinusa]

I just checked my wiring diagram 91 AARE, the oil pressure sender wire is indeed the purple one. The diagram I have was drawn in 1988 with last change done int in 1990. They really don't change all that much. It's the one the dealer sent for my 91.

[somewhereinusa]

I have a chassis diagram dated 1991. I can send them if needed.

[miscrms]

Originally Posted by somewhereinusa

I have a chassis diagram dated 1991. I can send them if needed.

Sent you a pm.



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[cadillackid]

I thoiught it was a 3 wire sensor and you are testing with only 2 wires back to your gauge.. you may need the 3rd wire as a reference..
-Christopher

[miscrms]

That is correct. Part of the hypothesis though is that the gauge was showing high when the switch is open and low when it’s closed. Unless my 1 Ohm was too high to simulate closed that doesn’t seem to be the case. That makes me wonder if there is the a break in the sensor wire somewhere between the gauge and the sensor. Or I am not understanding the wiring correctly yet.


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[miscrms]

From the generic examples I’ve seen, a 3 wire sender is usually ground, sense, and warning. With sense being resistor to ground that varies with pressure, and warning being a switch to ground that closes below some set pressure limit. My thought was that sense and warn wires had been combined in the harness, and that combined signal and ground were the 2 wires coming to the replacement switch. That might not be right though.


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[miscrms]

Here’s kind of what I’m assuming:



This would be a two wire case, with the second wire just being the case/chassis in this example. If it’s a sensor/sender you would get proportional output on the gauge, if it’s a switch you would just get low high.

A three wire would have both a proportional resistive terminal and a switch in addition to ground. Like this fairly generic one I just ordered from Amazon, so I’ll have something to try this weekend:

VDO 360 025 Gauge Pressure Sender https://www.amazon.com/dp/B001P71WZO..._ExqbAb55SFSJ8

The 11psi switch is a little high, but I’m mostly interested in the sensor, which should be the correct 150psi 10-180 Ohm type. I’m still confused about the threading, but hopefully it will work. The HF gauge says it’s 1/8 27 NPT, and most of the adapters say they are 1/8 27 NPT female. The existing pressure switch I removed threads into those correctly, and the gauge threaded into the engine port correctly. However the one adapter that says it’s 1/8 27 NPT male is considerably smaller than the hose or the removed sensor fittings. Hopefully it’s just a typo on that one and it’s really 1/16th or something.




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[miscrms]

Well I was at least able to confirm the basic operation of the gauge. With the key at IGN On and the purple wire disconnected it’s at max, and goes to zero when that lug is shorted to ground. When I put in 100 Ohms to ground it goes to just under 120psi so the 10-180 Ohm scale seems about right, like the later ones. I also got the calipers out and confirmed the sender I took out is 1/8” npt.

The wiring is still a mystery, but hopefully I can get somewhere on that this weekend. Worst case I now know the sensor I ordered should work with one new wire run from the sender to the gauge.





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[somewhereinusa]

Did the wiring diagrams come through?

[miscrms]

Originally Posted by somewhereinusa

Did the wiring diagrams come through?

Got it, thanks very much!



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[miscrms]

Interesting, first observation is the rear oil pressure gauge is only powered when the ignition switch is set to rear. That explains why I was sometimes seeing it get power, and sometimes not

There are also two separate senders shown for the front and rear gauges!




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