electronic thermostat case video

Hi scanner danner I'm a subscriber to your premium channel and I have to say that your material is about the best about and I have progressed a lot with help from your channel.
I have a case study here that I would like a better understanding of
2001 bmw 316i 1.9 litre engine with a fault code stored for 115 electronic thermostat circuit, the fault will not clear. the thermostat has been replaced with an aftermarket unit and still no change in fault code. I have attached a wiring diagram of the circuit .
With the sensor unplugged the green wire reads battery voltage and the black wire reads ground, when I back probe the green wire at the pcm and then plug it back into the thermostat the green wire drops to near o volts. Unsure if I have a pcm fault with a transistor or if there is possibly voltage drop somewhere? any help would be great with this.

Hey Boylejohnstone! Moved this to the Repair section, FYI.

Thanks for including the wiring diagram, super helpful. Mitchell diagrams are great, but I also really appreciate OE diagrams like this one, since it gives us a better idea of what's going on inside the DME.

It looks to me like this is a power side switched heater, as the diagram shows a transistor between the green wire and key on B+. The battery voltage you observed could possibly be a bias voltage from the DME, and the voltage going to zero with the thermostat plugged in could be a normal behavior. :huh: It's tough to know for certain without checking service info.

How quickly does the 115 code come back after clearing? I ask because I'm wondering if this DME has to energize the heater to set the code, or if it's looking at the bias voltage you observed. Knowing how the DME tests this circuit will be a big help in diagnosing this code.

As an experiment, I'd be interested to know what would happen if you were to connect an incandescent test light in place of the thermostat heater. The test light lighting during heater activation would tend to prove out the wiring and DME. Plus, if the code stops setting, then that'd suggest the aftermarket heater is no good. Make sense?

What was the the fault code? there a few codes for this system . and with the thermostat heating element unplugged and the codes cleared is the ]same code returning ?
Examine the wiring for damage due to coolant contanination.
Also I would mesure the ressistance of the thermostat element, and also the old one if you still have it. I have been told that the on BMW of this era the PCM is very intolerant of after market thermostats and will flag up a code if the ressistance isn't within spec. I don't have the correct specification but I have seen values of 12 and 40 ohms quoted.
I would try substituting an ordinary tungsten test light into the plug in place of the thermostat clear the code and run the engine.

It looks like the circuit might be very similar to this GM design

thanks for the reply. I have attached the operation condition of the thermostat in this which seems to indicate that the thermostat is power side switched? but unsure due to the transistor paths.
The fault code will not clear what so ever and also with it unplugged there is no other fault codes stored. Using the verus pro the data pid for the thermostat does not change, and when controlling the thermostat with the bi-directional control it doesn't react.
To clear things up on the operation of the thermostat heater by looking at the wiring diagram I'm a correct in thinking that pin 1 of the thermostat should be constant battery power KOEO/KOER with the flow through the transistor towards pin 1, and pin 2 being possibly ground switched through the transistor to complete the circuit and heat the element within the thermostat?
I did try and check the data pid for the thermostat with a test light in place connect to ground and probing pin 1 and seen no change but as I say new to this so I could be way off.
And also my thought about aftermarket parts when it comes to any type of sensor etc is that is it correctly manufactured to work in the circuit

Hi
the only code that shows is the 115 electronic thermostat which will not clear and also makes no difference if unplugged same code stays.
the customer had fitted a new thermostat but as I say it was aftermarket at this was because of the same code 115 which he had googled and it said to replace the thermostat which he then done as you do. There is no signs of any coolant contamination on the connector etc the customer did say that there was no leaks etc just the check engine light on.

Boylejohnstone wrote: thanks for the reply. I have attached the operation condition of the thermostat in this which seems to indicate that the thermostat is power side switched? but unsure due to the transistor paths.

Thanks, sir! Just read over it, good info in there.

To clear things up on the operation of the thermostat heater by looking at the wiring diagram I'm a correct in thinking that pin 1 of the thermostat should be constant battery power KOEO/KOER with the flow through the transistor towards pin 1, and pin 2 being possibly ground switched through the transistor to complete the circuit and heat the element within the thermostat?

I think it's the other way around. :silly: The operation condition document you posted had this to say:

BMW wrote: In the event of control intervention by the DME control module, power (12 V) is applied to the heating element integrated in the thermostat.

This, together with the wiring diagram, makes me confident that this is a power side switched circuit. When commanded, the thermostat will get 12V (or a pulse width modulated 12V, not sure yet) on pin 1. There should be a constant ground on pin 2, as you saw during your testing.

I did try and check the data pid for the thermostat with a test light in place connect to ground and probing pin 1 and seen no change but as I say new to this so I could be way off.

Oh cool, you already tried this! Did you happen to see if the code would clear with the test light installed?

really doesn't leave much guess work its power side switched, using a multimeter front probing might get you into trouble. a test light will place a load on the circuit,. but if it were me id do what tyler mentioned usin the test light in place of the heater, OR you can use a resistor in place. maybe its a snap on issue that wont clear the code. maybe try a cheap hand held code reader