temperature indicator failure still not solved

graham.hassall · 3 Jan 2013

This is turning into a difficult problem the story so far

I purchased my 2004 c4s tiptronic in Sept and the only issues I discovered on the drive home was an intermittent fault on the temp gauge which went to zero and the red warning light in the gauge would start to flash. It would also not get to normal working temperature when it was working and settle at 60 deg C.

The outcome of this was that I took it to a local independent who advised me that it was just a matter of replacing the sensor which he did and on the drive home the fault came back. I took it back and he fitted a new level sensor again this did not cure the fault. I lost confidence in the independent at this stage and started to do some research on this problem myself and discovered that it is a relatively common fault which can in some instances be cured by cleaning contacts in the path from sensor through to ecu or in some instances only by replacing the whole instrument pod. I obviously hoped for the former and with some time on my hands between new year and going back to work I have set out to sort this problem.
Firstly I set about cleaning the contacts on the multiplug (the lower one of the two) using a glass fibre pen as recommended by yo clarkie and quicksilver this was quite easy once the air filter was removed, but after a test drive regrettably the fault was still present, consequently I figured I hadn't cleaned them well enough so I decided to bypass the multiplug by making up a connector to go to the sensor and run some wires directly to a point close to the ecu where I spliced into the blue orange and brown pink cable wires after checking for continuity. Very disappointingly this did not cure it either as it failed again after the same amount of time on the test run.

I have now removed the instrument pod as I figure there must be a fault here and I will send it to BBA reman for checking out and repair if possible but before I do this is there anything else I can try?

One thought I have had is that the water temp is low at 60deg C possibly because the valve is not closing on the cooling circuit to the tiptronic thus overcooling the engine possibly because of a problem with the vacuum pipe but in addtion could this also be confusing the ECU and creating this fault?

To simulate correct operating temperature I could put the temp sensor which was removed by the independent (which I retained) in hot water (80deg C) and measure the resistance I could then put a resistor of this value in the circuit in the car. Will this create any unforseen problems?

Sorry this is turning into a bit of a long post, any help would be much appreciated as although this is a trivial problem it is spoiling my enjoyment of the car.

cheers

Graham

GT4 · 4 Jan 2013

So neither new nor original sensor results in correct gauge function?

Conclusion: it is likely neither sensor is at fault, therefore likely both work.

Assumption is that you believe above is true or you wouldn't be suggesting calibrating a "dumb" resistor with the original sensor's characteristics.

It it not safe to assume the car is NOT running at operating temp (both Tip and block bypass thermostats would corroborate, along with oil pressure dropping to circa 2 bar, and even a toasty cabin heating would indicate correct temp, and ultimately the diagnostics circuit/DME reading the sensor temperatures - ie PIWIS could confirm all these).

It is more likely the car IS running at operating temp (if anything M96/M97 blocks have trouble not overheating under even ideal conditions, and any issues tend to actually raise the temp - there is no known under temp scenario after warmup).

Therefore, if two working sensors under at least operating temp conditions don't result in a correct gauge reading, neither will a substitute resistor looking just like a working sensor and installed in its place.

The gauge itself almost certainly needs replacing, although PIWIS will confirm in any case.

Waz · 4 Jan 2013

I don't know if this will help you may have already done it. Borrow a set of clocks you can lend mine if it helps.
Mine did what you described after I removed the clocks and put them back.
It has stopped doing it now after fiddling with the block connector on the rear of the clocks. Worth a try :thumb:

graham.hassall · 5 Jan 2013

I will try and read the actual temp from the OBD socket as I am sure that GT4 is right re the apparent low temp and gauge fault and taking the instrument pod out and disconnecting/reconnecting is no big issue which has got to be worth a try (thanks Waz)

Out of interest I assume the temp should be 80degC or slightly lower as I have fitted a Hartech thermostat.

cheers

Graham

infrasilver · 5 Jan 2013

these niggling problems can be a PITA.

