1991 Bobcat 753 auxiliary hydraulics not working

[Trapper Dan]

Trapper Dan here from the Yukon. I recently bought a 1991 Bobcat 753 skid steer from Hinton Alberta and hauled it back here with a bunch of attachments. I have a farm with enough driveway to make you tired just walking it and my master plan was to use a machine to help around here rather than my back.

Everything worked as it was suppose to for the first three days once I got up the highway and then it started to go pear shaped. The auxiliary hydraulics has two LED's in the top right corner of the cab that come on with a switch just below it. Push the switch once and one light comes on and you can hear the pump engaging and everything worked for the first two outhouse holes I drilled. If you push the same button twice, the second light would come on and put the pump into "float" mode or at least that's what I assumed. I used this with the power broom attachment so the bucket wouldn't dig into the ground when I was cleaning up debris.

Everything worked for the first couple of days and then the light became temper mental ( or maybe I was just becoming more mental with my temper ) and the switch didn't allow me to turn on or off the light unless if it sat for a few hours completely shut down. At one time there was a code of HP 4 meaning "hydraulic pressure broken wire" but since then it hasn't decided to turn on at all by itself. I looked for the illusive "fuse box" but it doesn't have one that I can see. There's two fuses suspended off a tab in the engine compartment above the battery on the left side and two repays in the same position. I checked both fuses and they're good and I switched the relays over. I also opened all of the wiring looms from the ignition switch on the left side in the cab and traced everything to the right side where the auxiliary hydraulics button is. I cleaned every connector along the way and put dielectric grease in every singe connector throughout the entire machine, under the seat and otherwise.

There's no obvious broken wires to be seen and I also opened up the controller hand lever and cleaned up the contact points in the control head. The only other places I can see that may have mystery electrical parts in on the back side of the seat where the wiring harness terminates into an epoxy sealed computer board and also the same type of computer board on the right hand side of the frame under the cab when it's flipped up. I'm at a loss where to look now or how to test the modules.

I've had it for less than a month and only got three days of use out of it and winter will be shortly and I don't want to be working on this thing in the cold when I need to use the hydraulics for the snow blade. Any advise would be appreciated.

 

[fortworth-cowboy]

Can you take a picture of the Lights from the AUX hydraulics.
Where did you get the code HP4 from?
Do you have a serial from the machine?

 

[Trapper Dan]

Can you take a picture of the Lights from the AUX hydraulics.
Where did you get the code HP4 from?
Do you have a serial from the machine?

The HP 4 code shows up on the lcd screen when I first start it and the switch just below the lcd is where the button with the LED lights are that indicate when the auxiliary hydraulics become functional after you push the button. It all worked as it should have at one time.

 

[Trapper Dan]

 

[Trapper Dan]

Does anyone have a used PWM 6668773 or 6673757 PWM ?

 

[KyMadMan]

Does anyone have a used PWM 6668773 or 6673757 PWM ?

Check with Isoscell. I believe they do repair work on these as well as the B.O.S.S. units.

 

[Trapper Dan]

After many, many days of working on this unit I finally took it into the dealership and had a technician check it out. He came up with a failing PWM being most likely what’s causing these problems. There wasn’t another PWM unit to test this one against in their inventory, but was 99% sure this is the culprit since all the solenoids, fuses, wiring, relays, joystick etc checks out.

I ordered a new one one through the dealership and it’s on its way now and I’ll let you know if that fixes a problem. I was hoping they could be 100% certain that this is what’s causing all these issues by swapping out a used one to double check before I put down money for the new one since it’s one of those “nonrefundable “type of purchases. It should be in later this week and fingers crossed it all works.

I’ve got a lot of work lined up with a tree planner and grapple bucket and I’d like to get this machine making money rather than costing me time and money.