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Old 08-23-2019, 02:50 PM   #1
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3126B temp sensor

Here's a head scratcher for everyone. I have been having a tough time recently with rough starts on the cat. Grey smoke etc. I have that issue where the wait to start light comes on for the instrument cluster check but then goes off right away.

I've been researching it and troubleshooting stuff like ohms on the heater element which were literally 0. Stuff like the relay was multimetered by me and everything seemed to be fine. No burnt wires anywhere etc etc.

Anyway, I unplugged the temp sensor and lo and behold the wait to start light comes on and stays on for a full 8 seconds. I start it with no issues at all. This of course leads me to believe the temp sensor is faulty but if its unplugged should the wait to start light stay on the entire 30 seconds?? I've turned the key to the on position at least a 1/2 dozen times and it always goes out after 8 seconds now?? I mean it's great that it's a work around the problem for now but why 8 seconds and not 30? Am I missing something? Is it something else too?

Thanks all.
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Old 08-23-2019, 06:08 PM   #2
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It not only uses the water temperature it uses air temperature and engine rpms and hmmm something else but I don't remember right at the moment But anyway no it shouldn't go a full 30 seconds. A bad temperature sensor can cause all kinds of starting and rough running problems. The ecm is looking at the coolant temp along with other parameters to also control Heui pressures. I had one in a excavator that would start and run terrible. Heui pressures were all over the place. I saw my coolant Temps were erratic so I changed the temp senor and the problem went away. So change the sensor and see what happens. Safe Travels
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Old 08-23-2019, 10:24 PM   #3
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Thanks for the help cat. I mention 30 seconds cause this troubleshooting guide I'm reading says 30 second cycle time over and over

Troubleshooting
3114, 3116 and 3126 Engines
Media Number -SENR6502-06

So, Yeah, according to this document the coolant temp, oil pressure switch, power relay, and control module are the factors to look for.

My coolant temp probe looks like this:



But yet it looks updated by cat now:



Shouldn't be an issue though I guess. Maybe cats sites using a generic picture.
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Old 08-23-2019, 11:49 PM   #4
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Quote:
Originally Posted by New 2 Newmar View Post
Thanks for the help cat. I mention 30 seconds cause this troubleshooting guide I'm reading says 30 second cycle time over and over

Troubleshooting
3114, 3116 and 3126 Engines
Media Number -SENR6502-06

So, Yeah, according to this document the coolant temp, oil pressure switch, power relay, and control module are the factors to look for.

My coolant temp probe looks like this:



But yet it looks updated by cat now:



Shouldn't be an issue though I guess. Maybe cats sites using a generic picture.
That picture the updated one does not look a coolant sensor. Make sure you have Caterpillar double check that part number. Good luck. Let us know how it comes out.
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Old 08-25-2019, 10:30 AM   #5
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Yup you're right, it looks just like the first picture. 264-4297 right from cat an $80 CDN non-returnable part.

And the head scratcher continues. I plugged in the new sensor early yesterday morning but didn't thread it into the engine so I could test it. I also made sure I didn't handle it with my warm hands on the metal sensor part. It was nice and cold outside only 9C/48F and the wait to start light only came on for 2 seconds. This of course worried me cause that's exactly what its doing with the original temp sensor when its plugged in. I unplugged the new sensor and the wait to start light came on for 8 seconds again. It started just fine.

So then I left it till this morning, 8C/46F weather conditions and with the new sensor still plugged in the wait to start light came on yet again for only 2 seconds, I started the MH and harrrdddd start, grey smoke, chugging bad.

So how could a temp sensor plugged in cause the wait to start light to NOT come on longer than the 2 second instrument light cluster check and yet when its not plugged in the heater stays on for 8 seconds longer?????? I really don't want to just leave it unplugged all the time do I?.

Where can I look now before I have to take this to a cat center? If I plug in a ET program what should I be looking for?

Thanks

PS: No check engine lights or anything else that indicates an issue anywhere to me.
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Old 08-25-2019, 01:00 PM   #6
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Quote:
Originally Posted by New 2 Newmar View Post
Where can I look now before I have to take this to a cat center? If I plug in a ET program what should I be looking for?

Thanks

PS: No check engine lights or anything else that indicates an issue anywhere to me.
Could be that another sensor is bad and when you unplug a sensor (any one; coolant, air temp, etc.) the ECM defaulters to a programmed heat time (like a limp mode). I don't know.

When connected to a scan tool, you look at the senor readings to see if "they make sense". Like a coolant sensor that reads -10 degrees when it's 80 degrees out.

There are three kinds of sensor failures 1) Sensor fails "open", 2) Sensor fails "shorted", 3) sensor is within allowable limits but wrong - off by a large factor (the sensor readings are 50 degrees off).

If the sensor fails open or shorted the ECM will detect it and set a code. If the sensor is stuck on some number or wrong by a large factor (like 50 degrees off) the ECM doesn't know the sensor is bad.
That's what your looking for.
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Old 08-27-2019, 10:10 AM   #7
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Originally Posted by New 2 Newmar View Post
Yup you're right, it looks just like the first picture. 264-4297 right from cat an $80 CDN non-returnable part.

