Using VAGCOM and similar diagnostic toolsThe Digifant I ECU doesn't store sensor faults in the way more modern ones do. It will register a fault if it is getting a reading out of bounds or no reading at all from a certain sensor. These aren't stored and when the fault is fixed, the ECU stops registering it after a couple of minutes with the engine running. Also, the fault code registering system is next to useless, sensors can be totally knackered causing all sorts of issues and a fault code reader will show nothing. Because of this, the handheld code readers you can buy on Ebay and similar are a waste of money. By far the best tool is a £5 multimeter from Maplins. What you can do with VAGCOM and similar programs is monitor the sensor readings the ECU is seeing.
VAGCOM does work with G40 Digifant, you need the normal ODB lead and a 2+2 adaptor (also available on ebay for less than £10). You may have to mess about with serial port speeds to get it working. I've not tried using a USB cable, I only have an RS232 serial cable and it works fine, this is with version 311. You might also have luck using a piece of software called VWTool.exe which I've put online here:
http://www.polog40.co.uk/VWTool.zipThis is a pretty old piece of software (well, from the mid to late '90s) but is still new enough to read Digifant one. It should run fine under Windows XP even though it's an old 16bit program. I believe the guy who wrote this software died a few years ago so RIP whoever you are and thanks!
The only useful parts of either software is the "measuring blocks" section. This allows you to monitor (and graph) the readings from various engine sensors as the ECU sees them. VAGCOM is much better as the VAGSCOPE software that comes with it plots nice graphs for you (with VWTool you can save the values and make graphs in Excel but it takes a bit of messing about). It wont tell you what values mean what but here is a list as they appear in a VAG workshop manual:
display group 01
1 coolant temp (blue temp sensor)
2 engine speed (RPM, from the hall sensor)
3 lambda signal
4 injection period
02
1 coolant temp (blue temp sensor)
2 engine speed (RPM, from the hall sensor)
3 not for service dept (seems to be ignition advance, units in BTDC anyway)
4 engine load (map sensor)
03
coolant temp (blue temp sensor)
engine speed (RPM, from the hall sensor)
not for service dept (this appears to be inlet temp)
c0 pot voltage
04 the same as 03
05
coolant temp
not for service dept (seems to be inlet temp)
not for service dept (seems to be a temperature but not sure what)
not for service dept (seems to be battery voltage)
Some of the values say "not for service dept" but are clearly meaningful sensor values, I guess it just means they are readings the VW dealers aren't interested in. One of these is the inlet temp sensor, this seems to always read around the -20deg mark (yes, minus!), other people have found the same thing too so you can pretty much ignore that.
The best thing to do is to get a laptop, get vagcom/vwtool running and logging values and drive about a bit (once up to temp or whenever you problem happens). Try to use the full rev range and put the engine under load at different RPMs. Then when you get back you can look at the logs. The best thing to do is to plot them into graphs (either with vagscope or Excel), then you can look for any reading disapearing or going mad at any point. You need to use common sense, i.e. it should be obvious that coolant temp should be around 80deg to 110deg and not really above or below those figures, although they are only rough values, you are looking for things that are way out. Also check the lambda singal under partial throttle (it's ignored on full throttle or no throttle). Watch out for low battery voltage too, shouldn't be going under 13.2volts, if it does you've probably got a worn out alternator.
