Outside thermometer sensor question vs ECU

[beetle928]

Hi, I'm new to the Beetle world. I like these cars and especially TDI's
I just bought what the budget could afford a 2003 New Beetle TDI.
I'm satisfied with but I have a couple of problems I would like to solve.
My outside thermometer don't work and I found the sensor was missing under front bumper.
Also I have a problem when starting the car warm.
Is I start it the morning when cold like -15 Celsius I make a few keyturns to get more glow plugs and it starts well.
If I start when hot it's ok too.
If I start when cold outside and the car has sat for 4 hours I must crank a looong time to be able to finally start it with a lot of smoke. The glow plugs lamp is almost not there at keyturn when I think it should. So no difference if I make many keyturns to get more Glow plugs it's looong to start.

I wondered if the computer (ECU) considers the outside temp when calculating the time the glow plugs should be ON ??
If yes it would explain my starting problem no?
I would appreciate any help
Thank's

 

[dweisel]

temp sensor

Ambient temp sensor located down low on the driver side bumper is just for the clock/temp display. The Coolant Temperature Sensor (CTS) on the rear part of the engine does have to do with starting glow time. Try unplugging the CTS and see if you get better all around starts. Could be a bad CTS. Also you can change glow time with VCDS program.

Hope that helps.
dweisel

 

[beetle928]

Thank's a lot for the help. If I unplug the CTS will the glow plugs heat a long time hot or cold?

 

[Vince Waldon]

The ECU only uses the coolant temp sensor to trigger the engine glow plugs... and if you unplug it the ECU will think the coolant is really cold.

 

[dweisel]

Thank's a lot for the help. If I unplug the CTS will the glow plugs heat a long time hot or cold?

You'll get max glow time.

 

[beetle928]

Yes Yes Yes this is a logical test for me Thank'S But where exactly is this sensor?

 

[beetle928]

Is it the one on this image?

 

[dweisel]

Is it the one on this image?

No, if you remove the accordion air duct from the air cleaner, you can reach it better. It's on the back right side of the engine. You can Google it.

 

[beetle928]

The above image is the result of my Google search
I will take a look at this during the weekend and I'll let you know the result if ever it could help someone else with same problem. Lately when it was suppose to be very cold I connected the heater of the car over night. The morning after the engine was easy to turn but the glow plug time was ridiculously short so the engine did'nt start I've been able by turning the key many many times to get enough glow plug and it started after a looong crank. That's silly.

 

[dweisel]

My mistake. Yes, that is the sensor. I was confused because the pic is shot from the side of the engine and not down. The pic makes the sensor look like it's in the wide open.

 

[beetle928]

Then it explains why when the heater is connected the glow plugs don't work long?? The computer thinks the engine is hot. This is a silly configuration no? The starting problem I had after a night connected is because means my CTS works no?

Thank's a lot for your help.

 

[Vince Waldon]

If you are using a coolant heater then then engine IS hot...and doesn't need the glowplugs, exactly as designed.

That said, if the car has only been plugged in for a short time and the ambient temp is low convection can warm the coolant by the CTS faster than the rest of the block and fool the ECU a bit. If the car has been plugged in for a couple hours everything equalizes.

Also worth noting that a TDI in good condition will start well even without glowplugs...so a hard start when cold often has a non-glowplugs-related cause...retarded timing or a weak battery/starter being two common issues.

 

[beetle928]

Thank's a lot Vince. I understood well. As soon as I can I'll check that.

 

[Vince Waldon]

No worries...will be interesting to hear what you find.

 

[beetle928]

Ok I've been able to disconnect the coolant temp sensor. Odd place and odd kind of connector. I get full glow plug now I'll try it well in the next days and let you know.
Altough I have another question. Is the CTS used for something else tahn the glow plug time? Like the blue temp light or engine temp indicator?

 

[Vince Waldon]

Yes, but the one physical CTS body has two internal sensors. One is used for the glow plugs, the other for the cluster (blue temp light, gauge if equipped).

Very common for one side to fail and the other to work perfectly well, if that's what you're wondering.

 

[beetle928]

Thank you Vince that's exactly what I was wondering.
Thank's again.