And the head scratcher continues. I plugged in the new sensor early yesterday morning but didn't thread it into the engine so I could test it. I also made sure I didn't handle it with my warm hands on the metal sensor part. It was nice and cold outside only 9C/48F and the wait to start light only came on for 2 seconds. This of course worried me cause that's exactly what its doing with the original temp sensor when its plugged in. I unplugged the new sensor and the wait to start light came on for 8 seconds again. It started just fine.

So then I left it till this morning, 8C/46F weather conditions and with the new sensor still plugged in the wait to start light came on yet again for only 2 seconds, I started the MH and harrrdddd start, grey smoke, chugging bad.

So how could a temp sensor plugged in cause the wait to start light to NOT come on longer than the 2 second instrument light cluster check and yet when its not plugged in the heater stays on for 8 seconds longer?????? I really don't want to just leave it unplugged all the time do I?.

Where can I look now before I have to take this to a cat center? If I plug in a ET program what should I be looking for?

Thanks

PS: No check engine lights or anything else that indicates an issue anywhere to me.
With ET as stated see what the temperature is reading versus ambient temperature after sitting all night. See what the air temperature is reading. You will be able to see the actuation of the heater solenoid. The only accurate way I know to test the heater element it self is to test the amperage draw of the heater. It should be 92 amps +-10. The heater will not come on if the sum of the coolant temperature and the air temp is not below 109f.
How long have you had the hard starting? The hard starting can be caused because of low fuel cranking pressure. You should have a min 15-20 psi fuel pressure cranking.
If that's ok, and I hate to say this it could be injector wear problems. Worn injectors will cause hard starting and smoke when starting cold. The engine will start normally after it warms up slightly and when hot. But if it sits for a couple days or a cold night the hard starting and smoke will return. Hope that helps. Safe Travels
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Old 08-27-2019, 11:26 AM   #8
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One other thing it is doing a self test when it comes in for 2 seconds. On some models the engine light will flash quickly if there is a detectable fault and log the code. On other models it will just log the diagnostic code. Saying that the system will not detect if the heating element is broken, it also will not detect the heater solenoid- heater contacts are bad (it will detect if the solenoid is not being energized.)Safe Travels
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Old 08-29-2019, 12:05 PM   #9
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Thanks for the information guys. Here's what I did yesterday. I thought I would mess around with the other sensors that affect wait to start time and see if I could luck out and stumble across a faulty sensor. I have ET but I don't want to use it till I get back from our camping trip next week cause if I screw up the ecm just before this trip i'll never hear the end of it.

I plugged back in the original coolant temp sensor that I never pulled out of the engine yet and unplugged the air intake sensor, went up front and cycled the key to the on position and the wait to start light came on for THIRTY seconds. Interesting. I did an amp test on the heating element during a 30 second cycle and it seemed to be working just fine. I even felt some heat in the metal around that area. This would also eliminate any relay issues too cause powers going through it. It started up with no grey smoke but the rad fan wouldn't shut off and the engine started revving higher and higher till I plugged the air intake sensor back in. The WTS light stayed on the whole time while running too. Oddly enough the coolant and air intake sensors have the same plug-in connector. I'm wondering if its the same sensor? 264-4297 on cat parts says:

Applications:
• Low temperature sensors are used on machines and engines in air systems, fuel systems, engine oil systems, cooling systems, hydraulic systems etc.

I can't seem to find anything specific on cats site about the "coolant sensor" or "air intake temp sensor" these are the results I get:

No Matches Found
We were unable to locate any verified parts that match your serial number. This could be because we do not have information about this serial number, or we cannot currently verify parts within this category.

So now I'm second guessing myself on if I even have the right replacement sensor? My family is 8YL. I also don't get very solid positive results doing web searches on 3126 coolant sensor or 3126 air intake temp sensor. Any ideas?
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Old 08-29-2019, 01:36 PM   #10
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Give me complete serial number and I will check it out.
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Old 08-30-2019, 06:08 AM   #11
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You should be able to go the CAT Parts Store https://parts.cat.com/en/catcorp
enter your engine serial number, then pull up the part numbers for the temp sensors (then you can see if the sensors in question have the same part numbers).
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Question - you said you have "ET". Is that the "CAT ET" program you have (the one where you need to pay an annual subscription to use it)?

I've seen all kinds of "ET" scanners/programs for sale, ranging from a couple of hundred dollars, to a couple thousand. Some say "annual subscription not required".

So what tool/software does it actually take to access the CAT ECM (do injector leak tests, IAC tests, etc.) not just read codes?
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Old 08-30-2019, 09:10 PM   #12
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Thanks Cat. The complete # is 8YL67426

Country. Yes the complete ET program. It's a annual sub not required version thus the delay to plug it in until the seasons over. I had one of those types at work for the Cummins engine in the truck and it always programmed just fine. It was called a inline 6 data link adapter.
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