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Username Post: why use relay        (Topic#359287)
995jim 
Contributor
Posts: 569
995jim
Loc: Ohio
Reg: 12-17-06
12-29-19 01:35 PM - Post#2783134    

Hi, Im installing an electric fuel pump for a nat aspirated engine which draws 4.5 amps. I would like to know why I would want to use a relay harness setup vs wiring straight to battery with a fused link. Is it so that the ignition is involved to eliminate a dead battery? help thanks
jim

Jim

65' Impala SS, 65' Belair, 89' Silverado Z71, 95' Silverado Z71, 12' Silverado, 15' Silverado LTZ, 16' Tahoe LTZ


 


wagonman100 
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Loc: Baltimore, MD
Reg: 11-27-04
12-29-19 10:00 PM - Post#2783180    
    In response to 995jim

No, it isn’t for the battery. If you wire to a circuit that only is live with the ignition on, the fuel pump won’t draw on the battery with the ignition off. A relay just lessens the load on the switch that activates the device it is running.

Jay
Friends don’t let friends drive Fords.

1999 Silverado Z71 4X4 extra-cab short bed
1983 Malibu Fauxmad - tubbed
1978 El Camino Kustomized
1972 Monte Carlo
1957 210 handyman wagon
1957 Nomad sport wagon
1957 Cameo Carrier


 
CowboyTrukr 
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Loc: Salt Lake City
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12-29-19 10:40 PM - Post#2783182    
    In response to wagonman100

^^^^ what he said.

Greg

‘01 Silverado 2500HD 8.1/Allison 1000 Xcab/LB
'00 Explorer XLT 4.0 SOHC V6 Auto
'95 K1500 Z71 EC Short Step 5.7L+0.040/NV3500 SOLD

"The only thing necessary for evil to triumph is for good men to do nothing" Sir Edmund Burke


 
995jim 
Contributor
Posts: 569
995jim
Loc: Ohio
Reg: 12-17-06
12-30-19 10:26 AM - Post#2783214    
    In response to wagonman100

So is it recommeded to use this relay method for the fuel pump? Im just trying to understand why then all switches dont have relays. Or do they?
Thanks jim

Jim

65' Impala SS, 65' Belair, 89' Silverado Z71, 95' Silverado Z71, 12' Silverado, 15' Silverado LTZ, 16' Tahoe LTZ


 
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
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grumpyvette
Age: 72
Loc: FLORIDA USA
Reg: 03-16-01
12-30-19 02:07 PM - Post#2783230    
    In response to 995jim

http://garage.grumpysperformance.com/index.php?thr...

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


Edited by grumpyvette on 12-31-19 03:52 PM. Reason for edit: No reason given.

 
Chevelle 
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Posts: 1060
Chevelle
Loc: Dana Point, CA
Reg: 08-31-13
12-30-19 02:46 PM - Post#2783233    
    In response to grumpyvette

Comes up with an error page.

Chevelle
Community Experience Manager

I'll be happy to help just ask.

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grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17545
grumpyvette
Age: 72
Loc: FLORIDA USA
Reg: 03-16-01
12-31-19 03:53 PM - Post#2783337    
    In response to Chevelle

is anyone else having an issue?
yes you have to register, but its totally free and rather easily done.
my only goal here, is to help the max number of people avoid problems

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


Edited by grumpyvette on 12-31-19 03:59 PM. Reason for edit: No reason given.

 
george88gta 
"16th Year" Silver Supporting Member
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george88gta
Loc: new york
Reg: 04-23-03
12-31-19 05:21 PM - Post#2783343    
    In response to grumpyvette

No trouble at all. I signed up a long time ago and had no trouble accessing your site. Thanks, Paul, lots of good info.



 
george88gta 
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george88gta
Loc: new york
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01-02-20 04:02 PM - Post#2783518    
    In response to 995jim

  • 995jim Said:
So is it recommeded to use this relay method for the fuel pump? Im just trying to understand why then all switches dont have relays. Or do they?
Thanks jim


Here is my two cents. Automakers have to keep costs inline. As a result, they will use the smallest awg to save costs and reduce weight. It is not cost effective to run lengths of 12 - 16 awg wire when you can run 18 - 22 awg wire. If a circuit requires a heavy load, then they install a relay. They can activate the relay with a much smaller wire and let the relay feed a much shorter circuit with the properly sized wire ( think of a headlight circuit). Additionally, the switches can be of a lighter construction since they are only switching milliamps or only 1 or 2 amps.
So the answer to your question is go with the relay and you can feed the relay from a switched ignition source. Hope this is helpful.



 
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17545
grumpyvette
Age: 72
Loc: FLORIDA USA
Reg: 03-16-01
01-02-20 04:29 PM - Post#2783521    
    In response to george88gta

just a thought?
would not adding a relay and the more complex wiring to feed the device and relay far out cost the simple use of a marginally heavier gauge wire that would eliminate the need for the relays use?

