Tracing Voltage Drop to Engine Coolant Temp Sensor

[VaTechE36]

Good morning all...figured I'd reach out to the experts as I'm trying to help a buddy of mine get his racecar back up and running (which is actually not an M3) His car is a 93 E36 325i w/M50 that is running rich and have really lumpy idle. Lots of parts thrown at the car, but we are currently trying to understand what would cause the Engine Coolant Temp Sensor to only have 3.5v supplied to instead of the required 5v. Issues with the ECT can result in a rich mixture and lumpy idle from what we have been able to find out


We have tried multiple DMEs, and have hacked the wiring harness to attempt to isolate potential draws to where the only things going to the DME are the power wires, ground wires, and the wires running straight to the ECT.


Any suggestions on things to check? I've attached a few pictures to help illustrate what we're messing with.

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[tjm3]

First thing, the voltage at 3.5v is correct and it will drop as the engine heats. That's normal. You're also reading the schematic incorrectly. The little symbols inside the control unit indicates a voltage drop, a voltage split actually, is taking place. See the resistor connected directly to the output at terminal 78 with the little arrow that's actually providing the signal to the control unit. There's two loads: one before the arrow and the sensor after. It's providing a voltage differential signal that will vary between about .9V to near 3.5V. It'll never get to 5V. That's not your problem.

By the way, if you try to hot wire the 5V you stand a better than 50/50 chance of frying the circuit. And, you can see resistors connected inline with the 5V power supplies any way you look at the schematic, so right off the bat you can see that you're not gonna get the full 5V anywhere but inside the ECU on the 5V sensor bus.


Why are all those wires cut and labelled? What's the project? Different ECU? I see some crimp connectors. Bad idea. More voltage drops with those than any naturally occurring problems. Solder and heat shrink, especially for a track car. Those crimp connectors won't last. You're adding unreliability with crimp connections.

[VaTechE36]

Tony, thanks for the reply. The harness is all hacked up due to troubleshooting.

What is weird with the 3.5v is that on my car (also a 325) and another friend's 325, we both get almost a full 5v (4.96v and 4.9v) with just the ignition on sitting probably around upper 70* ambient temperatures.

The harness will definitely not be staying the way it is lol.

I'll pass the info along though and see he this helps him diagnose a bit.

Thanks again! Any additional thoughts would be greatly appreciated!

Andrew

[tjm3]

The voltages depend on how you measure and what's connected or not connected and whether your measuring a complete circuit or just part of it. On that temp sensor circuit, if you take measurements on a complete circuit then you'll measure the voltages correctly but if you disconnect the sensor then you're not measuring accurately. With the sensor out even with the resistor in the circuit (inside the ECU) there's no complete circuit and so no voltage drop. You'll get the full 5V or what appears to be something very close to that. Honestly, most people don't understand what they're looking at when they read a diagram or schematic and it's easy to be led to the wrong conclusion because of it. You need to be asking first 'what reading SHOULD I have here', then measure and decide if it's correct. I'm guessing you took readings from different cars different ways and got different results. What years were the other 325's?

[VaTechE36]

This is all very possible. I know mine is a 93 just like the car in question and I have 4.96v. I am not sure about the other but it would be somewhere between 93 and 95 OBD1 car.

My harness is not at all hacked up in any way and just unplugging the connector, stuffing my voltmeter prongs into the plugs, flipping the ignition on, results in around 5v. I believe the same process was performed on the car measuring 3.5v, but not sure if the harness is in the same condition. I also think the same process was performed on the other car with an unconfirmed year which resulted in something also very close to 5v.

I will say that all the cars had the sensor plug disconnect and measured and we had 2 around 5v and one around 3.5v which I'm assuming is a pretty significant change in the car that is currently not running correctly.

Definitely some variables out there for sure, and I, at least, am not well versed in the changes of electricity outside of V = I*R lol.

[AJLM34A]

What is the resistance on DME pin 78 when the engine is cold vs engine is hot? Is the thermostat stuck open?(upper and lower radiator hoses different temps)

[E34V8]

Did this ever get properly sorted? Have the same issue after shorting pin 14 to 19 in obd1 connector. Car runs at 8.5:1 afr. Barely idles. Coolant temp sensor blue at 5v and black at 3.5V. 1995 m50tu