Coolant Temp sensor voltage is 3.4 not 5v please help

[Serge88]

Hi guys my 318is with m42 1994 is throwing coolant temp sensor code, I replaced the sensor but still the same drama, I tested the voltage from the harness and it is 3.4v instead of 5v? what do I do in this situation? thanks.

[flyfishvt]

There are a couple sensors. Where was the one you changed located? Near the throttle body?

[electrofelix]

With the M42 I believe there are only 2 coolant temp sensors, both in the cylinder head.

One gives the reading for the temp gauge (furthest from the front) and the other gives the reading used for fuel supply (i.e. AKA choke switch).

I suspect that he's referring to the second one, which is closest to the front and the plug colour is generally blue.

Unfortunately I can't help with the actual problem, only with pointing out which sensor he's likely to be taking about.

[Serge88]

thanks guys electrofelix is correct, I just need to know what could cause the voltage to drop? bad ground? split wire? if anyone can help that would be awesome.

[randomy]

Resistance. Where are you measuring the voltage (ground point and hot point), and what documentation is advising you the voltage should be 5v?

[Serge88]

I'm figuring bad ground, will confirm when done.

[95325i5sp]

you realize it gets 5v from the ecm then modifies it? thats how a temp sensor works, its a variable resistor that changes resistance in proporion to temp. ti recieves the 5v reference voltage from the ecm then depending what the temp is it drops the voltage a corresponding amount which the ecm interprets as temp. 3.4v sounds about normal for a temp sensor. dtheway to test it would be findwhat the resistance should be at a specific temp adn if thats good the sensor is fine, its the wiring to the pcm thats bad or has excessive resistance, you'd have to test for continuity between the connector on the sensor to the appropriate pins on the ecm connector.

[Serge88]

thanks for the last reply mate, I'm still struggling it is getting 3.4v at the connector when stone cold, traced all the wiring back to the ECU and fuse box and all seems fine? I am still getting a coolant temp sensor code even though I have replaced the sensor? would the signal be thrown off if its 0.1 - 0.2v off? cheers.

[95325i5sp]

no the computer would just think the engine was cooler/hotter, whatever degree change that small amount of voltage would translate to. you seem to have everything checked out if you've measured the resistance between the sensor connector and ecm/dme connector and it wasnt high(no more than at most one ohm for that short a wire but probly more like .5 ohms.) and have a new sensor. that leaves only the dme to be bad. its the only thing left in that circuit unfortunately. im pretty new to bmw's(had my 95 for only a couple weeks) but have read on here that for our year cars that the dme/ecm is succeptable to water damage/corossion because of where its at or a design flaw or something. also read about guys opening them up and cleaning the circuit boards with electrical contact cleaner, and clearing up some crazy problems. whether or not u wanna try that is up to you lol , im a mechanic not a computer expert, but at this point id at least open the dme to see if there was obviosly some kinda corrosion, not sure how id go aboutgetting it off though.

in any case randomy up there is right- your problem is resistance that wasnt designed to be in the circuit causing a voltage drop, whether it be in the harness or the ecm itself.

[Naush]

Hi dears.
What can cause a Sudden Bad or Fluctuated idle on M42 ?
Other than ICV, Airflow Meter, Blue Temperature Sensor or Vacuum Leak ?
Any ideas ?
Its been a challenge for me.
My solution was :
( I disconnected the ICV wire and put a small carburettor jet in between the Airflow Meter and the ICV just to let a tiny air stream to pass through for idle and raised the RPM a bit to 900 -1000 by tightening the accelerator cable.Thus i can the car with A/C on also )
But i know its not ok. And i have like a drop in power in Mid Revs when the Disa is being opened....
THANKS FOR YOUR RESPONSE.

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[ultimatetester]

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