ryanlafayette

Hello All,

Apologies in advance for the long post. I am new to posting on forums and want to make sure I provide enough information.

High-Level Overview:
I have been dealing with a P0115, engine coolant temperature sensor circuit 1, check engine light for the past three months on my 2000 ML320. I have replaced the temperature sensor and am still getting the issue. When the check engine light originally came up and after replacing the sensor the dash engine temp gauge and OBD live data while driving has looked to proved accurate temps. The temp will rise to around 80c within about 10-15 min of driving. After replacing the sensor did not work, I took the car to the dealer to diagnose and the estimate to diagnose came back at about $1600 due to needing more time to diagnose. This was too much for me to spend and is too close to the car's current Kelly Blue Book value.

Background on the car
I was gifted a 2000 ML320 that my grandmother owned. The car did not move a lot as she was not driving much in her older age and had sat for about a year after she passed, and because it made my grandfather really happy seeing the car I have been trying to fix it up little by little. Here is a bit of background on how the car what when I picked it up to now.

• I picked up the car
• It had rained the night before and there were some noticeable wet floor mats in the driver and passenger footwell.
• It had a check engine light for the Brake Fluid Level Sensor
• The BAS/ESP light would come on after a couple of min of driving.
• The car was leaking windshield washer fluid
• The fuel gauge was not working, though would some times come on after filling the car.
• The AC would work sometime, and other times only pump hot air.

• I fixed the Sunroof leak by removing the front fenders and plumbing a new tube from the A-pillar down and out the fender hole. Confirmed the fix by pouring warm water in the sunroof catch basin, and verifying the warm water came out the front fender and there was no warm water near the A-pillar or footwell.
• I fixed the Brake Fluid Level Sensor check engine and leaking windshield washer fluid by getting replacement parts at the junkyard. I lucked out with getting the smaller capacity washer reservoir since my ML does not have the headlamp sprayers.

• I got the registration renewal and noticed it is a smog year.
• That weekend I went to get the smog check done, but the battery died. My grandfather mentioned the battery was old.
• I then replaced the Battery.

• After completing the drive cycles, I was on a work trip and the check engine light came on for the P0115 while driving through some mountains
• I changed the temperature sensor with this GDP Sensor. I ended up draining the coolant and removing the Thermostat housing to get the Temp sensor clamp off. I figured the coolant had likely not been changed in a long time, so I did a cleansing flush with Coolant system cleaner and distilled water, then replaced with new coolant (removed ~9L and replaced ~9L)

• After driving for ~150 miles to complete the smog drive cycles with the car operating at normal ~80c temp, the check engine light came back on for P0115.
• I did some digging and found this forum post though it seemed different from my error since I have always received the read out of the temp on the dash and through the OBD Live Data while driving.
• I also found this article, and did my best to test the harness. I was able to locate the pin locations on the ecu connector for pin 28/29. Disconnected the harness from the ECU and Temp sensor to confirm there was continuity in the wires showing no break. Then did a voltage reading at 66F (~18C) cold motor and read 3.1V which seemed close to the read outs listed by Maj. Dundee. Though in all honestly, I am not as experienced with electrical testing using the multimeter so I am not sure if my settings were correct.

• The car needed to be smogged so I paid the registration and took it to fail smog to get a Temporary Operation Permit for the next 60 days.

• While checking the fuse boxes and relays in the car I noticed corrosion in the relay box near the passenger footwell.
• I ordered a used relay box on ebay, disconnected the battery and swapped that in.
• After completing a few drive cycles, the fuel gauge began working and the AC also began functioning normally each time. However the P0115 check engine came back on.

• With no luck I took the car to the dealer to diagnose the P0115 and BAS/ESP lights.
• BAS/ESP seemed to be the Brake Light Switch
  • REQUESTING FURTHER DIAG TIME AND SUGGEST REPLACING BRAKE LIGHT SWITCH -WE FOUND CODES IN ESP CONTROL UNIT ( C1200 BRAKE LIGHT SWITCH S9/11 ) AND ( C1210 BRAKE FLUID INDICATOR SWITCH S11 ) WE ALSO FOUND THE BRAKE ASSIST SYSTEM CONTROL UNIT WITH CODES ( C1000 CONTROL MODULE N48 ) AND ( CAN SIGNALS STOP LAMP SWITCH )
    * DIAG AND REPLACEMENT OF BRAKE LIGHT SWITCH
• P0115, they needed more time to diagnose and the bill estimate was ~1600 which was way to high for me.
  • REQUESTING FURTHER DIAG TIME DUE TO ME CONTROL UNIT HAVING A STORED CODE FOR COOLANT TEMP SENSOR - NOT CURRENT - FOUND COOLANT TEMP SENSOR PINS WET NEED TO DIAG THE ENGINE HARNESS FOR WATER DAMAGE - WILL PRESSURE TEST COOLANT SYSTEM AND CHECK FOR AIR BUBBLES - FUSES/RELAY CHECK FROM PAST WATER DAMAGE

Best Next Steps?
Since the car has sentimental value, I don't want to throw in the towel before giving it one more shot at troubleshooting at home. Any recommendations would be greatly appreciated, and if it is related to electrical testing through multi-meter any articles or YouTube references that can help give background or walkthroughs on how to complete would be awesome.

The part of the mechanic's message that throws me off is, "REQUESTING FURTHER DIAG TIME DUE TO ME CONTROL UNIT HAVING A STORED CODE FOR COOLANT TEMP SENSOR - NOT CURRENT." If it is a stored value, does that mean I may need to complete more drive cycles to verify the issue is no longer present? I remember reading somewhere that it takes 5 drive cycles to clear a check engine light not through an OBD tool. Also, when I have checked the Temp Sensor connections male/female, as well as, the connections at the ECU I have not noticed any indications that looks like a wet connector.

My father-in-law, also thinks it could be related to a partially stuck open thermostat since I did not replace the thermostat and only replaced the gasket when I removed it to replace the temperature sensor. This article seems to support that could cause the issue, https://www.benzworld.org/threads/p0...ml320.2617354/.

vesiadog

Please read the post on the following link . It explains how a low temp coolant is treated as a Fault code.
By chance have you been able to get a temp reading at all? Using an outside source temp reader ?
https://www.benzworld.org/threads/th...BF%BD.1829169/
I also want to add if you remove the top hose from the thermostat housing you should replace the Oring or just buy a new hose with the Oring etc… probably a good idea if the hose is old.

ryanlafayette

Thank you for sharing the link! After reading this ended up being similar symptoms I was seeing. The temp was reading a little low around high 60s to low 70s in celsius. I changed the thermostat, cleared the check engine light, and the light did not come back up.

After inspecting the old thermostat, it did look like there was a small gap between the thermostat and the housing which was likely leading to the cooler temperatures and check engine warning on lower than expected coolant temperature.

vesiadog

If you want to waste some time and test your old thermostat you can get a digital readout and out the thermostat in an old pan and heat up the water … it sounds like you have a failed thermostat that was staying open .. cooler water thus the code!