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I have a caterpillar 120-H motor grader ID # ***** model…

I have a caterpillar 120-H...
I have a caterpillar 120-H motor grader ID # *****
model #3126 serial # ***** ARRENGEMENT #(###) ###-####
transmission # *****
this cat goes forward it has first-second-third- fourth and no fifth-sixth-seventh and eighth
reverse it has first-second and third no fourth-fifth and sixth
is there a way to do diagnostics on this thing manually ?
and can some one help me to get a diagram of the transmission to solenoids and computer
anything any info will be appreciated thanks
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Answered in 5 minutes by:
10/23/2014
Mechanic: catmastertech, Technician replied 8 years ago
catmastertech
catmastertech, Technician
Category: Construction and Road Equipment
Satisfied Customers: 4,036
Experience: Field service technician for CAT 19 years.
Verified
catmastertech :

Hello. If your transmission warning light is on, you have an electrical fault causing the problem.

If the light is not on, you have a control valve problem.

catmastertech :

Here is how to check the fault codes for H models. This list is old but, may work for your grader too.

Flash codes for older H motor grader <<<click here.

catmastertech :

Here is the 12H trans wiring

Full Size Image

catmastertech :

Should be the same for a 120.

catmastertech :



























































































































































































































GEAR ENGAGED CLUTCHES
DIRECTIONSPEED RANGE
Forward HighForward Low Reverse Second SPEED Third SPEED First SPEED Low High
Solenoid 1 2 3 4 5 6 7 8
Park X X
Neutral above
0 °C (32 °F)
X X
Neutral below
0 °C (32°F)
First gear forward X X X
Second forward gear X X X
Third gear forward X X X
Fourth gear forward X X X
Fifth gear forward X X X
Sixth gear forward X X X
Seventh gear forward X X X
Eight forward gear X X X
First gear reverse X X X
Second gear reverse X X X
Third reverse gear X X X
Fourth reverse gear X X X
Fifth reverse gear X X X
Sixth reverse gear X X X

catmastertech :

Chart does not transfer well but, I hope you can see the clutches and the gears used.

catmastertech :

No fifth forward and no fourth reverse means, your issue is will solenoids 6 or 8.

catmastertech :

I assume you do not have any of the gears above these so, the trouble is in solenoid #8

catmastertech :

This is at the bottom of the stack and is sticking out different from the rest of the solenoids.

Customer:

hi cat master I have no power to solenoid # ***** is a orange wire

Customer:

now from 24 way connector at shift lever I have two orange wires

catmastertech :

Forget the shifter, they rarely fail.

Customer:

can you explain command to solenoid # ***** from ECM

catmastertech :

You will likely have a harness problem from the ECM.

catmastertech :

The power train ECM responds to decisions by sending electrical signals through the various outputs. The outputs are pulse width modulated. The ECM operates the PWM outputs in an on/off manner or in a proportional manner. This depends on the requirements of the output device.

catmastertech :

Start testing the harness from the connector at the transmission control valve. This is the most common fault I have seen.

catmastertech :

Then trace the wire back to the J1 connector of the ECM and pin 11.

catmastertech :

You must have less than 5 ohms resistance for a good circuit.

catmastertech :

Higher resistance should give you a fault code.

catmastertech :

Some models will even give a flash code with the transmission light.

Customer:

ok so # ***** solenoid orange wire goes to ecm j1 connector pin 11 ?

catmastertech :

yes

catmastertech :

It should be marked 755

Customer:

so when I command rev 4th or forward fifth on shift lever that inputs to the ECM right then ECM commands orange wire to # ***** solenoid right

catmastertech :

yes

catmastertech :

should be 24 volts on that solenoid.

catmastertech :

the direction solenoids are proportional and will have a variable current.

catmastertech :







Forward HighForward LowReverse
catmastertech :

solenoid 1, 2 and 3

catmastertech :

4 thru 8 are 24 volt

Customer:

great I will fix that cat tomorrow thanks cat man you are it bud will let you know how it goes have a good night

catmastertech :

If your ECM does not output the voltage, it may have failed. I have seen this on lower gears that are used often. If this machine is on the highway and uses high gears the must, then it is possible for this solenoid driver to fail.

catmastertech :

Glad to help, already past my bedtime. Thanks and I will look for your reply tomorrow night.

Customer:

if I replace ecm do I need to reprogram it ? but I don't thing so right budy

catmastertech :

All new ECM's must be programmed. Some can be ordered from the factory with the software. You will still need to have the clutch pressures and fill times calibrated or the shifting will be rough.

catmastertech :

Even if you find a used ECM, it still needs to be calibrated to your transmission.

catmastertech :

CAT recommends calibration about ever 5000 hours for best operation. It is not mandatory but, just recommended.

Customer:

ok bud I hope this box is good will chat tomorrow ok I got to go night night

catmastertech :

Ok, goodnight.

Customer:

thanks a lot cat man I found the ECM to be the problem I'm waiting on a new one

Customer:

thanks cat man I found the ECM not to be sending 24 volts to solenoid valve #8

Customer:

at fifth to eighth speeds how ever I checked orange wire from #8 solenoid to ECM and is got good continuity and also checked shifter sensor to ECM and found that sensor gets 24volts from ECM pink wire and ground on black wire so ECM sends 5 volts to all the rest of wires and when I go to like first gear that 5 volts the ECM sends goes to ground letting the ECM to know is in first so when I shifted to fifth to eight the same thing happens so I know my shifter like you said is fine thanks a lot can man do

catmastertech
catmastertech, Technician
Category: Construction and Road Equipment
Satisfied Customers: 4,036
Experience: Field service technician for CAT 19 years.
Verified
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catmastertech
catmastertech, Technician
Category: Construction and Road Equipment
Satisfied Customers: 4,036
4,036 Satisfied Customers
Experience: Field service technician for CAT 19 years.

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