Intermittent Coolant sensor problem? -GT3200

Ian H

Member
Messages
167
Hi
Haven't been on forum for a while due to best part of a years faultless GT3200ing.
Just about to book her in for MOT when she throws a small wobbly-

On starting I noticed coolant temperature gauge doesn't register and after a while I get rough running on acceleration. After ~10 mins. tempr gauge jumps to normal (maybe as thermostat opens or maybe just co incidence) and engine settled down to usual purr.

Next time out coolant tempr gauge starts ok and engine runs smooth but after ~40 mins. Coolant gauge drops to zero and engine then bit rough on acceleration. 10 mins. later comes back to life again.

I assume this is intermittent fault on coolant temperature sensor or associated wiring which is sending wrong data to both the engine ECU and in dash gauge?

Before I start digging under the bonnet -

Is this the likely cause or something else (changed the coolant thermostat and radiator cooling fan resistor last year so I assume these are still ok)?
Is there just one sensor feeding both the engine ECU (if indeed it uses one ) and the in car gauge?
Given the signal comes and goes is it likely to be the wiring or can a dodgy sensor behave like that?
Where do you find the sensor if it needs replacing?
Is it worth checking/trying to fix or should I just source a new one?
Does anyone know of an alternative source for new sensor eurospare look cheap ~£7 but non on the shelf

Many thanks for the help as always

Ian
 

Trev Latter

Member
Messages
1,213
It does sound like either a failing sensor or associated wiring. I'd check all of the connectors are good and tight first. There may be an alternative part which is more readily available, but I can't help with that. Extract from the manual below.

3200gtec-043.jpg
 

Ian H

Member
Messages
167
Hi
Trev many thanks for the manual page.
Davy83 yep coolant levels all good.

You can just about get to the senor from the front of the engine so had a quick look. Plug contacts seem fine and no obvious issue with probe.
Ran the diagnostics and get error code P10115 - Engine coolant temperature - when probe is disconnected which is encouraging that the error codes work.

Checking the error code on http://www.datsc.com/tabid/122/OBD-Error-Code/P0115/Diagnostics/Maserati/Default.aspx provides some great further diagonistics help.

You can check the wiring by ensuring there is 5v background voltage supplied to probe with key on engine off.
You can check the sensor by measuring resistance which should be between 200 ohms at 100c and 10,000 ohms at 0c (yep resistance should drop with increased temp).

Both these checked out ok with background voltage and resistance ~2000 ohms at 20c - which just confirms fault is intermittent and more hunting to be done.

Unless anyone has a better idea think I'll push ahead and replace probe and hope this cures the problem.

Still not quite clear where dash temperature gauge gets its signal and whether it is from the same source. Error codes commentary (see below) suggests an alternate source but that may just be for Ferrari.

The wiring diagram (p15) suggests signal source for dash gauge is from Bridge Can Controller but I don't know whether this is picked up from ECU which receives the feed from this sensor or a separate feed which is not shown.
Could be looking at completely the wrong sensor?

On the Diagnostics I also picked up P0351 to P0358 errors which are all the Ignition coil Primary/ Secondary Circuit malfunctions. This may just be due to some disconnections made during gaining access to the sensors so have ignore to date.


Ian



Generic OBD Error Code Details for P0115


Engine Coolant Temperature Circuit


What does this mean?
OBD Code P0115refers to
The ECT (Engine coolant temperature) sensor is basically a thermistor that changes resistance with temperature. Usually a two wire sensor, a 5Volt reference from PCM (Powertrain Control Module) and ground signal to PCM. This is different from a temperature SENDER (which usually operates the dash temperature gauge and operates in a similar way as the SENSOR, only it's a different circuit than what a P0115 is referring to). As the temperature of the coolant changes, the resistance changes on the ground signal to the PCM. When the engine is cold, the resistance is high. When the engine is warm, the resistance is low. If the PCM detects a voltage condition that seems abnormally low or high, P0115 will set.

Symptoms
Possible symptoms of OBD code P0115
Symptoms of a P0115 DTC code may include: MIL (Malfunction Indicator Lamp) illumination will always occur Vehicle may be hard to start May blow a lot of black smoke and run extremely rich Engine may want to die or backfire in tailpipe Engine may run lean and increased Nox emissions may be apparent (Requires gas analyzer) Cooling fans may run all the time when they shouldn't be, or not at all when they should be

Causes
Possible causes of OBD code P0115
Usually the cause can be traced to a bad ECT sensor, however that doesn't preclude the following: Wiring or connector damaged at sensor Open or short in reference or signal circuit Open or short in signal circuit Bad PCM

