F30 Erroneous Coolant Level Warning

Hector Mechanic Commack, New York Posted   Latest   Edited  
Tech Tip
Autologic
2012 BMW 320i 2.0L (N20B20A) 8-spd (GA8HP45Z)
Low Coolant 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!

+9
Jeff Owner/Technician
Baldwinsville, New York
Jeff
 

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.

+4 Ð Bounty Awarded
Jack Technician
Tulsa, Oklahoma
Jack
 

Good job Hector, Aftermarket parts can save you money and waste a lot of your time!

+1 Ð Bounty Awarded
Dmitriy Analyst
Toronto, Ontario
Dmitriy
   

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

0 Ð Bounty Awarded
Rudy Technician
Montebello, California
Rudy
 

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.

-1 Ð Bounty Awarded
Dmitriy Analyst
Toronto, Ontario
Dmitriy
 

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.

0 Ð Bounty Awarded
Rudy Technician
Montebello, California
Rudy
 

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

+4 Ð Bounty Awarded
Chris Diagnostician
Commack, New York
Chris
 

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!!!

0 Ð Bounty Awarded
Dmitriy Analyst
Toronto, Ontario
Dmitriy
 

"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

+1 Ð Bounty Awarded