With the low temp stat the clocks gauge needle should read between the 8 and 0 (higher without) but the digital readout will show it at a slightly higher temp.

It does sound like the engine is running at the correct temp and there is a fault somewhere else.

I know you have run a wire to bypass the engine loom multi plug but have you checked the supply voltage at the wiring plug and resistance at the sender on the engine?

graham.hassall · 5 Jan 2013

hi infrasilver

I checked the resistance of the new sensor when it was connected at the ecu and it was 3.5kohms cold. I have not checked the supply voltage at the wiring plug as I am a little unsure what you mean. Is the temp sensor powered? I assumed it was passive and the change in resistance was all that was measured. Do I simply measure a voltage between the blue orange wire and brown pink wires used to connect to the sensor? Should it be 5volts or 12volts?

cheers

Graham

GT4 · 5 Jan 2013

On mine standard was 90°, which corresponds with needle straight through "0" of 80.

With Hartech LTT the needle pointed just left of "0" in "80" (although actual OBD signal was 83°)

graham.hassall · 5 Jan 2013

A quick update

I removed the instrument pod and reconnected last night and horror of horrors this morning the oil pressure indicator, oil level indicator, fuel level sensor and washer bottle sensor all gave a failure indication, however it started so I drove the car to a friendly garage who measured the temperature of the coolant flow via the OBD port and his Snap On diagnostic unit and the temperature after a 10mile drive indicated 62deg C.

This has left me most confused as the temp indicator failure message came up during the journey to join all of the other failed indicators but when it was working it tallied with the signal from the OBD port. Gloom and doom I thought I understood the issue but now I have to rethink. Firstly I wanted to get back to a situation where only the temp indicator fails not every other sensor on the car so I presumed maybe a fuse so I checked the obvious candidates in the fuse box but all these were fine I then disconnected the earth on the battery and removed the instrument pod expecting to see a cable not connected or hanging off. All seemed fine so I released all the connectors checked for bent pins and all seemed fine so I then offered a short prayer to the gods reconnected everything and all the previous messages of doom disappeared and all the gauges are working.

So I am at least back to where I started from in having this weird temp indicator fault and low temperature measurement.

I still think however that GT4 is right as even with the tiptronic coolant circuit also cooling the engine I am sure that after a while this would rise to 80deg C which should be the case after a 10 mile run. So I have to ask why is the obd port reading 62degC could it be that rather than reading directly from the ecu it reads via the temp gauge giving a false reading? On pre 2001 cars I think the temperature gauge took its signal directly from the sensor from the wiring diagram I have whereas 2004 cars the sensor signal goes to the ecu and from the ecu to the instrument pod.

I think my next step is to get the car up on some ramps remove the covers and check that the valve closes when the engine is started from cold.

Any other suggestions?

Jethro · 5 Jan 2013

When you took the readings from the OBD port of 62degC did you leave the car idling for long enough so that the fans came on ?

infrasilver · 5 Jan 2013

graham.hassall said:
hi infrasilver

I checked the resistance of the new sensor when it was connected at the ecu and it was 3.5kohms cold. I have not checked the supply voltage at the wiring plug as I am a little unsure what you mean. Is the temp sensor powered? I assumed it was passive and the change in resistance was all that was measured. Do I simply measure a voltage between the blue orange wire and brown pink wires used to connect to the sensor? Should it be 5volts or 12volts?

cheers

Graham

I can't remember what the wire colours were but I was getting (i think) 5 volts supply from the wiring, that wiring also runs quite tightly from the engine wiring loom down behind the power steering pipes and near the inlet on the O/S and mine was slightly worn through so I cut the cable and re-soldered to make the connection better, I didn't cure my problem though at the time but could of caused an issue in the future.
Trace those cables back to your loom, its worth eliminating anyway.

infrasilver · 5 Jan 2013

The red oval is the cable and the yellow oval is where mine was chafed slightly.

graham.hassall · 5 Jan 2013

Well its been a long day but I think I may have solved it.

