help with getting 12volts to fuel pump

Ken Seegers

SCCoA Member
Does anybody here who has changed wiring to fuel pump to get a full 12 volts have procedures and pictures on how they did it? I get full 12 volts to relay but after that it drops to 9.7 volts
Thanks
Ken
 
A properly wired relay should just pass the main 12v feed right thru - is there a chance you're using the trigger feed, post relay?

Is there a diagram you're already using or can you photo the wiring now?

Example:
fuelpump_relay_wiring.gif
 
Does anybody here who has changed wiring to fuel pump to get a full 12 volts have procedures and pictures on how they did it? I get full 12 volts to relay but after that it drops to 9.7 volts
Thanks
Ken

At what point are you measuring 9.7V, and what gauge wire are you using?

RwP
 
I get full 12 volts to relay but after that it drops to 9.7 volts
Thanks
Ken

Not really understanding that. I know when I checked power coming to my stock fuel pump it was about 9.7 (was very surprised) It is the reason to upgrade to get full juice from the battery. It seems that if your using the stock power to trigger your relay it would be 9.7v to the relay to open it and 12 to 13V flowing to the pump from the battery.:confused:

Ken
 
Not really understanding that. I know when I checked power coming to my stock fuel pump it was about 9.7 (was very surprised) It is the reason to upgrade to get full juice from the battery. It seems that if your using the stock power to trigger your relay it would be 9.7v to the relay to open it and 12 to 13V flowing to the pump from the battery.:confused:

Ken

Ken,
I measured the voltage going to the relay....it is 12v going out of the relay is 9.7v. I believe the relay might be the issue. I have another SC to pull the relay to find out if the relay is the issue.

Ken
 
Ken,
I measured the voltage going to the relay....it is 12v going out of the relay is 9.7v. I believe the relay might be the issue. I have another SC to pull the relay to find out if the relay is the issue.

Ken


I thought you were running a harness to pull power from the battery direct vie a relay that is triggered by stock power wire running to the pump. The stock wire that powers the pump, on my car was only 9.7 volts and after installing an aftermarket relay to run the pump I was getting 12.5V or better to the pump. I take it you are just talking about a stock relay for stock wiring?

Ken
 
Ken,
I measured the voltage going to the relay....it is 12v going out of the relay is 9.7v. I believe the relay might be the issue. I have another SC to pull the relay to find out if the relay is the issue.

Ken

Ignore the new guy :) I did say that the relay was bad LOL
 
I thought you were running a harness to pull power from the battery direct vie a relay that is triggered by stock power wire running to the pump. The stock wire that powers the pump, on my car was only 9.7 volts and after installing an aftermarket relay to run the pump I was getting 12.5V or better to the pump. I take it you are just talking about a stock relay for stock wiring?

Ken

Ken,
Right now everything is stock for wiring and harness. Do you have pics or diagram on how you ran the wiring to the pump?

thanks
Ken
 
Ken,
Right now everything is stock for wiring and harness. Do you have pics or diagram on how you ran the wiring to the pump?

thanks
Ken

I dont beleive the relay is bad. The wire providing power to the pump isnt a full 12V at least mine was not. Mine is 9.7 like yours. I bought a pump relay harness aftermarket kit which has a relay in it. The 9.7V power wire provides power to the relay in the kit triggering power to run from a independent wire from the battery allowing 12+V to flow to the pump. I bought mine years ago and cant tell you off my head the brand. Is was like 45 bucks. Made a big diffrence powering the pump with 12+V rather than 9.7V One of the old timers here can probably tell you off the top of their head which kit. This old timer is loseing his memory:rolleyes:

Ken
 
Went to my Garage and looked at the mods. The harness was a Racetronic brand. It comes complete with all the wiring, relay, connectors. It was easy to install and does what it is suppose to

Ken
 
Does anybody here who has changed wiring to fuel pump to get a full 12 volts have procedures and pictures on how they did it? I get full 12 volts to relay but after that it drops to 9.7 volts
Thanks
Ken

Im a little confused as to your setup - is this on your stock IRCM ?? You should be getting 12 volts on both sides of the relay .. almost sounds like excessive resistance in the relay contacts. Have you done a voltage drop test ( test probes wired in parallel between source and output voltage - 12 and 9 volts ) - if youre getting a 3 volt drop on the relay itself, the relay is no good and should be replaced. Or un-plug the fuel pump and see if youre getting 12 volts instead of 9.7.

If the fuel pump has less resistance than the relay ( many aftermarket pumps do ) , that could explain the imbalance between the voltages - Ohms law, Volts / amps / resistance in series .. as the fuel pump warms up during operation, it should increase in resistance and pull more voltage than the relay is using.


- Dan
 
If the relay is supplied with 12 volts it should also be 12 volts out of the relay. A slight variance is normal but down to 9.7 indicates a faulty relay or too much resistance. Like I said just one week ago I had the exact same problem and replacing the relay solved the issue.
 
Im a little confused as to your setup - is this on your stock IRCM ?? You should be getting 12 volts on both sides of the relay .. almost sounds like excessive resistance in the relay contacts. Have you done a voltage drop test ( test probes wired in parallel between source and output voltage - 12 and 9 volts ) - if youre getting a 3 volt drop on the relay itself, the relay is no good and should be replaced. Or un-plug the fuel pump and see if youre getting 12 volts instead of 9.7.

If the fuel pump has less resistance than the relay ( many aftermarket pumps do ) , that could explain the imbalance between the voltages - Ohms law, Volts / amps / resistance in series .. as the fuel pump warms up during operation, it should increase in resistance and pull more voltage than the relay is using.


- Dan

Dan,
I am testing the power at the fuel pump relay in the trunk. I take the relay out and test the power going to the relay and it is 12v. I put the relay in and test the power going to the pump and I get 9.7v.
I have not tested my other sc, but I get 40psi at idle with vacuum disconnected. I figured that car is getting full power.
Ken
 
If the relay is supplied with 12 volts it should also be 12 volts out of the relay. A slight variance is normal but down to 9.7 indicates a faulty relay or too much resistance. Like I said just one week ago I had the exact same problem and replacing the relay solved the issue.

You obviously didnt read or comprehend my post above yours .. ohms law .. basic voltage rules. Were talking stock for stock, you're correct - but when you have aftermarket fuel pumps you may see voltage drops on circuits that would otherwise have no voltage drop. It should be 12 volts out of the relay with the fuel pump unplugged - the digital volt ohm meter has a ton of internal resistance to get a true reading on the circuit, if you're still getting 9.7 out of the relay with the fuel pump unplugged, you have a bad relay. Check the resistance on the fuel pump and the relay contacts while they are isolated from the vehicle. Standard diagnostics procedure and all you need is a volt meter.

What may have worked on your car isnt necessarily true for all others, its easy to throw parts at it, to see if it worked - a relay is fairly inexpensive. But thats an amateur's way to fix something.


- Dan
 
I should have made it clear that I was referring to testing the voltage drop thru the relay only with the pump unplugged. Sorry about not making that clear :)

I don't throw parts at my car without first thoroughly trying to diagnose the problem.
 
Last edited:
9.7v is at the pump and the the other side of the relay....I am using stock wiring right now.

OK. I have a silly question. Why use a relay with stock wiring?

The PROBLEM is that the wire gauge is too small to get anywhere near 12V at the pump.

Try a heavier gauge wire. A relay helps some, but not as much as, say, 8ga or bigger wire to the fuel pump would help.

Also, run a new ground line back to the front - same size wire.

RwP
 
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