 

[beetle928]

After a few tests it starts very well since I disconnected the CTS. I give it the glow plug time I want
My check engine lamp is On since I disconnected the CTS. Is it possible?

 

[Vince Waldon]

Yes, the ECU will have:

- detected the missing CTS
- stored a trouble code in non-volatile memory
- turned on the CEL to tell you that there's a code waiting to be read
- used a default coolant temperature to allow the engine to run in the meantime

You'll need to actively clear the code with a scanner... it won't go away when you plug the CTS back in.

 

[dweisel]

Yes,disconnecting the CTS will throw a fault code.

 

[beetle928]

Jeeee you're fast and precise guys Thank's

If I may push it a bit further. You said that there were several informations (at least 2)coming from this sensor assy. My next question : What if I disconnect only the part that helps the computer decide the glow plug time? Is it possible ? Will the check engine still lite? I would like to drive the glow plugs by myself all the time. I could cut the right wire If I find which one it is
Thank's again

 

[dweisel]

I didn't say that someone else did. No matter what wire you would cut,you're going to get a CEL. Just set the glow time to MAX with VCDS. Start it whenever YOU feel it's right. The plugs will still be ON after you start. Seems to me you're going through lots of trouble to save a few seconds.

 

[Vince Waldon]

Jeeee you're fast and precise guys Thank's

If I may push it a bit further. You said that there were several informations (at least 2)coming from this sensor assy. My next question : What if I disconnect only the part that helps the computer decide the glow plug time? Is it possible ? Will the check engine still lite? I would like to drive the glow plugs by myself all the time. I could cut the right wire If I find which one it is
Thank's again

Cutting wires is of course possible, but this would also mean:

- the ECU will always have to use a fixed default temperature for fueling and timing decisions... far from optimal
- if your car's a manual the ECU will run the car's coolant glow plugs non-stop during the winter... big extra load on the alternator
- your CEL will be on all the time, which may mask other more important issues

Your car your rules... IMHO the glowplug system on these cars is well designed and probably capable of making better glow plug decisions than a human. IMHO of course.

If your car is having winter starting problems with the stock glow plug system in proper operational order you've got other issues... timing, compression, fuel system.

 

[beetle928]

Thank's a lot for all infos. I think the fact that the glow plugs continue their cycle after the engine has started is wrong. It is as odd as putting a sensor on the coolant to determine if the combustion chamber is hot enough to burn diesel. I'm far from a genious and maybe I missed something. I will certainly change the CTS and I must say that I did'nt used the car yet with a good CTS and correct glow plug time from cpu. Maybe once repaired I will be pleased with it
Since the CTS is disconnected it's starts fine everytime. If I don't like it after changing the CTS I could run a separate manually switched 12 volts line to the glow plugs and do what I want with it without any CEL ?

 

[dweisel]

Thank's a lot for all infos. I think the fact that the glow plugs continue their cycle after the engine has started is wrong.

I'm wrong and i'm done. Sorry for the misinformation. Believe whatever you want.

https://forums.tdiclub.com/showthread.php?t=352922

 

[beetle928]

Sorry I did'nt mean that you're wrong I meant that it's not well done. Sorry but I'm french native and sometimes it sounds odd.

 

[dweisel]

It's designed that way to smooth out the engine after a start. Years ago on Cat heavy equipment you manually glowed for a set time depending on ambient temp. Then once the engine fired and was running you manually turned the glow plugs back on until the engine smoothed out. Sounds to me like you need one of those old Cat start/glow switches on your car. LOL

 

[Vince Waldon]

It also significantly reduces emissions by getting the combustion chambers warmed up as quickly as possible... so that *all* the fuel burns.

Since the glowplugs have an effect on emissions they are carefully monitored by the ECU. So far all of the changes you're proposing will throw a permanent CEL.

 

[beetle928]

Thank's to both of you for your help time and patience. I should be ok.

 

[beetle928]

Finally I changed the Temperature Coolant Sensor and erased the code with my obd2 code reader. I still have problems starting when warm. While I was retrieving my codes I saw that the advance was set at 4 degres. Is it too much or not enough?