Hydraulic pressure warning 742B

wings5j · Nov 13, 2010

Hi to all,
All of my hydraulics seem to be working fine but it seems like I am seeing the hydraulic warning light more often and more steady than before. The hydraulic filters are fairly new (guessing about 150 hours on them).
Tazza has made some interesting posts about the "bronze filter". This I have never looked into. Could it have an effect on the "idiot light".
Thanks, John

jerry · Nov 13, 2010

I think your light is for hyd oil temp and charge pressure so since the bronze filter is in the suction line, if I remember right, it could affect charge pressure.

wings5j · Nov 13, 2010

jerry said:
I think your light is for hyd oil temp and charge pressure so since the bronze filter is in the suction line, if I remember right, it could affect charge pressure.

Thanks Jerry,
I won't be home for a few days, but when I do, I think I will clean or put in a new bronze filter. Certainly that would be good to do just on general principles. ;-)
John

Tazza · Nov 14, 2010

wings5j said:
Thanks Jerry,
I won't be home for a few days, but when I do, I think I will clean or put in a new bronze filter. Certainly that would be good to do just on general principles. ;-)
John

Just remember, not all machines had this setup. Its only there if you have the square block on the hydraulic pump. Middle hose on the left. If it has a fat one that goes to a block on the chassis, it has no filter.
Hopefully its just the filter, nice and simple fix.

frogfarmer · Nov 15, 2010

Tazza said:
Just remember, not all machines had this setup. Its only there if you have the square block on the hydraulic pump. Middle hose on the left. If it has a fat one that goes to a block on the chassis, it has no filter.
Hopefully its just the filter, nice and simple fix.

Hydrostat pumps get expensive quick if they lose their oil. I would run the machine a minimal amout until the cause is determined. Your lift and tilt operate seperately of the hydro system and may seem fine even if the hydros are starving.

wings5j · Nov 25, 2010

frogfarmer said:
Hydrostat pumps get expensive quick if they lose their oil. I would run the machine a minimal amout until the cause is determined. Your lift and tilt operate seperately of the hydro system and may seem fine even if the hydros are starving.

Well, I made it home and had a little bit of time to investigate. Per the parts book, it looks like my s/n (11381) would have the bronze filter, but after raising the cab, it appears that I have the newer style port block and no bronze filter.
I stopped by the dealer and talked with one of the technicians. He said there are three sensors in series, a differential pressure sensor at the main filter mount; a pre-charge sensor at the hydrostat; and a temp sensor at the port block. From the way I described its operation, he didn't think I had any pre-charge pressure problem.
I changed out the main filter to eliminate it from the equation. The technician also said that one of the sensors could be bad so I will at least need to check these out and hopefully by process of elimination will get it sorted out.
We are just getting into the snow season, so I would sure hate to have anything serious go bad.
The service manual does not seem to have any troubleshooting section related to the warning light so I guess I will have to try and follow out the wiring diagram and go from there. Any other advice from you guys would be appreciated.
John

frogfarmer · Nov 25, 2010

wings5j said:
Well, I made it home and had a little bit of time to investigate. Per the parts book, it looks like my s/n (11381) would have the bronze filter, but after raising the cab, it appears that I have the newer style port block and no bronze filter.
I stopped by the dealer and talked with one of the technicians. He said there are three sensors in series, a differential pressure sensor at the main filter mount; a pre-charge sensor at the hydrostat; and a temp sensor at the port block. From the way I described its operation, he didn't think I had any pre-charge pressure problem.
I changed out the main filter to eliminate it from the equation. The technician also said that one of the sensors could be bad so I will at least need to check these out and hopefully by process of elimination will get it sorted out.
We are just getting into the snow season, so I would sure hate to have anything serious go bad.
The service manual does not seem to have any troubleshooting section related to the warning light so I guess I will have to try and follow out the wiring diagram and go from there. Any other advice from you guys would be appreciated.
John

