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Discussion Starter #1
Trying to pinpoint the wire that goes from the BECM to the instrument cluster for the water temp gauge (2002 Bosch Motor). I've found the one that goes from the engine to BECM (green wire, pin 19, on 20way green connector).

Any help appreciated.
 

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From memory, the fuel level and water temp levels are sent from the BECM to the instrument cluster over the serial link, rather than having separate wires. Speedometer and Tachometer have separate feeds to the cluster, but if my memory is correct, pretty much everything else goes over the serial link. (I'll check my paperwork/notes later and confirm)
 

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Discussion Starter #3
Appreciate it. Can you elaborate on the serial link in this case. I understand in theory what a serial link is. I've never heard it mentioned on the P38 before.
 

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Serial link basically means that information is communicated between ECU's on a few wires (generally a signal ground, clock (for timing), data, and sometimes direction).

It means that more information can be sent over less wires - so instead of 4 wires for microswitches, another 3 for locking/unlocking, a couple for the window, and a number more for the mirror - it can all be done locally in the door, which means thinner wiring looms, less cable etc.

Newer vehicles use things like CAN bus which is more of a 'networked' solution between ECU's and uses even less wiring. The P38 was before CAN bus was even made, so the serial link is somewhere inbetween having an individual wire for everything and a fully networked communication system.
 

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Discussion Starter #5
Appreciate the info. My temp gauge is not working (only registers at bottom of the blue). Everything I can think of has been tried - new temp sensors, BECM checked (including different BECM), new instrument cluster, grounds cleaned. Was hoping there was a simple bad wire from BECM to cluster. I can get a temp reading via OBDII so there is a signal from the sensors to BECM/ECU I assume. Meaning there's an issue from there on.
 

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I've just had a double-check and the separate wires from the BECM to the cluster are for tacho/speedo. The serial link takes the information for the fuel and temp gauges - so if the other functions in the cluster are working OK, then the issue isn't the wiring between BECM and cluster.

If the BECM and cluster have been swapped and no change - then your issue has to be in the wiring from the sensor to the BECM.

The temperature read on OBDII comes from the engine ECU only, if you are using a generic scanner. I think you can read a temp on the BECM but would need something like a Nanocom to access that info as generic scanners wouldn't be able to get the data from the BECM. But if the engine ECU is reading a temp the the wire to that is OK. The BECM uses a different temperature sender (in the same sensor housing on a Thor vehicle though) so from the sensor there are 4 wires - 2 will be sensor grounds, and then one goes to the engine ECU and the other to the BECM.

I would be guessing that there is an issue in the wiring from the sensor to the BECM if everything else is checking out. Reading a temp on OBD from the engine ECU just proves that the sensor and wiring for that is working OK, but could still be an issue in the other wire from the other part of the sensor to the BECM.
 

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Discussion Starter #7
I've just had a double-check and the separate wires from the BECM to the cluster are for tacho/speedo. The serial link takes the information for the fuel and temp gauges - so if the other functions in the cluster are working OK, then the issue isn't the wiring between BECM and cluster.

If the BECM and cluster have been swapped and no change - then your issue has to be in the wiring from the sensor to the BECM.

The temperature read on OBDII comes from the engine ECU only, if you are using a generic scanner. I think you can read a temp on the BECM but would need something like a Nanocom to access that info as generic scanners wouldn't be able to get the data from the BECM. But if the engine ECU is reading a temp the the wire to that is OK. The BECM uses a different temperature sender (in the same sensor housing on a Thor vehicle though) so from the sensor there are 4 wires - 2 will be sensor grounds, and then one goes to the engine ECU and the other to the BECM.

I would be guessing that there is an issue in the wiring from the sensor to the BECM if everything else is checking out. Reading a temp on OBD from the engine ECU just proves that the sensor and wiring for that is working OK, but could still be an issue in the other wire from the other part of the sensor to the BECM.
Appreciate the info. I'll have to figure out which wires go from sender go to the BECM directly.
 

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Discussion Starter #8
Am I correct in that pin 19 green wire on 20 way green connector to BECM is from the temp sensor? If I can trace that one and the ground I maybe be able to bypass them.
 

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Sounds to me like you are over thinking this. If you've tried a different BeCM, instrument cluster and temperature sender, it has to be a problem in the wiring. As Marty has said, the sender is two in one, one to feed the engine ECU (and that is the one that supplies information to OBD) and one that feeds the gauge. You have a problem with the gauge sender side.

Unplug the temperature sensor connector, confirm you have a connection to earth on the black wire and short the green and black wires together. That should cause the temperature gauge to read hot. If it does, only the ECU side of the sender is working and your new sender is faulty. The green wire goes via pin 12 on a pair of connectors, C0162 and C0448, a pair of large round connectors that live under the coolant header tank. Check that and try earthing the green wire there and see if the gauge reads hot. If it still doesn't, try earthing the green wire at pin 19 on C1289 (20 way green connector on the front of the BeCM) at the BeCM. Check for continuity between the green wire at the BeCM and the green wire in the plug to the temperature sensor.
 

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Yes,
Pin 19 - Green wire on the 20 way green connector is the wire from the temp sensor in the engine bay.

The ground for the sensor is shared with a few others (including the BECM) on a stud under the carpet at the back of the BECM.

It also goes through C0448 on the way there, which is the big round connector under the coolant tank (connects the engine ECU/harness with the body loom/fuse box/becm etc). It has been know to get corrosion on those pins too... Pin 12 on that connector is where the green wire goes through.

If you disconnect the connector and check the resistance on the wire with a multimeter, between the green wire and ground, you should see somewhere around 5K ohms I would expect at normal temperature before starting/running the vehicle. As the coolant warms up, the resistance drops.

If you can use a thin piece of wire to poke in the back of where the wire goes into the connector, with is still connected to the BECM, then you should be able to read the voltage between the green wire and ground. At ambient temperature (before startin engine, but with ignition on) I would expect to see about 4v on the green wire, and again this would drop as the coolant heats up - to around 1V with the coolant at operating temperature.

Hope this helps with your troubleshooting!

Edit - Richard beat me to it... grounding the green wire at various places might be quicker - though if that all checks out, then check the resistance of the green wire, as the resistance range of the sender is pretty low in the operating temps of the coolant (it's not linear resistance change vs temp)... so if the wire is OK, but high resistance due to corrosion, then it can cause it to read low - but grounding it solidly might give it enough to make the gauge move. but it's quicker to do than looking for voltages at the BECM
 

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One thing to remember, the gauge is a heavily damped, hot wire gauge, so grounding the green wire will not cause it to shoot up to the top of the scale immediately. It will gradually climb over a period, so give it 30 seconds or so to see if it moves.
 

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Discussion Starter #12
Am I crazy to try and swap the green wire all together with a new wire from the sensor to BECM pin 19 as a test. If the grounds are shared and my only issue is the gauge I would assume it has to be the wire (assume of course).
 

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Not crazy at all. It will confirm (or not) a wiring problem and allow you to fault it at your leisure.
 

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Discussion Starter #14
Thx. I've been trying to confirm the other 3 wires on the sensor, which are the grounds and which is the feed to the ECU. Can anyone point me to a schematic as I can't seem to find them.
 

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The ETM for the instruments only shows half of the sensor but the full thing is shown in the section on the fuel injection.

sensor.jpg

Plug layout

plug.jpg

From these you can see that the Green/Blue and Red/Black both go to the engine control module (so are the ones that send a signal to the OBD port) while the Green goes to the BeCM and the Black is grounded.
 
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