electrical problem
Thread Starter
Joined: Nov 2001
Posts: 1,711
Likes: 6
From: Lawrenceville, Ga
electrical problem I have my 2nd fuel pump and my a/w water pump triggered off my Hobbs switch at 2 psi. I use a single ground wire signal off the Hobbs to trigger the 2 separate relays. I also have a manual switch to turn them both on if I choose, it also simply sends a ground signal to the separate relays. When the car is running everything works as it should.
Here's the problem, when the car is off if I manually switch on the water pump to circulate the water it won't turn back off with the switch. But if I pull the second fuel pumps relay out of the system it works as it should and turns off fine.
It seems like something about the second pumps relay is backfeeding a ground and keeping the water pump running even after I've switched it off.
Here's the problem, when the car is off if I manually switch on the water pump to circulate the water it won't turn back off with the switch. But if I pull the second fuel pumps relay out of the system it works as it should and turns off fine.
It seems like something about the second pumps relay is backfeeding a ground and keeping the water pump running even after I've switched it off.
TECH Regular
Joined: Jun 2013
Posts: 487
Likes: 6
From: Orange, Texas
One relay is backfeeding to the other, put a diode on there and it should stop it. I had the same problem when triggering my intercooler pump and fans off the same output ground and a diode on each relay fixed it.
ETA: this is what I used https://www.radioshack.com/products/...nt=20332048261
ETA: this is what I used https://www.radioshack.com/products/...nt=20332048261
Last edited by squarles67; Nov 7, 2017 at 11:36 AM.
TECH Regular
Joined: Jun 2013
Posts: 487
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From: Orange, Texas
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67 is talking about a snubber diode across each relay coil (in parallel). Check out this schematic showing position/polarity. http://circuits4tracks.daxack.ca/tips/SnubberDiodes.php
The diode stripe (as shown in radio shack component link above), is the negative side which connects to the + side of relay. You'll notice the diode icon is a triangle with a stripe on the tip. That stripe represents the stripe on the diode housing.
Hope this helps.
Since you have a common signal from hobbs switch to both relays, you'll likely need to isolate each relay using isolation diodes (same part as above is fine) so only the desired relay is turned on with the manual override switch.
FYI here is some other relay configs that may be helpful. http://www.gtsparkplugs.com/electric-fan-wiring.html
The diode stripe (as shown in radio shack component link above), is the negative side which connects to the + side of relay. You'll notice the diode icon is a triangle with a stripe on the tip. That stripe represents the stripe on the diode housing.
Hope this helps.
Since you have a common signal from hobbs switch to both relays, you'll likely need to isolate each relay using isolation diodes (same part as above is fine) so only the desired relay is turned on with the manual override switch.
FYI here is some other relay configs that may be helpful. http://www.gtsparkplugs.com/electric-fan-wiring.html
Thanks guys! 