Possible Solutions
First, visually check the sensor for damage to wiring or connector and repair as necessary. Then, if you have access to a scanner, determine what the temperature of the engine is. (If you don't have access to a scan tool, using the dash temperature gauge may not be an effective way to determine coolant temperature. This is because the P0115 code is referring to the ECT SENSOR, and the dash gauge is operated by, usually a one-wire SENDER. Basically a different sensor that the code doesn't refer to.) 2. If the engine temperature is abnormally high, around 280 deg. F, that's abnormal. Unplug the sensor on the engine and see if the signal drops to, say, negative 50 deg. F. If it does, then it's a good bet the sensor is bad, internally shorted, causing a low resistance signal to be sent to the PCM. However if you want to be sure it's the sensor and not the wiring, there's a couple tests you can do. With the ECT sensor unplugged, check that you have 5 volts on the reference circuit with KOEO (Key on engine off). Also you can check the resistance of the sensor to ground using an Ohm meter. The resistance of a normal sensor to ground will vary a little depending on the vehicle, but basically, if the temp of the engine is around 200 deg. F., the resistance will be about 200 Ohms. If the temperature is about 0 def. F., the resistance will be over 10,000 Ohms. With this test you should be able to tell if the resistance of the sensor matches the temperature of the engine. If it's not accurate according to your engine's temperature, then you probably have a bad sensor. 3. Now, if the temperature of the engine according to the scanner is around 280 deg. F. and unplugging the sensor doesn't cause the reading to drop to negative 50 deg. F, but it stays at the same high temperature reading, then you'll need to repair the short on the signal circuit(ground) to the PCM. It's shorted directly to ground somewhere. 4. If your temperature reading of the engine according to the scanner is showing negative 50 deg. F or so, (and you don't live in the arctic!) unplug the sensor and check for a 5V reference present at the sensor. 5. If there isn't, then check at the PCM connector for proper 5V reference. If it's present at the PCM connector, then repair the open or short on 5V reference from PCM. If there is no 5V reference present at the PCM connector, then you're done with your diagnosis and you may have a PCM fault. 6. If the 5V reference circuit is intact, then check the ground signal to the PCM using the previous ground resistance test. If the resistance isn't normal for the temperature of the engine, then ohm the resistance of the ground signal to the PCM by removing the ground signal wire from the PCM connector. The wire should have no resistance, unplugged from the PCM to the sensor. If it does, repair open in the signal to the PCM. If it doesn't have any resistance on the ground signal wire and the resistance test of the sensor is normal, then suspect a faulty PCM.



Ferrari OBD Error Code Details for P0115


Engine Coolant Temperature Circuit


Possible Solutions
Malfunction (sig) 1 Check the wiring 2 Check CAN line 3 Check CAN line impedence-60 ohm 4 Check lemperaatue between left and right. Has to be the same.



Sorry no other information is available at present


 

jluis

Member
Messages
1,703
I'd start with the thermostat first.

It's unlikely to be the thermostat as if that was the case the engine would run well (within the parameters for the given temperature) and would not throw temperature related codes.
It would also not cause the temperature reading to jump up and down as described.
 

Contigo

Sponsor
Messages
18,376
If the thermostat is sticking then it could affect the temperature in the way Ian describes. Also trapped air could cause a similar issue. Have you checked the water pump? is it noisy?
 

davy83

Member
Messages
2,809
yes i was worried that there was air around the temperature sensor which does very odd things to the readings but would jump from one reading to another as it changes from being in air to coolant? its probably the wiring though!
 

highlander

Member
Messages
5,214
yes i was worried that there was air around the temperature sensor which does very odd things to the readings but would jump from one reading to another as it changes from being in air to coolant? its probably the wiring though!

Wiring issue in a 3200? Surely not Davy? ;)
 

Rex B

Member
Messages
657
Intermittent faults can be difficult to find as they are often temperature related and will often cure themselves by the time you switch off the engine and get your meters out to start checking. I would think your problem is electrical and would start by changing the sensor and if this does not cure the problem then you need to trace the wiring through each point.

Rex B
 

Ian H

Member
Messages
167
Thanks guys
Instantaneous drop in temp reading does suggest wiring to me rather than vapour lock in coolant but hey never rule anything out :)
Will order sensor, replace and let you know out come.
Just double checking - the temp reading from this sensor goes direct to the engine ECU is this re used by the dash gauge or does that have a separate feed?
Many thanks
Ian
 

Ian H

Member
Messages
167
Hi
Fitted new coolant sensor today courtesy of McGrath (£25 + VAT and delivery) and all running sweetly - both dash gauge and engine.
Part number is 402 183 01. No stock at Eurospare show list it at ~£8. There seem to be a few alternative Alfa suppliers out there who can supply but not too expensive and good to know it is the proper part for a pucker supplier.
Now have a shiny new MOT.
Many thanks for all of your help.
Ian
 

Alana

New Member
Messages
1
Where is the coolant sensor located? The pic you posted is so small I cannot make it bigger.
 

davy83

Member
Messages
2,809
Is it just me or are those last few posts from Ian H and Stewart W dated in the future? I may have slept longer than I thought, but I believe its the 18th of August and these threads are dated 19th and 20th August? has the SM forum finally cracked time travel and can now predict when your 3200 will break down and how long it will take to fix it, excellent!!!
And Ian glad to hear its working again, and it was cheap!
 

saintetienne

Junior Member
Messages
242
Good afternoon everyone ,

over the last few days I am getting intermittent temperature readings as above , I reckon the coolant sensor is probably on the way out .

I had a peek this afternoon and can't see it for looking , would anyone be kind enough to point me in the right direction for it's location

thanks in advance


I have now found it - no thanks to the workshop manual diagram ! :)
 
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