Strange idle connected exhaust vibration

[driftin jo]

You have got me thinking hear, because when I removed the old engine I forgot to dissconect the earth strap on the left hand side looking fIom the front of the car,and it broke the eyelet of the chassy end.
I did not know this until we had the rebuilt engine in place,and tried to start it and could see the spark arching. We the put an intirely new earth strap from the battery to the engine mount,this was a very heavey duty strap that runs to both the underside of the battery tray,and to the engine mount,the old earth which was broken I did not reconnect
it.

If you have tried to start the car without the heavy gauge earth strap then you may well have melted all your earth wires as the starter would have tried to draw 300 amps through them.
I've seen it happen before, not on an s2 but the result was not pretty .

[twoqu]

I wonder if we can try that Final control element stuff to activate the ISV. It actually advises doing it so you can observe the ISV working. It is outlined in that Motronic manual. Just use the spare one to save disconnecting anything for the time being.
I can try some stuff at the weekend to see if it works subject to the weather! Never tried it myself.

[twoqu]

Right!
John has very kindly yanked the connector off his ISV and the idle goes straight up to 1050rpm! Sound familiar?
So effectively your ISV is dormant for some reason. Should be able to get somewhere by systematically checking everything.
Thats going to include the power supply (fuse S24) which looks like its fed from the fuel pump relay. The Idle switch, without that the ecu doesn't know whats going on. The wiring back to the ecu from the idle switch. The wiring to the ISV from the ecu and lastly if we can do that control stuff via vagcom that should have covered it all.
As is unlikely that both ecus have the same fault, it must be one or other of those.
Not saying that will solve all the problems, but if we can at least get it to idle, we are making progress.

[220bhp]

Robin,I have tested the wireing from these sensors,but interestingly the engine was cold at the time,and this problem occurs when the engine reaches tempreture,so I will check it cold,and again when the engine is hot.
Many Thanks for all your effort by the way, it is much appreciated.

[220bhp]

I tested the ISV,the TPS,and the lamda wires for continuity,both when the engine was cold, and then when it was hot, and I got continuity on all the wires.
When we took the car for drive to warm it up, we took noteon how long it was before it started to play up.From cold for the first mile and half the car drives realy well,then you start to notice the throttle getting a little jerky. In another mile.the drone is there at 2000 and the car is very difficult to drive.the car boosts ok but a little lower than standard with no popping and is very smooth, until you drop down to 2200.

[driftin jo]

Have you tried swapping the temp sensor on the back of the head ?
Have you looked in vag com to see what temp the ECU sees when warm ?

[220bhp]

I have change the tempreature sensor in the head.and I do not have vagcom at the moment,ebay cable did not last long.

[twoqu]

Thats a shame. I would have thought that the output test would have confirmed its function or not!
I think its dead hot or cold as the idle is the same.
I think you are just going to have to painstakingly and methodically check everything and report back.
Start with the TPS as without that it definately will not work.
Leave the connector on. Peel back the rubber boot and check for continuity or closed circuit between the idle switch contacts 4 and 6. Then go back inside the car, pull the ecu connector and test for continuity between pin 52 and 6 on the TPS. Then from pin 10 to TPS 4. According to the pin outs on Paul N site, pin 10 is also the earth on the Inlet manifold. So basically that looks like the ecu earths pin 52 by connection through the idle switch to pin 10.
If thas all ok, then move on to pin 4 of the ecu to pin 2 of the ISV. That should have continuity. Then test fron fuse 24 to pin of the ISV. That should have continuity.
Interestingly its the fuel pump relay that provides power to fuse 24. I am not sure this fuse is live with just the ignition on. It should be with the engine running. So you can test for power at fuse 24 and pin 1 of the ISV.

As far as I can tell that should just about cover it. The only other possibility being that the ISV is earthed out by a short from pin 2 to earth. If it was powered up it would then go fully open and not be controlled.
In an ideal world, doing the output tests with perhaps an LED test light would be the best way. You can virtually do it from under the bonnet yourself then. However I believe that output test does rely on the TPS idle switch being operative as thats how you start the test! As I said previously, I might be able to confirm the validity of some/all these tests at the weekend.

All this is basically covered in that Motronic Manual. Obviously it talks about the various VAG tools but you will just have to improvise. They are essentially basic automitive electrical tests! A reliable Vagcom set up is really a essential tool even on the clockwork 3B!

[newsh]

I tested the ISV,the TPS,and the lamda wires for continuity,both when the engine was cold, and then when it was hot, and I got continuity on all the wires.
When we took the car for drive to warm it up, we took noteon how long it was before it started to play up.From cold for the first mile and half the car drives realy well,then you start to notice the throttle getting a little jerky. In another mile.the drone is there at 2000 and the car is very difficult to drive.the car boosts ok but a little lower than standard with no popping and is very smooth, until you drop down to 2200.

Ok, just to check,

1/ Was the continuity test done with both ends unplugged? From ECU plug to ISV/TPS plug etc?
2/ Did you check for shorts between the wires?
3/ Did you check the temp sensor wiring? (continuity and shorts?)
4/ Is it the correct temp sensor part number?
5/ You need to measure the continuity of the ground wires under the Inlet manifold back to the respective pins on the ECU plug.

[twoqu]

As John says, if pin 10 of the ecu is the earth to the inlet manifold then check the continuity of that connection.
Its possibility that the decrease in drivability is just down to the mixture becoming leaner as the car warms up.
Again with Vagcom we can't be sure what the ecu is seing in terms of temperature.

[twoqu]

Here is Paul's info. It helps to know what pin is where. Just make sure you get the ecu connector orientated correctly.

http://www.s2central.net/motronic_pinout_3B.html

[220bhp]

Robin thanks for the link to to s2 central. As to the other tests.PIn 4
to pin 2 on ISV checked ok.The other checks I will perform when it stops raining.
John, Question 1. yes it was tested witj both ends unplugged
2,shorts between wires.
3. tempreature sensor at the back of the head,continuity check ok,again how do I check for shorts.
4 I will have to remove it to see .I assume you mean the sensor at the back of the head
5 as for testing the ground wires, do I have to remove them from the IM and test them individually,to the ecu, or is there just one earth pin on the ECU.sorry but I am a little out of my depth hear,I make furniture remember
Anyway many thanks to both you and Robin {two Q] for all your help in solving this very annoying problem

[twoqu]

Its a bit tricky at the best of times. That earth on the inlet, the thinner one is the one to pin 10 and that seems to have something to do with the TPS idle switch. In fact you may be able to check continuity between the end of that cable on the IM and terminal 4 on the tps. In theory it should be connected!
The probe just needs to touch the ringlet.

[twoqu]

Break in the wheather!
Yep, terminal 4 is connected to the IM earth wire!

[twoqu]

I think that should also mean that you can check between pin 52 and 10 of the ecu connector for continuity as thats efectively the idle switch circuit!