# 17748 - Camshaft Position Sensor (G40) / Engine Speed Sensor (G28): Incor. Correlation



## iang24 (Jun 24, 2020)

Yes I know. Here we go again with another 1.8T timing issue. I have searched this forum and others and there's a ton of information about the dreaded 17748 code - invariably the answer seems to be that the timing belt and/or chain setup is off. However please hear me out here - I swear the timing is spot on. It all started after swapping out the head after my chain tensioner broke and destroyed things on the old head. The engine mostly runs pretty strong although it will post the code after start-up and now and then the immobilizer will prevent the engine from starting. Anyway, with the engine at TDC (crank mark lined up and TDC gauge in #1 spark plug hole reading top of compression stroke), camshaft notches pretty much lined up and the required 16 chain rollers present and correct. Just from looking at it, I can see that if I were to move the belt a notch, or one sprocket tooth for the chain, things would obviously be off. So.... something else is amiss I think. Brand new G40 - no help. Ohmed all 3 wires from ECM to G40 connector - all good. G28 signal on VagComm scope looks solid. All that seems to leave me with is perhaps a bad VVT actuator. The tensioner is brand new and the coil is reading about 15 ohms, so electrically it checks good. I'm wondering if there is something going on with it mechanically though. 'Not sure about the engine oil pressure, but I did have the pan off and inspected for debris at the pump pickup. All was good.
I'm really flummoxed by this. Any ideas /comments would be welcome. I believe I saw on another post somewhere that you can check activation of the VVT actuator using measuring blocks. Any info on this is also welcome. Thanks guys.
Car is 2002 Beetle, 1.8T.


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## billymade (Jan 29, 2008)

http://wiki.ross-tech.com/wiki/index.php/17748/P1340/004928

Because of disassembly and head replacement; double checking everything, is recommended (sounds like you have done this). As the 1.8T's age, the tensioners wear out, the chain can stretch and throw a trouble codes. This can cause a clatter, right at the drivers side of the valve cover and in bad cases, actually make contact with the valve cover. Most 1.8T's I have seen, after 100k or so; the chain and tensioner, get noisy. 

Definitely, read the ross tech link and if you have vcds, you might see, if you can check things by using the Measuring Value Blocks (MVB) to Check Timing Chain. Another issue to consider, is oil pressure; the tensioner runs on oil pressure and if the pickup tube or sludge is/was a issue, that could cause problems with the activation of the tensioner. If you think, that could be a possibility; you could hook up a oil pressure test kit and confirm, it is in spec. 

Being such a common problem; you might watch, the numerous video youtube and compare the sound, of your 1.8T, to failed ones filmed there. 

https://www.youtube.com/results?search_query=VW+1.8T+cam+Chain+Tensioner+Noise+

I just did a head on 1.8T audi tt and a turbo s new beetle 1.8T; in both cases, the cam chain and tensioner, were replaced. Upon startup, the cam chain noise; was significantly reduced, both were over 100k and were the original tensioners/chains.


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## iang24 (Jun 24, 2020)

Hey. Thanks for your reply. 'Sorry it's taken so long to respond. I've taken a couple of weeks off of messing with this POS. Anyway, I know all about the cam chain tensioner failure issue - that's why I had to replace the head. The chain ended up breaking and blowing a big hole in the valve cover. The jammed chain caused the camshaft to stop rotating while the crank carried on turning! What a mess!
I am back to troubleshooting this now. I will check oil psi...


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## billymade (Jan 29, 2008)

Ugh, sounds like a pretty major catastrophic failure! :facepalm: Is the the tensioner or chain noisy (maybe low oil pressure)? The marks on the cams; is a bit off, on the one facing the firewall; the line drawings, in the service manual show this. It can be a bit confusing; when you are used to timing marks on a timing belt or crank pulley. 

https://www.justanswer.com/audi/88p94-audi-a4-quattro-similar-question-03-a4-1-8t-quattro.html

The 16 links, are not perfectly lined up; a bit to the left (exhaust side), as shown in the link above. 

Here are some service manual links; that might be helpful: (click on the arrows/next; to access all the pages): 

Camshaft Adjuster/Chain Tensioner: Testing and Inspection
Camshaft Adjustment Valve, Checking

https://workshop-manuals.com/volksw...nspection/camshaft_adjustment_valve_checking/

oil pressure specs and testing: 

https://workshop-manuals.com/volksw...ressure/component_information/specifications/

Let us know; how things go and anything you find, working through your troubleshooting and testing process, thanks.


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## iang24 (Jun 24, 2020)

Thanks again. Good information. When I run the engine, it is super-smooth. No rattles. The notches on the cams line up with the arrows on the caps more or less exactly as shown on all the illustrations I've seen and with 16 rollers between. I think there will always be some variance to exactly where the notches are due to chain wear, etc. BUT it's obvious that if I was to move a cam one sprocket tooth over in either direction, it would be way off. 'Same with the belt: I have the engine at exactly TDC on the compression stroke as measured by an accurate TDC tool in the #1 spark plug hole. The mark on the cam pulley lines up pretty much exactly with the mark on the valve cover. If I was to move it by a tooth, it would definitely look worse.
I think right now, my plan is to put it back together and run it again and use measuring block 91 on my Vagcomm to try to see what the tensioner is doing before I go to the hassle of ripping it out. I can't help but think that if the tensioner was bad, there would be more codes. One think I did find: with the tensioner solenoid removed and 12V applied across it, the plunger only moves about 1/8". I thought the stroke would be more than that, but there's no evidence that the solenoid is damaged so maybe that's normal and enough to let oil through to operate the tensioner. The mystery continues...


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## billymade (Jan 29, 2008)

Yeah, good question; after 100k, they seem worn out, i replace the tensioner and the chain (for good measure). I got mine from fcpeuro; Bapmic brand, seems good so far. FCPEuro, has a lifetime warranty on all their parts; so, that is reassuring and you get another one, if needed at no cost. The oil pressure and tensioner testing procures; might help diagnose the issue?

Using VCDS, sounds like a good and very accurate diagnostic procedure; please report back on that and/test share how to do that, as it can help the next person, having similar issues, thanks.


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