FredR

Paul,

I have had an ignition switch failure on my late 90S4 many years ago- it manifested itself as a problem starting - it would seem reasonable that a worn switch could drop out the ignition circuit but whether it could do that without signs of the problems I experienced I have no clue. Ironically a couple of weeks after I replaced the switch I lost the S4 in my big smash but I still have that new switch in my parts bin that I harvested from the S4- I figured the one in my GTS is bound to fail sooner or later

merchauser

anyway good way to diagnose without taking out alternator and bringing to a shop?

j.kenzie@sbcglobal.net

Hi Merch,
I'm sorry, but especially for your situation, with intermittent malfunction it might be difficult. Problems usually get worse, so when or if it becomes more consistent, you might bring to a shop. I've become pretty good at pulling the alternator, and it's not too bad. I'm sorry about the bum steer on the ignition switch.
Good luck,
Dave

merchauser

hey dave, all good. I agree that a diode could be at issue. since it is intermittent, would a local electric shop have a hard time finding, or would a weak diode be obvious? looking over my notes, I didn't realize that this issue has been at hand for over 2 years. didn't remember, and don't often have "full" loads while driving. point is, I don't think it is getting worse quick enough. lol

I cleaned up a few more ground points this AM, and clinging to optimism that roger's ground straps will help.

Hey_Allen

I don't know if you'll catch your problem, since it doesn't sound quite like a failed diode behavior, but there is a way to easily diagnose a failed diode.

If you connect an oscilloscope across your battery and adjust it to see a clean trace for the alternator ripple, you will see if a diode has failed.
A good diode bridge will show up as a smooth string of ripples, whereas a failed diode will show up as a sharp drop to 0v mixed in with the string of good diode ripples.

In the car that I learned this, the charging voltage looked normal, running high 13v at idle to low 14v when driving, but would never actually charge the battery (as shown by the resting voltage dropping each time the car was turned off and tested.)

merchauser

thanks allen. I have a friend who can help with that. BTW: what symptom might a bad diode exhibit?

moving along, something odd to report: there are times when adding my AC and blower fan (and/or lights) will slightly dip the volt gauge, and other times it will stay rock solid; however, today I noticed that one thing will ALWAYS make the needle dip, and that is putting the shifter in reverse. I did this over and over this AM, and each time the volt gauge would dip from center between 12 and 14 (lets call that 13) to about a 1/4 more towards 12 (i'll call that 12.5). something to do with grounding of the reverse switch?

j.kenzie@sbcglobal.net

Hi Merch,
Does the voltage drop when put into Drive? On my car, I drop about 200 RPM, and my voltage drops by 0.5 volts.
Allen's suggestion sounds excellent.
Good luck,
Dave

merchauser

hi dave, I have a 5 speed, so there is no RPM or voltage drop. I am thinking that the draw is either the reverse lamp switch, or the reverse lamps at the tail lights? grounds again?

Speedtoys

I still think that the subject title continues to lead the thread in a poor direction.

The engine should not stall when you lift the pedal.

That is not a voltage issue, short a massive problem with power feed or ground points.

The ECU isn't directing the IAC, or the IAC is slow when hot sometimes.

You don't hear the car struggle to speed up..it just goes flat and dies...right?

"Car stalls lifting off the gas when hot" would be a more productive discussion...here we are talking about reverse lights causing the stall.

merchauser

jeff, the car never struggles, but often recovers from that dive down. also, yesterday the stall occurred while stone cold on a cool day, so it is no longer exclusively a hot or heat/soak issue. FWIW: this issue was occurring 2 years ago before new MAF, IAC, TPS, CPS, and all proper intake refresh parts.

SeanR

If your TPS is only getting intermittent contact at idle, you will have this issue. Search out Gregs explanation on how to set the Idle and WOT contacts.

merchauser

thanks sean. hate to make absolute statements, because I have been wrong before, but I am near certain that TPS is set up properly. any other ideas? waiting for new ground wire from roger.
here is what mine looks like. corrosion a full inch back on cable

Speedtoys

"but I am near certain that TPS is set up properly"

But not 100% so.

Sean is someone you pay $$$ to to fix things like this...he's been there and done that.

Hard to go the next step, when a step is ignored.

Its an easy test with a multimeter at the ECU, even if you can HEAR it click properly.

Speedtoys

It's still not the backup lights.

j.kenzie@sbcglobal.net

Jeff is correct. You can test the TPS function at the 35-pin connector at the ECU. Pin 2 to pin 18 IIRC. 0 ohms at idle, open circuit off idle. Again, with an intermittent problem, it may test OK one day and fail the next. I can't think of a provocative maneuver for this test, but maybe there is something suspicious that you will notice.
Good luck,
Dave