D6H fuel pressure sensor issue

[BuckWild]

This is my first post, so just want to first thank all of you for this forum. It has already helped me A LOT! I can usually snoop my way through this forum and find the info to keep my going, but I finally have one I need to ask directly...so here goes!

The problem I'm having is on our D6H serial number 9KJ01166. I have not owned it long and have been slowly fixing a few things that have been neglected. The fuel pressure sensor, that is mounted on the secondary filter housing, did not have its wires connected. They were both hanging there like someone or something pulled them out of their terminals.

Apparently this sensor is normally closed and fuel pressure opens the circuit, and this sensor showing fuel pressure is what tells the dash monitor the engine is running, therefore if it thinks there is fuel pressure with the key on, engine not running, the oil pressure warning light will come on and alarm/buzzer will sound, which is exactly what happens, sense the wires are pulled off the sensor. (open circuit). Of course as soon as I fire the engine, oil pressure comes up and the alarms go off.

I can connect these two wires with key on, engine off, and the oil pressure warnings do not sound. This tells me the circuits are good, but once the engine is running, I should get some sort of warning light for no/low fuel pressure due to the wires being tied together, but I do not. Actually, I would assume as soon as I turn the key on, I should see a light for low/no fuel pressure until I start the engine. I thought maybe the wires have been pulled off the sensor for a LONG time and the warning bulb might be shot from being on all the time, but the lamp test switch lights up all bulbs. (They are probably LED anyways since its a circuit board in there).

So basically the dash monitor knows when there is an open or closed circuit on the fuel pressure circuit, as demonstrated by the low oil pressure warning, but yet does not display a low/no fuel pressure warning when the circuit is closed. Anyone have any insight on this?

Also, some of the lights on the monitor are not labeled with an image of any kind, I have ordered a operator/maintenance manual from the dealer, but if anyone has that info on hand and wouldn't mind sharing it I would much appreciate it. They make the manuals as needed so said it could be a few weeks until it came in.

Thanks in advance!

 

[Nige]

If you're not totally committed to obtaining a paper O&M Manual from the dealer, why not just download the same document in Adobe pdf format direct from the Cat Publications website..? Detailed instructions are available if you're interested. Should cost about $45.

From your screen shots it appears as though you have the SENR4955 Electrical Schematic for the machine. Is that correct.?

I need to read through what you've posted again, but IMO in your assumptions of how the system OUGHT to operate you are crediting the Monitor System with a level of intelligence that it doesn't have, especially in a tractor that was built 25 years ago when electronics were nowhere near the all-singing all-dancing systems they are today.

 

[Cmark]

The monitor panel is used in lots of different machines with the only difference being which switches are wired to it and the sticker on the front showing which LEDs are used in that particular application. Your machine doesn't monitor fuel pressure as a warning.
This is what it should look like. If your sticker for the monitor is missing, it's part number 6V7606.

 

[BuckWild]

Thanks Nige. Honestly I didnt know I could download that, but I've already ordered the paper manual. It was only 55 bucks and my ol man is partners with me in business, and let's just say paper books are more his style lol. So at least he will be able to find info if he needs it.

Yes that is the correct schematic. Believe it or not I found it online by searching google! One of the very few bits of CAT info I found online for free.

I may have given the monitor more credit than I meant to with my wording. What I assumed was; the pressure sensor being in the open circuit condition would open/close a simple relay that fed the oil alarm system. I didnt see any relays anywhere, so I thought there may be some transistors in that monitor instead. (I think I may be back to giving it too much credit again)

 

[BuckWild]

Thanks Cmark.

If it doesn't monitor it for warning, what is it there for? Also my dash looks a bit different, probably because it's a cab machine which I forgot to mention. I will try to attach a picture.

 

[Nige]

Cab or no cab, the dash panel will be identical. What you have in your photo as regards warnings is the same as the illustration that CMark posted from the Operator Manual which are for instructional purposes rather than a faithful representation of what's physically on the monitor panel in the tractor. The icons indicate which system is alarming. Google "Pictographic Symbols" and you should find all of them.

LH column top to bottom - Coolant temp, eng oil press, hyd oil temp
RH Column - trans temp, alt charging, trans oil filter

I've had a bit of a look at the Electrical Schematic and I'm going to have a stab at the function of the switches for fuel pressure and oil pressure and the alarms they might generate, especially regarding the fuel pressure switch.

I think the switch on the fuel filter head might be to generate a "Low Fuel Pressure" signal when the engine is running that would indicate a plugged fuel filter or filter, and maybe it is also intended purely to function as an indicator that the engine is actually running in order to generate a genuine low oil pressure alarm. Thinking about it a low engine oil pressure alarm with the engine stopped is nothing but a nuisance alarm.

So the oil pressure switch (OPS) is Normally Open, the fuel pressure switch (FPS) is Normally Closed when the engine is stopped.
The logic inside the Monitor Panel with the Key Switch in the ON position would be simple if it works the way I think it might. Again you have to remember we're talking about a 25-year old machine here. The logic is probably pretty basic: -

a) OPS open + FPS closed = engine not running - no alarm
b) OPS closed + FPS open = engine running normally (positive oil & fuel pressure) - no alarm
c) OPS closed + FPS closed = engine running with low fuel pressure - alarm (for plugged fuel filter)
d) OPS open + FPS open = engine running with low oil pressure - alarm (LH column centre LED) for low engine oil pressure.

Of the four lines above the only one I can find no reference to in the Operator Manual is an alarm for low fuel pressure. So the alternative explanation in that case would be that the sole function of the fuel pressure switch is to detect whether the engine is running or not. That would also make sense.

 

[BuckWild]

Nige, I think your dead on. Cmark mentioned this machine does not have a low fuel pressure warning, so its sole purpose must be prevent the low oil alarm from going off while key is on, engine off. Makes good sense beings the engine has no "true" logic or sensors, such as crank or cam, to know it's running. Just a good ol on off switch operated by the pressure increase that will only be there if the mechanical pump is turning (engine running).

So everything is working as it should. I reconnected the wires and I will probably add a pressure or restriction gauge (maybe both). I could have done this anyways, but I'd really like to keep all the factory system working correctly. Thanks so much for your help on this!