F30 Erroneous Coolant Level Warning
This customer called in with a 2012 320i 2.0. This vehicle had a Low Coolant light on. The customer replaced the level sensor with an aftermarket one. The light was still on.
Service information specifies that the Front Electric Module (FRM) will supply a 3.3v square wave pattern at 10HZ.
Not much to go on.
The customer sent me scope patterns of the signal wire with the sensor unplugged and plugged in
We can see here with the sensor unplugged, we are going from battery voltage and dropping approx. 3.3v. This tells me that the FEM and wiring is working properly
Here we can see the aftermarket sensor plugged in. We see the same 10HZ, but the signal barely moves.
Is this good? I don’t know.
We need a known good to compare against.
Luckily the customer had the exact vehicle outside. He hooked up and captured the following readings at various coolant levels
Here is half filled. We see 10 HZ and dropping a little over 2v
Here is empty. 10HZ and dropping about 3.3v
This is full. 10HZ and dropping a little under 2v
This vehicle needed an OE sensor. A sensor fixed the vehicle.
Thanks for reading!
We do not use any aftermarket sensors of any sorts for any application except brake pad wear sensors, aftermarket sensors have bitten me in the ass more than once and have learned my lesson, thats along with ignition coils as well.
Good job Hector, Aftermarket parts can save you money and waste a lot of your time!
Hector, interesting waveforms... What is the sampling rate of the oscilloscope in use? I think this sensor + circuit measure capacitance between electrodes immersed in coolant. To measure capacitance typically a charge-transfer method is used, which produces the following waveforms: media.digikey.com/pdf/Data%20She….pdf Page 2, Figure 2-1 If the
Hey Lou....er uh Dmitriy, are you saying we should try to emulate the capacitance measurement used by the DME? Why would one need to do that? The above measurements,using voltage, revealed the problem. I apologize if I misunderstood what you are saying.
Rudy, I’m not suggesting to emulate the capacitance measurement. My concern is that if another diagnostician uses a different scope (with higher sampling rate), they might get a different-looking waveform and be wondering why. Thus my question was about what scope was used and at what sampling rate.
The sampling rate wont matter as much as the set up. For automotive use, the majority of the scopes we use have very similar specifications. The big 3 scopes in use by the majority of automotive technicians are Snap-On, Pico, and Escope. Even if a tech wasn't using one of those,unless he has a known good wave form, he wont know if his signal is good or not. As it stands now,this wave form is
Agreed. I searched all over to find a known good for this vehicle. This also makes sense of the service information. It is also nice to see how something actually works, instead of reading a manual. Something like this will make getting a definitive diagnosis that much better. Awesome-sauce Hector!!! Keep 'em coming!!!
"The sampling rate wont matter as much as the set up." What do you mean by "the set up"? Scope settings (which ultimately affect the effective sampling rate) or something else? "For automotive use, the majority of the scopes we use have very similar specifications." Oh, so this myth has not been dispelled here yet? Alright then, I guess I'll have to start an epic thread someday... "As it