Cat 3126 B won't start ****HELP***

Discussion in 'Trucks [ Eighteen Wheelers ]' started by rpsunrise, Nov 29, 2009.

  1. DDS

    DDS Medium Load Member

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    May 20, 2008
    Toronto
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    The sensors are under the high pressure pump assembly, mounted in to the front cover. Do not bent the bracket. You need to remove the bolt that holds the bracket and you will be prying up on the bracket to un-clip it from the bottom sensor leaving the bracket on the upper sensor. You can fish out the lower sensor, unplug it, change it with the new one and reinstall with the upper sensor still in place. Now remove the upper sensor with the bracket installed, it with fish out, abit tight but will come out. Unplug and install new upper sensor with the new bracket. Install the upper sensor, now with two pry bars you hold the bracket against the lower sensor and push in on the bracket, this will clip the bracket back on the lower sensor. Try to clock the lower the sensor so the bracket will hit the right spot. Take a look at the new sensor, you will see what I mean. The bracket has to clip on to the sensor in the right spot.
     
  2. rpsunrise

    rpsunrise Bobtail Member

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    Nov 29, 2009
    Albany NY
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    DDS,
    Again a million thanks and again, you make is sound easy. I hope that after I remove the air intake pipe and other lines/hoses that are in the way I will be able to see it. Being under the bed in the RV makes it a bit difficult.

    Question – Started it again with either to move it out of the way, started fine with the either. After it was up to temp and moved I turned it off and tried to restart it after about 10 minutes and it started right up.

    If I had a bad slow speed sensor would it still start just because it was warm? An hour later I tried again and it would not start.

    Again, Thanks
    Bob
     
  3. DDS

    DDS Medium Load Member

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    May 20, 2008
    Toronto
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    Its going to be hard to change that sensor in a RV, even if you have done them before in a regular cab config. Yes, a weak number two engine speed sensor will cause an intermittant no start. I have had engines with no codes have a failing sensor with a no start. The engine can not find top dead center and will not fire the injectors until it see,s it. The sensor is weak so when you fire it with starting fluid, you speed up the rpm and the sensor picks up a signal.
     
  4. rpsunrise

    rpsunrise Bobtail Member

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    Nov 29, 2009
    Albany NY
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    DDS:

    It's starting to make sense why it would not start cold (not spinning fast enough) but starts when warmed up, engine spins a little faster. I disconnected the intake manifold pre-heater before using the either (don't want to blow it up...... yet). :biggrin_25523:It's in the backyard now so I can get at my snowplows and have some time to work on it.

    I hope I can remove the bolts that hold the compressor to the block and move it enough to access the sensor without disconnecting the lines that go to it.

    Again, thanks for all your help, wish you lived close, want to take a vacation to NY?

    Thanks
    Bob

    PS: And thanks for being so prompt with your replies.
     
  5. DDS

    DDS Medium Load Member

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    May 20, 2008
    Toronto
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    You will not be able to loose the air compressor that easly. There is a support bracket at the rear of the commpressor, you will need to pull the power steering pump to get at the bolts. That bracket must be torqued back up properly, or the front cover can crack.
     
  6. joeywalnuts21

    joeywalnuts21 Bobtail Member

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    Jan 30, 2010
    berlin, nj
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    if your still having the problem check this, injection actuation sensor on your 3126 it will be on the left side of the engine towards the front top next to the valve cover. Unplug this sensor if it starts then thats your problem, replace it and your good to go. It is extremely common, just happened to my 2000 F750 mechanics truck today, happens to CAT excavators quite a bit. alot easier then reaching behind the air compressor for the primary and secondary speed sensors.
     
  7. jcross

    jcross Bobtail Member

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    Mar 5, 2008
    Kingsland, TX
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    I have a 1998 FL70 with a 3126 and I'm fixing to replace both sensors tomorrow when they get here. Are there any tricks or things I should know about before hand?

    That space looks pretty tight in there.

    Thanks for any advice.....
     
  8. rpsunrise

    rpsunrise Bobtail Member

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    Nov 29, 2009
    Albany NY
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    Hello Everyone:

    I want to update the post I started last year and thank everyone for their help and suggestions.

    Since there was no rush last year to find the cause and fix the motor home as it was winterized, I decided to wait for warmer weather to attack it again. I had picked up the slow speed crank sensors and a new manifold pre-heater last year so I had the parts.

    Getting to the crank sensors was my biggest problem with the air compressor in the way and getting to the air compressor was another problem as the cat engine is below the bed in the back bedroom (it's a 40 foot diesel pusher):Bus:. My ribs are still sore :biggrin_25510:. My plan was to support the compressor (about 60 lb) in a sling to hold it up as I was unbolting it.

    I removed intake manifold tube, pre-heater off manifold, turbo intake tube to give me some room and drained coolant. I then removed the cooling lines off the air compressor and the air lines, not easy as they were corroded and no easy access. Used lots of PB Blaster.

    I have an auxiliary hydraulic pump powered off the back of the air compressor so I had to remove that to gain access to the back brace supporting the air pump. With compressor mounting bolts out I was able to move the compressor away from the crank sensors about 2 or 3 inches. This gave me enough room to replace the sensors.

    Putting it back together was a lot easier and I also put in the new pre-heater.

    Judgment time had come and I turned the key one notch and waited for the yellow pre-heater light to go off. It did and I turned the key one more notch and vroom, It started right up. I was so happy I took my wife out to dinner last night.:occasion5:

    Now can anyone tell me how to get the diesel grease our from my dried and cracked fingers and hands.

    Thanks again to all.

    Bob
     
  9. tallboy

    tallboy Bobtail Member

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    Aug 30, 2011
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    I have a 2003 GMCT7500 Pole Digger with a Cat 3126 which has thw same cold starting problem. I have replaced the fuel pump, IAP sensor, fuel pressure regulator and both speed.timing sensors. However it still only starts with starting aids if it is left for more than 3 hours. Any ideas?
     
  10. jcross

    jcross Bobtail Member

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    Mar 5, 2008
    Kingsland, TX
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    Tallboy......I'm going through this myself. I've replaced both speed/timing sensors, fuel pump solenoid, new fuel lines, fuel check valve on back of head. I just got some injector o rings and will try that before I replace anything else.

    I've read where a cracked injector o ring can create a no start (leaking fuel).
     
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