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Where ever you are! I have a 1994 312 A cat excavator,that i…

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Hi there where ever you...
Hi there where ever you are! I have a 1994 312 A cat excavator,that i am having issues with,I had a local Cat mechanic out and tracing all he was able to do on site, broken wires,dry joints, computer function,but could not solve the problems which seem to have evolved .The auto power up has stopped operating, or the decell is not working with the right hand button,originally noticed on occasion that it would not always power up on the pedals to walk, mainly on reverse mode. The governor/activator works fine in manual mode,the numbers show up on the dash whenyou turn the rotating switch from 1 to 10,but shows I assume a fault code 'H' or 'A' on the screen,have been having issues with moisture in the hydraulic oil and it recently threw the fan belts off, as I noticed the engine temp rising switched off before just hitting the red. The authorised Cat mechanic did every thing he could with no solution , I would be very appreciative if you are able to help .Thank YOU Richard. PS.the mechanic was only 6 years as a Cat mechanic and didn't know what the problem codes were?
Submitted: 5 years ago.Category: Construction and Road Equipment
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Answered in 4 hours by:
3/3/2017
Mechanic: catmastertech, Technician replied 5 years ago
catmastertech
catmastertech, Technician
Category: Construction and Road Equipment
Satisfied Customers: 4,036
Experience: Field service technician for CAT 19 years.
Verified

Hello,

I would agree the problems your are having are issues that have just added up over time.

The first thing I would address the letter codes you are seeing. The H means the backup mode switch for the hydraulic control is on. This is the single switch in the rear of the left console (if my memory is correct). The A is for the engine throttle backup mode switch is on. This is the two switches in the back of the other console. One switch turns on the backup mode and the other switch is for a manual control of the throttle. These switches should always be off or in AUTO mode. They should only be on for testing or when override of the ECM is needed.

The loss of engine speed when traveling is likely caused by the pressure switches that signal the ECM when you move the travel levers have failed or the wires are damaged.

The auto engine speed control also has a switch for the implement functions when you are using the joysticks to signal the ECM. Either this switch is bad or the wiring is damaged.

If you turn off the AEC function on the monitor and can control the engine speed with the rotary dial, the controller is fine, you just need to troubleshoot the switches and their related wiring. I often install a jumper wire in the harness connector to the switch to temporarily bypass the switch. If the engine speed stays up, the switch is the problem.

To troubleshoot the active errors and logged codes, the monitor must be in full working condition so the hidden service switch can be used to turn on the service mode. All errors are displayed on the monitor screen. No laptop is used to connect to the machine like newer units. (This is often a stumbling block for the new guys.) You will need the manual for this model and serial number to know for certain the correct key pattern to use all the diagnostic features of the monitor.

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Customer reply replied 5 years ago
Thank you for your reply,the young mechanic from our Cat dealership was from what I could interpret had little knowledge of this older machine,the previous guy who used to help me has retired and was excellent.I have been operating it in manual mode as I have needed this excavator. We checked all the wiring going to the ecu,all the wires to the rotary switch/ all the wires to the governer/activator and there was no breakages all signals were going through/ checked to see if the ecu was ok !. Today on the screen it was showing the letter U,it operates fine on the manual switch but wont power up in any function on auto! We did what you say for the AEC function, the switch is fine.?????It had been playing up on powering up for the walking mode mainly reverse,I changed the sensor but it only seemed to be ok for a while,worked fine when you first used it,but as soon as you were on steeper terrain it would"t power up, any ideas. thankyou Richard
Mechanic: catmastertech, Technician replied 5 years ago

H is shown when the pump backup switch is on, A is when the throttle backup switch is on and U is when both switches are on.

Also look at the ECM controller face, it has a small window with three lights. Green means the unit is powered, yellow means the unit is not communicating with the monitor and red mean the ECM has failed.

I am not clear on the throttle, I assumed you only had an issue with the AEC function. Are you also having problems with the dial control?

The dial wiring does not affect the AEC control, only the pressure switches for travel and implement and the wiring for the switches.

If the issue is only present when you go up a hill then, a wiring must be moving and shorting out or is broken and pulls apart.

If your monitor is not functioning when the switches are in AUTO, focus on the communication wiring first. You should also have a yellow light on in the ECM window.

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Mechanic: catmastertech, Technician replied 5 years ago

Feedback wiring should also be tested, this is only from the throttle control cable box (governor actuator) to the ECM (controller)

Governor Lever Control By Engine Speed

Illustration Of Controller Inputs To Outputs For Engine Speed Control (Partial)

(1) Monitor. (2) Controller. (3) Speed sensor. (4) Engine. (5) Engine speed dial. (6) Travel pressure switch. (7) Implement swing pressure switch. (8) Governor actuator. (9) Feedback sensor.

An exact engine speed control is performed by the combined operations of the components shown in the above illustration. A failure in any one of them could result in improper engine speed control.

This section describes the engine speed control only when there is a problem in feedback sensor (9) and/or its circuit. During a normal condition, the controller controls the engine speed by matching the feedback sensor signal and the engine speed dial setting signal.

If a problem occurs in feedback sensor (9) and/or its circuit and there is no hydraulic demand, the controller controls the engine speed by monitoring a signal from the engine speed sensor and moving the governor actuator until the engine speed is close to the engine speed selected by the engine speed dial. It will take five to ten seconds for correct adjustment to occur after the engine speed dial position is changed.

Immediately after the feedback sensor problem has been corrected, the controller reverts back to normal engine speed control using the feedback sensor. Control by this method overrides the functions of the AEC and the low idle switch.

When this function is activated, the character display reads "L" and "A" on its first and third positions.

For more details, see the Testing And Adjusting module for the machine being serviced.

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Mechanic: catmastertech, Technician replied 5 years ago

I must sign off for now. I am leaving to work out in the field too. I will be back tonight (US Eastern time) and can help more then.

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Customer reply replied 5 years ago
Hello catmastertech,hope you are still available,nothing wrong with the ecm(three green lights) numbers dial up on the dash (1) but no engine response,no response to any control except using the manual mode,registers current on the feedback wire(purple) have you any other ideas? thanks Richard
Mechanic: catmastertech, Technician replied 5 years ago
I'm still here. Are you familiar with using the monitor to access diagnostic information?
What is the machine serial number? (Not sure if you posted it earlier.)
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