Show full size 924Board.org
Discussion Forum of 924.org
 
 FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 
 Technical FAQ924 FAQ (Technical)   Technical924 Technical Section   Jump to 924.org924.org   Jump to PCA 924 Registry924 Registry

Key-On voltage does not drop to zero once car is running

 
Post new topic   Reply to topic    924Board.org Forum Index -> 931 Tech.
View previous topic :: View next topic  
Author Message
lildude4life  



Joined: 27 Apr 2015
Posts: 32
Location: Milwaukee WI

PostPosted: Wed Nov 25, 2020 2:48 am    Post subject: Key-On voltage does not drop to zero once car is running Reply with quote

So this is an odd one that I could use some help with diagnostic ideas and solutions.

So key ignition module under the dash, black wires are key-on 12v. I spliced into that wire for my ECU 12v signal wire. Works great.

However, once the car is actually running, if I turn the key off the ECU does not shut off. Grabbed a multi meter and the black wire coming from the key is at 5v almost exactly once the car is running and the key is off. If I click key back on it goes to 12v, then off goes to 5v and so on.I have to manually disconnect the 12v signal wire to the ECU to get the car to shut off.

So far I have disconnected every 5v sensor I can think of from the engine, tried looking for a 5v signal from other locations, started to try and trace the black 12v signal wire from the key to where ever it goes (not fun) and cannot find where I have a 5v leak coming from that ONLY when the car is running.

If I turn key on, ECU turns on, Key off ECU turns off.
If I turn key on, ECU turns on, if i then click it to start and have it crank once or twice but don't let it actually start, turn key off, ECU turns off.
If I turn key on, ECU on, crank, as soon as it fires and is reeving, key off and the ECU stays on and there is 5v in the line.

Now, as i was tracing wires around I somehow ended up looking at the wires for the center counsel switches and saw the same 5v signal there with the car running. Interesting. So I had the car running, turned key off and the car stayed running as ECU was on, flipped the switch for the rear wiper and HOLY MOLY the voltage drooped to zero and the car shut off. WTH?

So for the time being, to turn my car off I turn the key off, then turn the rear wiper on.

Ideas:
1. Leave it as is.
2. Add additional switch somewhere under dash as ECU signal switch.
3.Figure out where 5v leak is coming from
4. Try adding a pull-up resister to ground to see if that will pull the voltage to zero when key is turned off.

It feels like its something sensor related as its 5v. But I cannot find where 5v is getting fed back into that black wire.
_________________
"931 EFR" 81 931 S2 w/ EFR 6258 turbo
Back to top
View user's profile Send private message
Paul  



Joined: 02 Nov 2002
Posts: 9466
Location: Southeast Wisconsin

PostPosted: Thu Nov 26, 2020 9:27 am    Post subject: Reply with quote

Most likely a grounding issue or a bad ignition switch.

Use a test light to see if the 5V is a phantom current, a voltmeter will show 5v for both a milliamp and 30 amps of current.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.
Back to top
View user's profile Send private message Send e-mail
Display posts from previous:   
Post new topic   Reply to topic    924Board.org Forum Index -> 931 Tech. All times are GMT + 11 Hours
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group