How to test coolant sensor loom for 5v?

[TdiBlacky140]

I'm bit confused with regard to checking for 5v at the loom connector end? With the connector unplugged and the ignition switched on, how do I check for 5v, do I put the 2 probes of the multimeter across the 2 terminal pins of the connector at the same time? Or do I put 1 probe of the multimeter on the battery negative and the other probe on 1 of the terminal pins?

[heyescroft]

I would try going between -ve and each connector in turn and see what you get. You can’t do any harm if you try either way

[TdiBlacky140]

I would try going between -ve and each connector in turn and see what you get. You can’t do any harm if you try either way

I tried that now, 1 probe on battery negative and the other probe on the 2 pins of the connector and neither of the pins has a voltage. So there is no 5v getting to my coolant sensor.

Next question,,, how do I troubleshoot now for the missing 5v? Where is the other end of the sensor loom, is it going to be a 2pin plug connected into the Ecu that I can just unplug there for example?

I have continuity between the 2 pins on the loom plug when disconnected from the sensor so this would suggest the wires aren't broken, so is there any point checking the wires. I'm guessing it's best to try find the source of the 5v first? Is it the module, a simple fuse perhaps? Hmmmm, someone please help with these electrics Haha

[TdiBlacky140]

I checked the 2 wires terminals of the loom side of the connector and both have 11.7v when I put 1 probe on the terminal pin and the other on the battery positive terminal. So, there's definitely a short to ground?

Is it strange that both wires have a short to ground, does this suggest the problem is at whatever ECM / ECU they connect back to? Or if both wires are effected as they are does this suggest the fault is that these 2 wires are touching each other somewhere along their route?

[TdiBlacky140]

Maybe someone is still interested in troubleshooting this with me so I'll post my latest findings.

So,

- I have continuity between the two terminal pins with key out. (I done all my continuity tests with the ignition off)
- I have 12.3V on each terminal pin when I put the probe on battery positive terminal. (0v when I probe the negitive battery terminal)
- I have continuity beep on each terminal pin when I put probe on battery negative terminal / earth strap bolt beside battery / engine bolt and gearbox earth bolt.
- I also have continuity beep on each terminal pin when I put probe on battery positive terminal. (not sure if this means anything but thought I'd mention it)

[heyescroft]

What is the problem, I’m assuming that you are probably seeing the coolant warning on all the time? I don’t know how the circuit works but would assume that the probe gives continuity to earth (0 volts) when the coolant level is within spec so that you have current flow which is detected by the ECU. Having 12v on both sides would suggest a short but the best bet would be to run the same tests on a known good working unit as I’ve just seen that the resistance of the sensor changes with the level of the coolant so it may be a differential circuit and having 12v on both sides is normal. There are many posts on various forums on this and it may be the sensor itself that is faulty.

[philipharmes]

There shouldn't be a short between the two wires you are testing.

But be carefull that you have you're test meter set to the lowest ohms range, usually around 0 - 200 ohms, then read what it actually displays.
If there is a hard short it will read below 10 ohms.

Those wires will go back to the engine ECU, so my next step would be to disconnect the multi-plug at the ECU, check the pins for straightness & any corrosion.
Measure the resistance again at the coolant plug, if the short has cleared, the problem lies within the ECU ( very unlikely ), if the short remains the problem lies within the wiring loom & it will be time to inspect it carefully for damage on its complete route.

[Jayjay1974]

Hi there can I just ask what would be the cause of a ecu pin to go green and would or could this affect the 12 volt feed to the maf sensor ? Thanks

[heyescroft]

Green pins is usually the result of corrosion probably by a liquid of some sort and it will provide a high resistance point which generates heat and increased current flow and a lower voltage. Whether it would affect the MAF sensor will depend on what the pin is and what it does you’ll need to trace the cable.

[TdiBlacky140]

Ok, only got back to the car now, and test results are the following:

- With the ECU unplugged there is no continuity between the terminals in the the G62 sensor loom plug.
- With the ECU plugged back in, there is continuity between the terminals in the the G62 sensor loom plug.

With the ECU unplugged:
- there is continuity between the red/grey sensor terminal and the Ecu pin 52 in Ecu loom connector.
- there is continuity between the brown/grey sensor terminal and the Ecu pin 53 in Ecu loom connector.


I'm not sure what the meaning of these results are or what the next step is! hmmmm!