true ignition circuit
Printed From: the12volt.com
Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=113826
Printed Date: May 11, 2025 at 5:46 PM
Topic: true ignition circuit
Posted By: jseibold
Subject: true ignition circuit
Date Posted: May 15, 2009 at 10:04 PM
We do GPS tracking and other similar things in semis and straight trucks...Most of what we install requires a true ignition connection...the problem...08 Freightliners have a databus ignition wire that we have been told not to interface with...we have found a wire per Freightliner schematics that we have been using but the voltage drops out while cranking...does anyone have a simple tip that would enable this wire to trigger something (relay, capacitor, etc) that would hold 12 volts while cranking...we have considered pulse timer modules, capacitiors, latching relays, but I know there are a lot of brilliant minds in here and I am just looking for some help...something fairly simple that one of our "trained monkeys" could handle.
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Replies:
Posted By: howie ll
Date Posted: May 16, 2009 at 5:21 PM
What's coming out of the fuse boxes? I've used that method on M/Benz and Audi with data ignition switches. There always seems to be a live whilst cranking ign circuit at the fuse box.
Posted By: i am an idiot
Date Posted: May 17, 2009 at 7:39 AM
https://www.bcae1.com/temp/headlightdelay01.swf The above provides a delay on and delay off. The delay on can be eliminated by removing the 20K resistor. For a 17 second retention, change the cap to 330MicroFarad and change the 1K to 47K. Insert a diode in series with the wire labeled control, banded end nearest the circuit. 1N4001. The transistor in the diagram is a 2N6488, NTE331 or a TIP3055, or TIP31, the last 2 are available from RadioShack. Terminal 30 of the relay is your simulated true ignition. The diode is to be fed by the wire that drops out while cranking.
Posted By: dualsport
Date Posted: May 17, 2009 at 8:27 AM
This was actually covered just a few days ago-
delayed off relay control
Posted By: howie ll
Date Posted: May 17, 2009 at 9:41 AM
Thanks dualsport, I love it when you, "watersurgeon" and "I am an idiot" reply with your nice circuits which I shamelessly copy and file.
Posted By: i am an idiot
Date Posted: May 17, 2009 at 12:42 PM
Shame on you Howie.
Posted By: howie ll
Date Posted: May 17, 2009 at 1:08 PM
No shame, I'm the first to admit that if some one's got a better idea than me, I'll use it. That's not shameful it's resourceful. The very first thing vwe should teach youngsters in any walk is don't be afraid to ask.
Posted By: jseibold
Date Posted: May 19, 2009 at 10:20 PM
Thank you guys for the suggestions....I will try that circuit on the test bench and see if it will work out for what we need...Thanks again for the tips! -------------
Posted By: mobilefleets
Date Posted: June 02, 2009 at 3:05 PM
There is a circuit in the fuse box that you can use. Unfortunately I can't remember what it is. Next time I'm working on a newer Freightliner I'll make sure I post up the wire color and location. It will save you some time and money compared to using the above relay circuit.
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Posted By: howie ll
Date Posted: June 02, 2009 at 3:37 PM
Didn't I suggest the fuse box 2 weeks ago? Doesn't anyone listen or go out and test anymore, or are you all waiting for OBD ll plug-ins? For trivia's sake everything over here 7.5 tons and up runs 24volts, but some Scanias, Volvos and Mercs all have 12v+ perm and acc for the radio etc. The only "sleeper" circuits I've found so far on the trucks are on 44 ton Mercs and Renaults.
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