Issues using a time delay relay for my second fuel pump (diagram included)

[ohisofly]

I'm using the stock fuel pump wiring to trigger two different relays for my two fuel pumps on my car built for drag racing. The first relay is a normal relay and the second relay is a Beuler time delay relay (it's a BU-508TD model which has since been replaced by the BU-509TD, but they function the same). My issue is that the time delay function on the second relay is not working, and the relay is instead just functioning as a normal relay.


The whole point of the time delay relay is to prevent fuel pump #2 from coming on and off rapidly, as I have a hobbs switch that'll recognize when the car is in boost and ground out pin 85 on the second relay. For simplicity, I didn't include that in the diagram because I currently have the hobbs switch removed from the circuit and am still having the issue of the time delay relay not working as intended. For reference, the second relay isn't wired up to be on all the time as it'd be unnecessarily heating up the fuel instead of just coming on when that extra volume of fuel is needed (under boost).

When I bench test the time delay relay it works as intended (staying energized for up to 3 minutes) after disconnecting power from pin 86. I've confirmed that on the time delay relay pin 30 has constant power, pin 85 has constant ground, and pin 86 is receiving a 12-volt signal when the fuel pump is energized. If I keep pins 85, 87, and 30 wired up, remove the wire going to pin 86 (where the green arrow is on the picture) and instead just jump a wire directly from the battery to pin 86 for a brief second, the time delay relay will function as intended and keep the second fuel pump running for up to 3 minutes (with the 12v wire going to pin 86 removed).

I'm at a loss why it works when I use a wire to put 12v directly to pin 86 on the time delay relay, but the 12v coming through my wiring in the car just makes it work like a normal relay.

I apologize in advance if the diagram isn't the best, this is my first time trying to use software to make one. Any help would be much appreciated.

 

[Avo Mk8]

I gather with the earlier BU-508TD if the trigger is grounded after it's triggered, the relay doesn't delay, whereas the BU-509 doesn't behave like this. Could it be the coil of relay 1 ( when un-energised) is dragging pin 86 of relay 2 down and invoking this feature?
Could try a diode in the connection?

Warning - no experience of these - just noticed in the video below. Maybe irrelevant of course!!

see

 

[ohisofly]

That's good to know about the difference between the BU-508 and 509, thank you for the heads up.

I had a hunch that maybe relay 1 was causing this issue and initially considered using a zener diode but I think a normal one would have the same effect as you mentioned. I think I'll order the diodes below (I need at least 10 amp diodes because the fuse for the stock fuel pump wiring is 10 amps).



I really appreciate the help ?