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


 
george88gta 
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george88gta
Loc: new york
Reg: 04-23-03
01-03-20 05:48 AM - Post#2783577    
    In response to grumpyvette

  • grumpyvette Said:
just a thought?
would not adding a relay and the more complex wiring to feed the device and relay far out cost the simple use of a marginally heavier gauge wire that would eliminate the need for the relays use?


Absolutely. For a hot rodder, just going with a larger wire certainly would be less expensive and simpler. It's probably only $.03 -.05 per foot increase.
Just find a switched source at the main fuse block ( making sure that the switched source can handle the amps) and you are good to go. Down side is that as long as the ignition is in the run position, the pump will run.
If you add in the "rollover" or "kill"switch that mfgs are required to use, then the relay is the way to go. They can tie in a few different ways to de-energize the relay in case of an accident, etc.



 
bobb 
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Loc: paradise
Reg: 09-05-03
01-05-20 11:16 AM - Post#2783746    
    In response to 995jim

for an electric fuel pump i would always run a relay so that oil pressure is needed to turn it on. yes its more complicated but still simple wiring. reason is that in case of emergency, when the engine is off so is the fuel pump, avoiding a bigger emergency.

70 L camino, grampa engine, g-force 5 spd, road rage suspension. Pray first before all else fails.


 
george88gta 
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george88gta
Loc: new york
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01-05-20 03:40 PM - Post#2783759    
    In response to bobb

  • bobb Said:
for an electric fuel pump i would always run a relay so that oil pressure is needed to turn it on. yes its more complicated but still simple wiring. reason is that in case of emergency, when the engine is off so is the fuel pump, avoiding a bigger emergency.


That's kind of where I was heading. I will admit, that a long time ago ( 6 volt 51 Mercury) I did wire an electric fuel pump to a switched circuit. If I remember correctly, my 1988 GTA ( fuel injected ) used a relay. For the initial turn of the ignition switch, you got a timed pulse to the pump. After that, it relied on the oil pressure to control the relay and activate the pump. Ford also had an impact switch that would cut off the fuel pump.
Bottom line is that it is safer to use a relay.



 
65_Impala 
Very Senior Member
Posts: 4559

Reg: 12-29-02
01-06-20 08:13 AM - Post#2783806    
    In response to george88gta

  • george88gta Said:
If I remember correctly, my 1988 GTA ( fuel injected ) used a relay. For the initial turn of the ignition switch, you got a timed pulse to the pump. After that, it relied on the oil pressure to control the relay and activate the pump.



No GM ran the fuel pump via oil pressure. Some early GM EFI systems had a oil pressure switch as a backup to the relay but you could eliminate that oil pressure switch without causing any issues.

The relay simply allows you to switch a high current load with a low current signal. Why you would want to do that depends on what you are trying to do.

There is no absolute requirement to use a relay to add extra safety to a circuit. There are roll-over switches and oil pressure switches that could run a fuel pump directly. But, it's sometimes an advantage to use a relay.

OEM's generally switched to using relays for things like lights because the cars electronics will switch those on and off (for example lights flash when you lock the car) and car electronics can't drive those items directly.

You can use a relay to power the fuel pump so that the fuel pump switches on and off with the ignition switch without the fuel pump adding any extra load to the existing ignition switch circuit.






 
995jim 
Contributor
Posts: 569
995jim
Loc: Ohio
Reg: 12-17-06
01-06-20 05:52 PM - Post#2783851    
    In response to 65_Impala

So I would like to use a relay setup tied into the ignition switch. I cant remember if or which wire on a 65 impala ign switch I would want to use. Is it the purple one ? Or do I then lose power while starter cranks?

Jim

65' Impala SS, 65' Belair, 89' Silverado Z71, 95' Silverado Z71, 12' Silverado, 15' Silverado LTZ, 16' Tahoe LTZ


 
65_Impala 
Very Senior Member
Posts: 4559

Reg: 12-29-02
01-07-20 10:36 AM - Post#2783922    
    In response to 995jim

It's not the purple. That's the starter solenoid wire so it would only energize while cranking.

It should be a pink wire that is ignition switched.





 
george88gta 
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george88gta
Loc: new york
Reg: 04-23-03
01-08-20 04:01 PM - Post#2784003    
    In response to 65_Impala

  • 65_Impala Said:
  • george88gta Said:
If I remember correctly, my 1988 GTA ( fuel injected ) used a relay. For the initial turn of the ignition switch, you got a timed pulse to the pump. After that, it relied on the oil pressure to control the relay and activate the pump.



No GM ran the fuel pump via oil pressure. Some early GM EFI systems had a oil pressure switch as a backup to the relay but you could eliminate that oil pressure switch without causing any issues.

The relay simply allows you to switch a high current load with a low current signal. Why you would want to do that depends on what you are trying to do.

