Hi all,
OK I'm looking for some help with my 90 Jetta. It has the 16V 9A 2.0L Motronic engine. For the past year it has been working like crap, some day's it works fine others it runs poor, low power or won't even start. Recently I've been trying to trouble shoot with my trusty Bentley, muti meter, and led test light. Here is what I've found. On the day's it won't start I've found that there is no spark, so I went through the steps listed in the Bentley for this situation. Tested the voltage supply and ground to power stage, OK, next I tested the voltage supply and ground to the hall sender, OK, next the hall sender switching function, OK, then I tested the Motronic control unit(ECU) response to the hall sender signal, and here is where I got a bad result. The test light should flicker, but it does not. So then it says to check the wire between here and the ECU, the wire is ok. So it then tells you that your ECU is NFG. I did not want to believe this so I checked all inputs to the ECU and they were all ok. So I finally bit the bullet and picked up a used ECU. Just as I thought I plugged it in and it still didn't work. The test I spoke about above (Motronic control unit response to the hall sender signal) came back bad again. So I thought great, this used ECU is NFG. Here is where it gets really weird. Just for the heck of it I went on to the next and last test in the Bentley and acted as if all pervious tests were OK. This test asks you to put a volt meter across the coil terminals, remove the hall sender plug and ground the center terminal with the ignition on. The volt meter should go up to at least 2V then drop to 0. It did this and at the same time a whole bunch of other things happened, clicking, pings and buzzing of other components on the engine. Just for the heck of it I then tried to start the car and viola it started! Now every time the car won't start I do this one test and she fires up and works just fine! What the hell is going on?!?!?! I am really anxious to find out what is wrong as I'm in the process of selling the car. I doubt anyone would want to go through the process of removing the hall sender harness and hitting to ground every time the car won't start!
Can someone please help! For the record both ECU's work in the car and all the grounds have been cleaned. I've also ruled out the Hall Sender and the power stage being bad, remember if the car won't start and I remove the power stage plug (so the power stage is out of the loop) and there is no signal getting from the ECU to the power stage, next I plug the power stage back in and now remove the hall sender plug (now the hall sender is out of the loop), then I put the center terminal of the hall sender plug to ground with the ignition on and this action sends a signal back through the ECU and some how "excites something?" and the car starts right up and works great!
What's going on! Holy crap! Help!
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