Crank position sensor fixed cold start!

jeffmon

Active member
Joined
Mar 14, 2010
Location
Missouri
TDI
98 Jetta
For about three years, when my 98 AHU Jetta had been sitting in cold temperatures (30F or lower) for several hours, I had starting problems. After the glow plug light went out, I cranked the engine and it started immediately. It ran for one to five seconds or so, then died. After that, it was very difficult to start.
I replaced many things, including the MAF, injection pump, coolant temperature sensor, intake air sensor, ECU, and probably some more stuff I can't remember.
The problem seemed related to the ECU switching from "start" to "run" mode, if there is such a thing. I suspect the controller uses some default settings at startup, then adapts to the running conditions that it senses.

Then I discovered G28 - crank position sensor. I just figured since the injection pump is timed to the crank, it could inject based on its own position. Not so, it uses the signal from G28. When I looked at the G28 signal with an oscilloscope when cold, it looked ok, the peak-to-peak voltage was about 10 volts. It goes up higher with engine rpm to about 50 volts. I replaced it anyway, thinking maybe when it was cold something weird was happening.

The problem went away. It starts fine in 15F weather without stalling. It hasn't been colder than that since I replaced the sensor. The sensor was relatively inexpensive, but I had to remove the front engine mount (and starter) to get to it.

I hope this helps someone at his wit's end.
 

ToddA1

Top Post Dawg
Joined
Aug 3, 2011
Location
NJ 08002
TDI
'96 B4V, '97 B4, '97 Jetta
Did you get a reading with a multimeter? I had a recent "no start" issue and the sensor was to blame.

-Todd
 

TonyJetta

Top Post Dawg
Joined
Sep 15, 2005
Location
Tucson, Az
TDI
'15 Jetta TDI SE / '06 Jetta TDI DSG Pkg0 / '96 Passat TDI
Thanks for posting that.

Did you ever look at the new sensor with the scope?

Tony
 

jeffmon

Active member
Joined
Mar 14, 2010
Location
Missouri
TDI
98 Jetta
I got about 1100 ohms across the sensor, which is within the range that would indicate it was OK. I couldn't believe that was the problem, but I had already ordered the new one. It makes me wonder if the magnetic part in the center wasn't as strong as it needed to be. That would explain why the resistance was OK but it was still causing trouble.

Just today it was pretty cold out, and it started like a champ!

I haven't looked at the new one with the scope yet. I was just so happy that it started and ran when it was cold outside. If a cold weekend morning presents itself, I'll check it, but I suppose that could be a while.
 

ToddA1

Top Post Dawg
Joined
Aug 3, 2011
Location
NJ 08002
TDI
'96 B4V, '97 B4, '97 Jetta
When I was Googling my G28 issue I found a thread on Vortex where someone else was having a no start issue with a G28 code.

His tested fine, but he replaced it anyhow. His car started. His fix seemed like a fluke...

Maybe the multimeter test isn't a foolproof way to test these.

-Todd
 

jeffmon

Active member
Joined
Mar 14, 2010
Location
Missouri
TDI
98 Jetta
In my case, I didn't even get a code. The ECU probably only tests for resistance, like a multimeter.

A G28 problem can be masked because the ECU can use the needle lift sensor signal if it can't get a good signal from G28. I suspect in my case the engine died when it tried to use the G28 signal to adjust injection timing after startup, and never got the chance to look at the needle lift sensor. If the engine (or intake air) isn't too cold, perhaps not much timing adjustment is necessary, and it runs long enough to substitute the needle lift signal for the G28 signal. When everything is cold, it just dies when the timing goes from the default startup value to the value calculated from the unsatisfactory G28 signal. But that's just speculation.
 

ToddA1

Top Post Dawg
Joined
Aug 3, 2011
Location
NJ 08002
TDI
'96 B4V, '97 B4, '97 Jetta
I've read that in the Bentley, but there seems to be controversy whether the needle lift sensor will take over.


I've read that it'll use the crank sensor if #3 sensor fails and vice-versa. I know my AHU won't run without the #3 plugged in. Never tried unplugging the crank sensor. This was in a VW or maybe Bosch publication.

fwiw my car will quite running with either sensor unplugged.
-Todd
 

soup nazi

Veteran Member
Joined
Nov 29, 2008
Location
Australia
TDI
A5 Golf 2.0 DSG, MkIII Tdi Manual
Anyone got the part number for the G28 crankshaft position sensor? Is it easy to change?
 

soup nazi

Veteran Member
Joined
Nov 29, 2008
Location
Australia
TDI
A5 Golf 2.0 DSG, MkIII Tdi Manual
Hey Todd thanks for that. My CPS checks out fine T 1250ohms. I'm chasing a low rpm power issue so it maybe a long shot. I believe the ECU uses CPS data at low rpm and switches to IP data at higher rpm.
 
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