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Clayton
Clayton, Mechanic
Category: Heavy Equipment
Satisfied Customers: 755
Experience:  5+ Years at Freightliner, Detroit & MBE Certified, ATD, and Electrical
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2004 Freightliner School bus with Mercedes Benz engine... high

Resolved Question:

2004 Freightliner School bus with Mercedes Benz engine... high temp light/buzzer comes on in the dash after driving approximately 15 miles, when slowing for a stop. Coolant temp is only 185 at the time. Have changed temp sensor, and thermostats. Also has PID code 110
Submitted: 5 years ago.
Category: Heavy Equipment
Expert:  Clayton replied 5 years ago.

Clayton :

Hi, Welcome to Just Answer, I am Clayton. I can help you with this.

Clayton :

Hey, Thanks for joining us

Customer:

this is Don

Clayton :

Nice to meet you Don, I am Clayton. I can try to help you with this

Clayton :

The high temp light and the buzzer gets the signal for the engine ecu VIA J1587 datalink

Clayton :

from the engine ECU I meant

Clayton :

Same way with the temp guage, I am almost stumped, if the engine ECU is reporting a high coolant temp for 185, the buzzer should not sound and the warning light should not come on. Do you have another truck to swap guage clusters with?

Customer:

No none of the other spare buses have that cluster.

Clayton :

do you have access to any diagnostic software?

Clayton :

Oh ya, PID 110 is an Engine coolant temp fault

Clayton :

It would come up on the cluster as ECU 128 PID 110

Clayton :

Does the check engine light come on when this happens?

Customer:

Yes check engine light comes on and we are reading the engine temp thru a data link connector with our OTC scan tool.

Clayton :

what fault code are you getting with the scan tool

Clayton :

PID 110 code?

Customer:

yes

Clayton :

And the OTC scan tool is showing the temp is only 185

Clayton :

?

Customer:

right, is showing 185 when light and buzzer comes on, I also covered the radiator and got the temp to over 190 and light and buzzer did not come on.

Clayton :

that does'nt make any sense

Clayton :

Is this a mbe 900?

Customer:

light and buzzer only comes on at 185 randomly, when slowing and coming to a stop, after it has been driven approximately 15 miles.

Clayton :

is it a MBE 900 or a 4000?

Customer:

not sure of engine model

Clayton :

You have the vin #?

Customer:

4UZAAXCS65CN77477

Clayton :

Ok, you have an MBE 900

Clayton :

So you replaced the sensor that is on the front of the engine below the oil seperator?

Clayton :

2 pin round connector

Customer:

I replaced a temp sensor on left side up by the thermostat

Clayton :

right above the water pump?

Customer:

yes

Clayton :

Alright, well the only thing left is ECU or wiring. Your PID code 110, was there another number after that like a 2,3,4 or 14?

Customer:

don't recall any numbers after it, bus not here at the moment

Clayton :

The format should be PID 110 04 for example, this will tell us its an open circuit, or a short to ground, or short to battery. When you go back to the bus, take your scan tool back with you. Remove the connector from the coolant temp sensor, short the two pins together, wait a few seconds with the scan tool hooked up to see if the code changes, If the code changes, then the circuit to the ECM is good...and you might have a faulty coolant sensor(again) or a Faulty ECU

Customer:

temp goes to 33 degrees when I plug it or short it, thats what the scan tool reads

Clayton :

when you just unplug it, what does it go to

Customer:

unplugged or shorted it stays at 33

Clayton :

does the fault code change?

Customer:

didn't check that

Clayton :

We will need that, otherwise we are shooting in the dark. Sounds like there is some resistance somewhere in the circuit

Clayton :

I would check the circuit next from the coolant sensor to the engine ECM

Customer:

I will have to get the bus back in here and check that fault code again.

Clayton :

OK. This is what I would do 1st

Clayton :

when you get the bus in here, hook the scan tool up and monitor the fault codes, short the connector to see if it changes, if it does not change, the circuit has some resistance somewhere, or it is simply open

Clayton :

Give it time to change. Give it 10-15 secs atleast

Clayton :

Unplug the engine coolant temp sensor connector, and unplug the engine 55 pin connector

Clayton :

Take the volt meter and check the resistance red lead on pin 1, and black lead on pin 2 of the ECT sensor connector, you should have no resistance. If you do, it is shorted somewhere

Customer:

but if there is a problem between the temp sensor and the ECM, wouldn' t the coolant temp be off?

Clayton :

Ya, it does not make sense to me either

Clayton :

If we verified the wiring is good, the only other thing is the engine ECM

Customer:

ok we will try what you suggested, thank you for your assistance.

Clayton :

Before you go.

Clayton :

55 pin connector, pins 1(ECT CONNECTOR) goes to PIN 34 (55 pin ECM connector) they are labeled,but small

Clayton :

Pins 2(ECT connector) goes to PIN 3 of the (55 pin ECM connector)

Clayton :

Ohm those out really well, tugging and pulling on the harness to see if your resistance changes..

Customer:

ok

Clayton :

Make sure you do not see any spread pins as well

Customer:

ok

Clayton :

If the circuit checks good, the only thing left is ECM

Customer:

ok

Clayton :

Thanks for using JustAnswer. If you have any more questions, let me know.

Customer:

ok

Clayton :

You can post up with follow up questions after I leave the chat, please accept my answer if you are satisfied

Clayton :

Talk to you later.

Clayton and other Heavy Equipment Specialists are ready to help you

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