Coolant temperature and intake air temperature

The -40 at any time is an issue. Probably the default low reading and the sensor is going open from time to time. We will remain confident it is not a harness issue. Cheapest easiest diagnosis is to replace all 3. While the manifold is off replace the coolant hoses. If the 2 big ones have the excellent factory silicon hoses you can probably skip those but the small air bleeder hose to the coolant tank needs to be replaced.
Removing the intake manifold every few years should be considered routine service on them. If it has any oil leakage at the valve covers consider doing that at the same time. Spark plug change too if close mileage wise.

Air temp sensor is easy to get to. While reading its value try thumping it and shaking the wire and connector and see what happens.

Starting on one bank is not common or normal nor would I consider it OK. If it was my car I'd want to know why before it snowballed into something serious or something that would leave me stranded.


By the way. When you say air temp, do you mean intake air temp or ambient air temp?

The car has an ambient air temp sensor that feeds info to the HVAC, TPMS system if so equipped and on some models to the engine management system. Typically if that goes bad you get lots of warning lights.

 

I don't see a connection in the wiring diagrams between the outside air temperature sensor under the driver's mirror and the engine management system. I can only see a connection to the HVAC ECU. The LH Bank Coolant Temperature sensor also sends data to the HVAC ECU (but not to the corresponding Motronic ECU).

I believe the RH bank coolant temp sensor is the only one used for engine management (and emergency fan control). The RH ECU sends its coolant temperature sensor data to the LH ECU on inter-ECU wiring.
The intake air temperature on the rh manifold appears to be feeding the LH Motronic ECU, so I assume the LH ECU feeds the data to the RH ECU on the inter-ECU wiring. Some of the diagrams in the WSM seem to contradict each other. I guess you'd have to confirm the wiring with continuity checks.

If you're losing a single bank because of loss of coolant temperature data, it may not be the coolant temp sensor, but wiring between the ECUs.

There is a temperature vs resistance table for coolant temperature sensors, but if you have an intermittent problem, the resistances may be ok when you check them. If it happens when the car is cold, I would check them then.

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If you can't get to the sensor plugs to do resistance checks, I suppose you could do checks at the ECUs.

e.g.

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The factory diagrams for 550 engine management is full of errors. Every time they tried to fix it they made it worse. One version had Motronic running the wipers.

 

Given the readings and that you lost a bank when starting ..have you check battery

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Sounds good as long as cranking voltage is above 10.5 11

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Is there any chance you know when it starts on one bank which it is starting on? Sometime when it does it put your hand behind the tail pipes and see which is warm and which is cold.

 

If it only does it very briefly it is most likely fuel bleeding down and not electronic. You didn't say that before. Huge difference.

 

Sorry to sidebar this thread, I assume these checks works on a 2.7 456 car as well? Or is there another way to diagnose if the primary coolant sensor is operating correctly without accessing the sensors?

 

Do you mean using the pins on the ECUs to check sensor resistance? Unfortunately, I don't think we have pinouts for the 2.7 ECU and I don't know if the temp sensors are used for the same functions. I haven't even seen a diagnostic chart for the 456 which would tell us the pins. The 456 manual I have doesn't include the 2.7 engine management system. On the 456 5.2, the RH coolant temp sensor seems to be the only one which feeds into the engine management system, but you shouldn't assume the 456 2.7 is the same. I would check to see if the same part number sensor is used on both cars. If they are, you could use the chart above.

 

The part number seems to be the same (125769), although the 5.2 only has one sensor, the 2.7 has two, so the 2.7 may use both for engine management (like the F348 2.7 and F355 2.7)

 

You are the best! I appreciate it.

 

Send to Rod Drew at FAI in Costa Mesa. Far better chance of actually getting fixed.

 

Trying to test my old sensors right now but not getting anything close to this chart

 

Are the sensors in hot water or are they touching a hot metal surface?

What is the resistance at room temperature?

 

86 (?) ohms suggests around 130deg C.

 

In heating (up) water. At ambient temps it was around 30ohm raising as temperature increases

 

165*F water around 60ohm

 

Weird. They should have a negative temperature coefficient. Hot = lower resistance.

If they are both the same, then you would have to assume they are ok, but there could be something wrong with your meter. Is it the same on the 200 ohm scale?

 

Are your ohmmeter leads plugged into the right holes?

 

Usually the black lead goes into the COM hole. The red should be in volts, not amps.