If any of the sensors are leaking that can be a sign of a failed and possibly shorted sensor. Each sensor can be bypassed in the system to verify which one is the culprit. A good initial test would be to verify charge pressure with a guage. If everything checks out the wiring may be chaffed and shorting and that can be a real delight to find. The wiring on that machine is very minimal and should be easy to diagnose.

wings5j · Dec 24, 2010

frogfarmer said:
If any of the sensors are leaking that can be a sign of a failed and possibly shorted sensor. Each sensor can be bypassed in the system to verify which one is the culprit. A good initial test would be to verify charge pressure with a guage. If everything checks out the wiring may be chaffed and shorting and that can be a real delight to find. The wiring on that machine is very minimal and should be easy to diagnose.

Thanks again for the help, sorry it has taken so long to post again. If I am at home, I don't seem to have time to interract on the forum.
Today I am out of town with time for the computer but my Bobcat is 2,000 miles away. Once I get home, I will try to trace the wiring and go from there. Hopefully you all won't get tired of this thread before I get it figured out. Thanks for your patience.
John

under the bus · Dec 25, 2010

wings5j said:
Thanks again for the help, sorry it has taken so long to post again. If I am at home, I don't seem to have time to interract on the forum.
Today I am out of town with time for the computer but my Bobcat is 2,000 miles away. Once I get home, I will try to trace the wiring and go from there. Hopefully you all won't get tired of this thread before I get it figured out. Thanks for your patience.
John

on your machine there are 3 sending units that will light the light . 1 charge pressure switch located under the seat on top of the hydrostat.2 high oil temp located on the port block. 3 plugged filter switch located on the filter mounting housing.

wings5j · Dec 26, 2010

under the bus said:
on your machine there are 3 sending units that will light the light . 1 charge pressure switch located under the seat on top of the hydrostat.2 high oil temp located on the port block. 3 plugged filter switch located on the filter mounting housing.

Yes, I will have to isolate each one and verify their individual condition. Time to brush up on my wiring schematic reading schools and get out my VOM and go to work.
Thanks for the help. John

under the bus · Dec 26, 2010

wings5j said:
Yes, I will have to isolate each one and verify their individual condition. Time to brush up on my wiring schematic reading schools and get out my VOM and go to work.
Thanks for the help. John

should be an easy check out . hopefully it will be a loose wire or a sending unit.

wings5j · Jan 25, 2011

under the bus said:
should be an easy check out . hopefully it will be a loose wire or a sending unit.

Here's the latest,
I confirmed the pressure differential sensor on the filter housing was bad and had Bobcat send me a new one and will put this on when I get home.
I decided I should check out the other two sensors while I was at it. My wiring diagram does not show the sensors. When I lift the cab I can see the pre-charge sensor on top of the hdrostat. It has two wires connected to two seperate terminals (Are these supposed to be "normally closed" between them?) and I am not sure of the basics on this one.
As to the temp sensor on the port block, I have not located it yet.
I am surprised that my factory service manual does not include this system in the wiring diagrams. I would appreciate any more advice you might have.
Thanks,
John

Tazza · Jan 26, 2011

wings5j said:
Here's the latest,
I confirmed the pressure differential sensor on the filter housing was bad and had Bobcat send me a new one and will put this on when I get home.
I decided I should check out the other two sensors while I was at it. My wiring diagram does not show the sensors. When I lift the cab I can see the pre-charge sensor on top of the hdrostat. It has two wires connected to two seperate terminals (Are these supposed to be "normally closed" between them?) and I am not sure of the basics on this one.
As to the temp sensor on the port block, I have not located it yet.
I am surprised that my factory service manual does not include this system in the wiring diagrams. I would appreciate any more advice you might have.
Thanks,
John

I believe it is indeed a NC connection.
The temperature sensor shouldn't be on the port block, check where the fat hose goes from the hydraulic pump. It should be screwed in there. Its simply wired in parallel to the pressure sender. When it gets hot, it closes.

Hydraulic pressure warning 742B

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