I got the car up on some ramps and removed the plastic guard to give access to the tiptronic coolant valve and solenoid, I then rigged up my Christmas present a boroscope for attachment to my pc and got a friend to start the car (I didn't fancy lying underneath it when it was running) . I expected to see the valve close but it didn't move, I then got my friend to rev it but still no movement on the valve. I then decided that I would manually close the valve by wrapping it in wire (there was very little resistance to me pushing it closed).

I then restarted the car and let it idle for 10minutes and noticeably the temperature increased faster and settled at 80degC. I then took it off the ramps and went for a 5 mile test drive and the temp stayed at 80, and there was no gauge failure! This is my first run in 4 months of ownership without a temp indicator failure and to be honest I am hugely relieved, I appreciate that I haven't sorted out the vacuum problem to close the valve but at least I have discovered the issue for my car which I would surmise as follows

Tiptronic valve does not close creating excessive cooling which the ecu intetrprets as a temp indicator failure.

I will take it on a longer test run tomorrow however before I do this could somebody confirm that the tiptronic gearbox only needs cooling when under high loads, (does it really need it under normal driving conditions?)

Assuming that all of the above is truly a light at the end of a tunnel and not an express train, does any body know the routing of the vacuum pipe from the gearbox to the inlet manifold? As I think replacing the vacuum pipe is step1 for repair, failing that I guess I may have to replace the vacuum chamber on the valve and test the solenoid which I replaced earlier as I thought it was suspect.

I would also be grateful if someone could confirm my assumption that the valve should close when the car is idling is correct?

Thanks so far for all of the help

Graham

GT4 · 5 Jan 2013

I'm glad if that is the solution, but I am surprised.

The reason for both the 1) Tiptronic spur on the cooling system (via Tip heat exchanger matrix and valving) and 2) the additional centre rad is that the Tip gearbox on average gets 1) too hot and 2) would the coolant temp too high without additional radiator.

ie it is expected to be a net contributor to heat, not a heat sink.

Obviously if the bypass open all the time then the time to operating temp may be extended, but it should not lower that actual temp.

I was assuming (in fact I think you mentioned it) that you had driven the car for a long period and never exceeded 60ish degrees.

If that long period was cruising on the motorway, rather than just extended "normal" use, then I guess it could get too cold.

Seems amazing to me a passive heat radiator (ie gearbox case) could possibly dump more heat than the front active radiators (ie finned and fanned) and overcome 320 BHP.

These engines are renowned for their overhearing, if the solution was just to buy a Tip and overide the bypass you have just doubled the value of any Tip 996 or 997!

(that last bit wasn't meant sarcastically either)

Just to clarify, if this is a real effect, as per one of my previous posts, did this have any effect on the cabin heating intensity?

graham.hassall · 5 Jan 2013

Thanks gt4 for the rapid feedback. I think that every point you make is correct, however it is the case that I have never seen the temp gauge over 60deg C in my period of ownership until today which covers all sorts of driving. I am not sure about the effect of the heater I will review it tomorrow.

Could you confirm that there is no problem with running the car with the tip valve manually closed while I test?

I have to say that it did occur to me that all of these overheating problems people have could be controlled by varying the flow through the tiptronic heat exchanger. Something is not right on this basis as a gearbox as you correctly point out is not a radiator it is a heat source and that eventually equilibrium will be reached defined by the cooling capacity of the radiators and thermostat which should be about 80deg C.

cheers

Graham

GT4 · 5 Jan 2013

The Tip gearbox will have it's own temp sensor and will both warna nd ultimately go into "limp" mode if it overheats, and that is unlikely this weather.

I would not seriously consider doing this for anything other than a temporary test run, and even then I don not know the long term effect on wiring a bypass shut.

And if there weren't some need for the bypass it wouldn't exist (although as suggested, this may depend on both load and weather - and Porsche sell everywhere from Dubai to Alaska)

Something still doesn't sound right and I would still suggest a respected indy or OPC really diagnoses the issue to your satisfaction.

temperature indicator failure still not solved

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