Oil pressure gauge quit, very odd problem.

[windtrader]

The oil gauge in the MC-8, 8V71na, worked fine for years. Recently, just went to zero and doesn't read at all. The engine does have proper oil pressure as shown by mechanical gauge in the rear compartment.


There is only ONE sensor in the block that sits between the engine block and the DN50 alternator. It has the mechanical pressure gauge thin piping and the 2 1/2 break 1509034. There is no separate oil pressure sensor.

The documentation states the break sensor is only on/off so it is impossible to set variable voltage yet it works. AC may have produced some that served both purposes but that is rare.


The bus has two wires in the same area, I assume one for the shutdown and the other for the pressure. The manual shows two, somewhere along the way, the pressure sensor was removed and plugged.

One other thought is the oil pressure gauge uses a pressure sensor located somewhere else. I looked and could not find another sensor in the engine bay. I could test the wire off the gauge to make sure it is the one that goes to break sensor and not somewhere else but this seems unlikely.

[luvrbus]

You should have 3 senders 1 for the tale tell light (3 lbs)one on the engine rail for the overrule and shut down and 1 on the oil manifold on the left side where a manual gage should be mounted ,does the tale tell light work?,just clean your connections on the harness from the rear panel to the frame and the gauge should come back to life unless you lost a ground,I use a jumper wire across the sending unit to check the gauge,if the gauge is bad I have a couple of MCI oil gauges if you need one,there is self-resetting breaker in the circuit too they can go bad   

[windtrader]

Hi Clifford,


Have you ever known the 2 1/2 break on/off low pressure sensor to work for oil pressure too? There is no separate oil pressure sensor.

1 for the tale tell light (3 lbs)one on the engine rail for the overrule and shut down.


1 on the oil manifold on the left side where a manual gage should be mounted ,does the tale tell light work?

The gauge wire works. It did peg the over when I connected it to the sender so the wire and dash gauge seem OK.

The shutdown circuit works as it stalls the engine when I ground that wire.

I don't know how the gauge was working all this time connected to the 3lb sensor unless it was dual purpose, supposedly there may have been some rare ones made by AC Delco.


Thanks for the offer on the gauge.

I am working hard through the remaining items to get on the road! Still hoping to meet you on this trip. Don

[luvrbus]

My MCI 8 had 3 senders ,I still have the OEM (not a copy) manual in the green binder that came with it and it shows the 3 senders ,dealing with MCI you never know they would change wiring from 1 serail number to next

[Dave5Cs]

Don what is a break sensor? Is that a Brake sensor?
Or breaker?

[windtrader]

Hi Dave,


There are usually two sensors - one is the low pressure sensor that shuts down the engine if the pressure falls too low. this is the 2 1/2 lb break  (generally referred to as 3 lb) switch. Its primary purpose is to shut down the engine if there is no oil pressure.


What I dsiscovered is, in rare cases, ONE sensor is dual purpose; it will shutdown the engine when oil pressure is lost ANDS it sends the oil pressure to the gauge. that is what my MC8  8V71na had. All the factory docs show two distinct sensors - one a low pressure switch and the other for oil pressure.


ALL the factory docs indicate two separate sensors. Here is the final state of the oil pressure and low pressure functions.  the first goal was to direct connect the manual gauge to the oil manifold directly but find the required 1/8" NPT compression thread to the gauge was a dead end; at least shopping locally.


Check it image and see that the low pressure ands oil pressure sensors are now seperated. Now, the low oil pressure light goes on when the start switch is on and engine is not running, as it should. Once the engine is started, the oil pressure indicated on the manual and electric gauges are not reading the same.


Given the oil pressure gauge and the low pressure light work as designed, I count that as a win.

[Dave5Cs]

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