The best investment you can make if you own an mk3 is a 7 quid maplins multimeter which is the most powerful diagnostic tool you can buy and will tell you far more than the OBD[VAG-COM/fault code reader].
If you have a running problem, let the car get up to temperature and check the following as they are the most common causes of poor running.

1) Test meter in DC volt mode and test voltage across the battery poles, you should see at least 13.2V with all electrical consumables such as headlights, stereo etc on. Anything less and there is a problem with the charging circuit meaning the ECU will get the arsehole and run rich to protect the engine from running lean from injectors which would struggle to fuel properly because of low voltage in normal working cycle

2) Blue coolant temp sender. Should be 200-250 ohms across the sender pins. Test with the meter in resistance mode. Any reading higher than this when the engine is hot then bin the sender.

3) Lambda probe. To test unplug the lambda from the harness and if it is defective the car will run a lot cleaner with it unplugged as it will revert to a failsafe map.

4) Dodgy earth straps. Check they are all intact and free of corrosion as a bad ground will give false reading back to the ECU and generally give the ECU the arsehole.

5) People plugging the wrong senders to the wrong plugs back to the ECU. This is your own or the last person to work on you cars fault. You deserve to have a problem and the ECU is entitled to have the arsehole.. How would you like you ears wired up to you eyes and vice versa? Plug to the correct plug of harness and your problems will go away.

6) Wrong spark plugs or cheap leads. They may work for a while.. But then they will bite back.. dig deeper next time and buy Bosch/OE items.

7) Broken wires in loom. Test the continuity of the loom from the ECU harness by unplugging it from the ECU and testing the corresponding end of the loom at the sender plug. I have a few Digifant pinout diagrams, but all Bosch pin outs are pretty similar from cars of the early 90's era. This is done by testing the resistance along the wire or by putting a more expensive multimeter is 'Beep' mode which will beep if there is continuity.


Cheers to Beavis from PSD for posting this info.

SUPERB

Nice and concise!!

I got my current multimeter for £3 from Maplin and it beeps and everything!!

Let the battery go flat on my old one tho and it started giving out dodgy readings!!

that voltage one is interesting. I think it just about manages to be that much on my GT. I've had to increase my idle to bang on 1000rpm so it doesnt start doing weird things with the relayed headlights on.

The best bit of advice i've read in a long time

Just bought a multimeter to test this, and though I'm not entirely sure I've got it right there appear to be some problems.

I'm getting a reading of 12.08V from the battery with the engine running, headlights, wipers and blowers on full. That's less than 13.2V. The battery's nearly four years old though, maybe that needs changing? Or is 12V ok?

I can't work out exactly how to use the resistance meter, but the resistance across the blue temp sender is .330 k ohms. Assuming k stands for kilo, which means 1000, then that's a resistance of 330 ohms once it's warmed up. Which again is not right.

More money to be spent here methinks!

The resistance across the temp sender changes with heat, tho I'm not sure if it goes up or down.

Should've said, that was fully warmed up.

yeah I tested mine with everything on (main beams, fogs, rear window heater, blower on 3) and it was about 12V again. I think I need a 90A alternator. It was ok till I put the rear window heater and the fogs on, and with normal lights and the blower it was about 13.5V so its just about ok for 99% of the driving i do. When it dropped to 12V though it started doing all sorts of wierd shit with the fuelling (lambda was all over the place) so i left the battery disconnected for several hours in an attempt to reset it. Its been ok since.

0.330K ohms is 330 ohms. I cant remember the figures from marks article exactly but that doesnt sound too far out.

some multimeters have diff resistance settings so if you have it set to 1000oms then it will read small readings in 0.330k but if you set it to 100ohms it will read right.

like when your mesuing voltage if you set it to 100v it will read 12v as 0.12 till you drop it to the 20v threashold(the only one i use)

Now I'm confused, it's a case of serious confusion between Mark's guide which says it should be 280 to 370 ohms at 80C and what this thread says.

I'm feeling pretty thick right now, after playing with my multimeter I left the blue temp sender earlier which shows me up to be a complete bonehead. What I then ended up with a car that had no power, was misfiring really really badly and even failed to start in Tesco car park. Took it for a drive wondering what I'd done to it, popped the bonnet and felt like a right tool when I saw that I'd left it off.

This is why I worry about working on my car, I suck.

Fuck it, all is fine now it appears.

Battery wise I recorded:
13.9 volts when idling, all electrics off
12.59 volts when idling with stereo on, wipers and blowers on full, fogs on, main beams and heated rear screen (which is a little unrealistic unless you're in really harsh snowy conditions).
12.08 with the engine off

Coolant temp sender was 224ohms when warmed up, and oddly enough it dropped to 219ohms while I was watching it. The engine was off, so I don't know why the resistance was dropping (surely it should be cooling down?).

I will be bringing my muitimeter to the rr day if you want me to give you some pointers. could do with a copy of that article though. is it still on the net?

do a search for "engine efficiency" on the PSD forum, it's still there

I've got a multimeter that has other car-related functions, if you can figure out how to use some of them it'd be very handy because the instruction manual is pants and I can't work it out.

It lives in my boot now.

mines in the passenger side door pocket along with a spare dizzy. What other car functions does it have? I only ever use voltage and resistance. Jamesd has one that measures dwell angle. Old Skool.

Mine measures dwell too, that and something else.

I can't remember offhand what it measures though.

Cost £14 from Maplins thanks to student discount.

http://www.pitstopdevelopments.com/forum/v...topic.php?t=166

Saves me searching for it again.

Marks guide is also on the Club G40 site:

http://www.polog40.co.uk/modification/engi...efficiency.html

Alex.. i'd replace that alternator as the figures with electrics on are too low.

D'oh, I was worried about that.

Looked pretty new though, like it had been replaced recently.

Is it worth buying second hand or should I just fork out for a new one?

And, random sounding question I know, would this affect the spark from the coil at all? I'm wondering if this may have been behind my ignition problems last November.

Can anyone confirm which connection unplugs the lambda sensor? Is it the one next to the TB (opposite side to air box) closest to the scuttle pannel? also...Anyone know how to test the other sensors?