relay latching problem
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=115212
Printed Date: May 08, 2025 at 11:56 PM
Topic: relay latching problem
Posted By: bigblockranger
Subject: relay latching problem
Date Posted: July 21, 2009 at 8:35 AM
I am setting up a cooling fan relay and a relay to activate my electric crankcase evac pump triggered by my ECM, (-) trigger.
Once these come on, they are on until you disconnect the battery.
I currently have it wired like this:
but believe it *should* be like this:
Will having the 30 & 87 flip flopped cause this latching condition?
------------- '86 Ford Ranger 545/C6/EFI
Replies:
Posted By: howie ll
Date Posted: July 21, 2009 at 11:46 AM
Either way makes no difference, try a diode across 86 and 85 with the band towards 86.
Posted By: bigblockranger
Date Posted: July 21, 2009 at 3:07 PM
I do have a diode (correctly oriented) and once you ground the coil, they are on all the time until you disconnect the battery. You can even disconnect the coil ground and it makes no difference.
Both of these circuits worked fine with a toggle wired with a switched (+). I may just configure it to work that way with the ECM since I know it worked well that way. Just did not want to have to rewire all that stuff again.
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'86 Ford Ranger 545/C6/EFI
Posted By: i am an idiot
Date Posted: July 21, 2009 at 5:40 PM
The lower picture will not latch. It will only be on as long as the - trigger wire is at ground. Do you need it to latch until you turn the key off? ------------- Let's Go Brandon Brown. Congratulations on your first Xfinity Series Win. LGBFJB
Posted By: bigblockranger
Date Posted: July 21, 2009 at 5:49 PM
No. Both are set up like the bottom pic and they definitely are latching. I do not want them to.
I have the ability to change from a (-) switching to a (+) so i will just do it that way and be done.
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'86 Ford Ranger 545/C6/EFI
Posted By: i am an idiot
Date Posted: July 21, 2009 at 6:22 PM
The bottom pic will not latch. If it is staying on, it is because the - signal is remaining on terminal 85. Upon further review, neither of them will latch. When it latches, remove the trigger wire from 85 and see if it turns off. If that is the case, your trigger wire is still at ground. How do you want the relay to react? Do you want it to come on and remain on until you turn the key off? ------------- Let's Go Brandon Brown. Congratulations on your first Xfinity Series Win. LGBFJB
Posted By: howie ll
Date Posted: July 22, 2009 at 3:05 AM
Meter 30 and 87 with nothing connected in case you have a sticky relay, is that possible. Otherwise I'm with Mr.Idiot on the 85 lead. Test it , also make sure the diode band faces 86 (pos side of coil), further reading makes think it's not a relay problem because from what you're saying you have two relays doing the same thing.
Posted By: bigblockranger
Date Posted: July 22, 2009 at 1:23 PM
Thanks for the input. In my haste I jumped to the conclusion that the relay must be the problem, but now think it lies in my transistor circuit that grounds the relay.
-James
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'86 Ford Ranger 545/C6/EFI
Posted By: hotwaterwizard
Date Posted: July 23, 2009 at 8:20 AM
I Have seen this before. The load keeps the contact from unlatching when the relay has a weak return spring. A different style relay will usually fix the problem. Try a Ford Starter Relay for $10 ------------- John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !
Posted By: bigblockranger
Date Posted: July 23, 2009 at 8:33 AM
Thanks.
I rebuilt the switching circuits last night using TIP120's instead of the smaller 2N2222A's and it now works perfect. I ran across another fellow that was having the same problem. Surprisingly, we independently came up with the same solution. haha
Thanks for the help.
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'86 Ford Ranger 545/C6/EFI
Posted By: hotwaterwizard
Date Posted: July 23, 2009 at 10:44 PM
Well??????? Where is the Schematic for your cool little circuit? 
------------- John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !
Posted By: bigblockranger
Date Posted: July 24, 2009 at 9:54 AM
There's nothing really cool about it, but this is the schematic.
Like I said, the 2n2222 was causing probs for some reason, and replacing them with TIP120's worked like a charm. ------------- '86 Ford Ranger 545/C6/EFI
Posted By: bigblockranger
Date Posted: July 24, 2009 at 9:58 AM
Ahh hell.
Try again.
 ------------- '86 Ford Ranger 545/C6/EFI
Posted By: hotwaterwizard
Date Posted: July 24, 2009 at 8:35 PM
Looks COOL to me. ------------- John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !
Posted By: dualsport
Date Posted: July 24, 2009 at 11:55 PM
Keep in mind it takes more current to turn on a relay than to maintain it once it's on.
I'd guess you have a high side drive signal, which is either high or open circuit.
If your driving signal doesn't drive to ground to cut off the transistor, it'll allow enough leakage current to maintain the relay in the energized condition, especially when the transistor has a higher gain. The TIP is lower gain, so it's apparently not leaking enough to hold the relay on.
If you put a small biasing resistor from the input to ground the other transistor probably should work also.
Posted By: bigblockranger
Date Posted: July 27, 2009 at 9:40 AM
Ahhh gotcha, thanks!
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'86 Ford Ranger 545/C6/EFI
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