Testing sensors

"Mungo \"Two Sheds\" Toadfoot" wrote in news: snipped-for-privacy@pipex.net:

Typically (but not necassarily always):

MAF sensor will increase in voltage as airflow rises, with something like

0.2v with the engine off and 5v at full blast.

CTS will drop in resistance as temp increases, say a few kohms when cold, gradually dropping to a couple of hundred when hot.

TPS increases in voltage as throttle opens - somewhere near zero fully closed and about 5v fully open.

Lambda should be 1v or less, and should switch rich-lean-rich-lean frequently. This may show as something like 0.2,0.8,0.2 etc

Of course, the make and model of the car are needed to give more accurate figures.

Stu

Brilliant, thanks Stu, that's a good enough start to see if one or more have failed. It's a s**te old Favorit of 1995 vintage, showing symptoms of overfuelling. I'm going a-testing tomorrow.

Si

Splendid, ta.

Si

Mungo "Two Sheds" Toadfoot presented the following explanation :

The ECU diagnostics should pick up any faulty ones.

I don't think the Favorit has any means of revealing recorded problems to the owner. If it has to go to a dealer to be read they're bound to charge more than it's worth! :)

Si

First thing I'd check is the coolant temperature sensor. That has a major effect on the fuelling.

Haynes actually do a pretty good generic manual on car EFI systems. Worth the money.

This will be Bosch mono-motronic 1.3, or multipoint- 3.2. Both are capable of fault codes. But always erase them, drive the car, and see which return before starting work.

Tim.

I've just checked the CTS (not the temp sender on the thermostat for the temp gauge!) and it's behaving oddly in that my meter's flashing the resistance then showing open circuit every time I touch the terminals. I think something's playing up inside the sender so I'll have a new one of those.

Also, the o2 sensor was reacting *very* slowly when I tested it with a blowtorch. It would eventually get up to ~0.8v then fall back to -0.2v (minus point 2!) when the heat was removed. Not good. I gave the vents a good blast with the torch and tested it a few more times during this cleaning process. It now reacts really quickly up to 0.9v and then straight down to 0.1v when the heat's removed. Spiffin'! Something to bear in mind for those whose car fails the MOT on emissions.

Thanks to all for the input - I'll report back any improvements.

Si

This is with it disconnected from the loom? Might be a form of diode - have you got a diode test function on your DVM?

"Mungo \"Two Sheds\" Toadfoot" wrote in news: snipped-for-privacy@pipex.net:

Don't mean to patronise, but make sure you get a good contact with the test probes. In autodata, the CTS values for your car are 2000-2750 ohms at 20C and 275-375 at 80C.

If you suspect it, I would suggest you substitue the sensor for a cheap potentiometer from maplins before buying a new CTS. A 2k5 pot. for about

50p will allow you to simulate any temp reading you want by setting it with your meter. Then you can see if it eliminates the fault.

Well done! Didn't realise you could revive 'em :-)

Stu

More usual is just to use a 180 ohm resistor with leads bent to insert in the plug. This simulates a hot engine.

I did think that the probes might not be touching properly at first, because of the strange behaviour, but it kept happening over and over again, at various temperatures (cup of 'ot water, cup of not-so-'ot water, cup of cold water). The resistance did change but it would just flash up and then show open circuit every time. This could, however, tie in with the symptom of a "cough" when trying to start from warm, then a complete fuel cut-off until the engine's considerably colder. Perhaps the ECU sees the water temp. sender first show a proper value then immediately go open circuit? What the ECU makes of that I don't know.

Too late, he cried. I've found a nice chap in Wales who will sell me a brand spanky new temp sensor for the princely sum of £10.50 delivered.

This is a marked contrast to Skoda themselves, who want £650+VAT for a new throttle body. They can FRO. A new o2 sensor can be had for £16 ish and a secondhand throttle body (£20) will be purchased soon too, in case it's that. Not bad at all. If it all works.

Si

"Dave Plowman (News)" wrote in news: snipped-for-privacy@davenoise.co.uk:

Quite so. I just thought the pot. would be more versatile, for the sake of a few more pence :-)

Stu