How is the ecu powered

Hi i swapped a 1.9 tdi ALH into my audi a4. I am mostly done now just the i cannot get a communication with the ecu. If i connect VCDS it doesn't find the engine module. Does anyone know how i can test if the ecu is getting power or which pins should be powered? Or does anyone have any other idea what i can do now?

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Which ECU are you using ? 68, 80 or 121 pin ECU ?

121 pins

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Ok, thats the 121 pin ECU. Pins 1&2 battery power from eng. control relay,
pins 4&5 are earth, pin 18 is the earth for engine control relay, pin 37 ignition switch
& pin 16 is for the dlc, normally a grey/white wire

Don't forget pin 88 if you want your glow plugs to work.

Pin 88 needs to have a 12v with ignition on, 0v when cranking and 12v
when running, mine would not start without 12v at this pin

So 1 and 2 have power and 4 and 5 are ground. For the glow plug relay it never gave 12v output. When i had it on the off position there was almost no power, and in the on position there was 6 volts and then it started going down till around 1 volt. And in the run position i think there was 3 volts or so.. Does that mean that the ecu is getting powered and it isn't connecting the the vagcom because of something else?

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The glow plugs sound right. I'm pretty sure the output is pulse width modulated so that it can deliver variable power to the plugs, so it would be normal to see the average voltage changing.

VCDS talks to the ECU using the K-Line. However, the K-Line is called the W-Line when it exits the ECU on pin 16 (gray wire with white stripe). The W-Line goes into the VW cluster on pin 5 of the green connector. The K-Line exits the VW cluster on pin 25 of the blue connector (also gray wire with white stripe). From there it goes to pin 7 of the OBD connector that you presumably have VCDS connected to. The cluster passes data back and forth between the K and W lines.

I don't know anything about how Audi clusters are wired but if the cluster does not pass data between the K and W lines, then that would explain why VCDS is not seeing the ECU. You may need to manually connect the K-Line of the OBD connector to the W-Line coming from the ECU.

gmenounos said:

The glow plugs sound right. I'm pretty sure the output is pulse width modulated so that it can deliver variable power to the plugs, so it would be normal to see the average voltage changing.

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Glow plugs are not PWM. Unless it is 4C or colder they just don't come on! Unplug the coolant plug to simulate -4.5C and they will come on.

If your OBD plug is connected to the ECU via pin 16 Vagcom will read the ECU
The cars I've been converting are not using immo's and the instrument clusters
are all analogue. Also where I live glow plugs not required as not really cold enough
for the engine not to start without them.
If you need the glow plug relay then the wire from the glow plug relay to pin 42 is how
the ECU switches on the relay. Permanent power is pin 30 on the relay, pin 87 feeds
the glow plugs, pin 85 comes from the Engine control relay pin 87.
Be aware that if the temperature is too high the glow plugs won't work so checking
voltages at the plugs will be difficult.
Or as Hasenwerk said unplug the sensor !

Hasenwerk said:

Glow plugs are not PWM.

Click to expand...

Newer ones are:

https://www.st.com/en/automotive-analog-and-power/l9524c.html

Hasenwerk said:

Unless it is 4C or colder they just don't come on!

Click to expand...

https://www.myturbodiesel.com/1000q/glowplugFAQ.htm

"After every engine start, hot or cold, whether temps are above or below 48F, the glow plugs are on. This is called afterglow. This helps emissions, idle quality, and warms up the engine faster. The faster an engine gets to it's normal operating temperature, the faster the engine seals up and closes the tolerances, creating less blow by gasses and burning less engine oil. Once it's warmed up the engine also becomes more fuel efficient and experiences less engine wear."

Ok thank you all.. It was just the k line wire i didnt have... I thought that the high and low-can were how you "talked" to the ecu. But now it works. Thanks again

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gmenounos said:

https://www.myturbodiesel.com/1000q/glowplugFAQ.htm
"After every engine start, hot or cold, whether temps are above or below 48F, the glow plugs are on. This is called afterglow. This helps emissions, idle quality, and warms up the engine faster. The faster an engine gets to it's normal operating temperature, the faster the engine seals up and closes the tolerances, creating less blow by gasses and burning less engine oil. Once it's warmed up the engine also becomes more fuel efficient and experiences less engine wear."

Click to expand...

I'll beg to differ on some ECUs. The ALH "CP" doesn't have "afterglow" - think about it - the instant the Diesel goes 'boom' it's hot in there - hotter than what a GP will produce! Perhaps someone is mistakingly referencing the coolant glow plugs that DO come on after the engine starts making the above reasons for getting to operating temperature valid.

Every TDI has afterglow... the plugs are run after the engine starts until the coolant gets to some temprature. Even the late IDI motors did this.

Having afterglow greatly reduces HC and PM emissions on a cold start.

compu_85 said:

Every TDI has afterglow... the plugs are run after the engine starts until the coolant gets to some temprature. Even the late IDI motors did this.

Having afterglow greatly reduces HC and PM emissions on a cold start.

Click to expand...

Yes, to the tune of about 45 seconds of afterglow I think.

My old MB S350 TD has even more afterglow than that IIRC.

Steve

MB used at max 60 sec. See page 67 here: http://eva2.homeip.net/MB91/1990-1991_turbodiesels.pdf

Afterglow on a TDI can last several minutes in the winter time.

I tend to agree with Hasenwerk on the after-glow of the engine starting GPs. My only experience that leads me to agree was working on friends 01 Jetta with ALH engine about a year ago (two-wire set-up). I opened the GP Relay. Using temporary wiring leads, by observation, I did see that the main contacts in the Rely that provide power to the GPs did not close (ambient in the 70s, Fahrenheit). And, there is circuitry in the GP relay that allows GP condition monitoring for an instant when the ignition is turned on without glowing the plugs, regardless of engine or ambient temperature. It's all right there inside the GP Relay.

So, how could there be after-glow if there was no glow?

The GP system in the MK1s keeps the GPs hot for a few seconds after the indicator light goes off. Back in the day when my hearing was good, I could hear the relay click off. Also, observing the aftermarket volt gauge indicated that voltage bounced back when the GP relay kicked out.

This discussion has enticed me to find that GP Relay in my stash of parts and do the test and observations again.