There is no absolute requirement to use a relay to add extra safety to a circuit. There are roll-over switches and oil pressure switches that could run a fuel pump directly. But, it's sometimes an advantage to use a relay.

OEM's generally switched to using relays for things like lights because the cars electronics will switch those on and off (for example lights flash when you lock the car) and car electronics can't drive those items directly.

You can use a relay to power the fuel pump so that the fuel pump switches on and off with the ignition switch without the fuel pump adding any extra load to the existing ignition switch circuit.





From the 2002 TransAm shop manual
Starting Mode
With the ignition ON, before engaging the starter, the PCM energizes the fuel pump relay for two seconds allowing the fuel pump to build up pressure. The PCM first checks speed density, then switches to the mass air flow (MAF) sensor. The PCM also uses the engine coolant temperature (ECT), throttle position (TP), and manifold absolute pressure (MAP) sensors to determine the proper air/fuel ratio for starting. This ranges from 1.5:1 at -36°C (-33°F) to 14.7:1 at +94°C (+201°F) running temperature. The PCM controls the amount of fuel delivered in the starting mode by changing the pulse width of the injectors. This is done by pulsing the injectors for very short times.

Havent yet found a reference to the oil pressure, but I agree, it is a rollover/engine protection type of safety provision. The injected fuel pump systems require 20 -25 amp circuits and the wires on the oil pressure sender couldn't handle the load. The fuel pump relay is energized by the pcm so I would bet that the oil pressure is one of the inputs required for continuous engine run time.



 
65_Impala 
Very Senior Member
Posts: 4559

Reg: 12-29-02
01-08-20 08:10 PM - Post#2784023    
    In response to george88gta

  • george88gta Said:
  • 65_Impala Said:
  • george88gta Said:
If I remember correctly, my 1988 GTA ( fuel injected ) used a relay. For the initial turn of the ignition switch, you got a timed pulse to the pump. After that, it relied on the oil pressure to control the relay and activate the pump.



No GM ran the fuel pump via oil pressure. Some early GM EFI systems had a oil pressure switch as a backup to the relay but you could eliminate that oil pressure switch without causing any issues.

The relay simply allows you to switch a high current load with a low current signal. Why you would want to do that depends on what you are trying to do.

There is no absolute requirement to use a relay to add extra safety to a circuit. There are roll-over switches and oil pressure switches that could run a fuel pump directly. But, it's sometimes an advantage to use a relay.

OEM's generally switched to using relays for things like lights because the cars electronics will switch those on and off (for example lights flash when you lock the car) and car electronics can't drive those items directly.

You can use a relay to power the fuel pump so that the fuel pump switches on and off with the ignition switch without the fuel pump adding any extra load to the existing ignition switch circuit.





From the 2002 TransAm shop manual
Starting Mode
With the ignition ON, before engaging the starter, the PCM energizes the fuel pump relay for two seconds allowing the fuel pump to build up pressure. The PCM first checks speed density, then switches to the mass air flow (MAF) sensor. The PCM also uses the engine coolant temperature (ECT), throttle position (TP), and manifold absolute pressure (MAP) sensors to determine the proper air/fuel ratio for starting. This ranges from 1.5:1 at -36°C (-33°F) to 14.7:1 at +94°C (+201°F) running temperature. The PCM controls the amount of fuel delivered in the starting mode by changing the pulse width of the injectors. This is done by pulsing the injectors for very short times.

Havent yet found a reference to the oil pressure, but I agree, it is a rollover/engine protection type of safety provision. The injected fuel pump systems require 20 -25 amp circuits and the wires on the oil pressure sender couldn't handle the load. The fuel pump relay is energized by the pcm so I would bet that the oil pressure is one of the inputs required for continuous engine run time.




You won't find anything saying that oil pressure is required because it never was. Pretty much every EFI system runs the pump at key-on for a short time to prime the system so that is no big surprise or anything unique.

I have re-wired these old injection harnesses (doing transplants and retrofits) and I can tell you with 100% certainty that the oil pressure switch is wired in parallel with the relay contacts. It is NOT wired to the coil of the fuel pump relay. I can also tell you that the relay IS energizes again once the ECM gets pulses from the ignition system which means the engine is cranking. It WAS NOT any type of engine or rollover protection.

Best I can figure, the oil pressure switch was wired in parallel with the relay contact as a backup for the relay to not strand anyone if the relay failed. But, GM had eliminated this backup aon most EFI systems by around 94-96 so they most likely tracked warranties and found that relay failures were not worth the backup.

A stock fuel injection pump doesn't require anything close to 20-25A. Try about 3-5A.





 
Texasray 
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Texasray
Loc: Texas
Reg: 11-01-16
01-16-20 04:01 PM - Post#2784492    
    In response to 65_Impala

  • 65_Impala Said:

No GM ran the fuel pump via oil pressure.
  • Quote:


Opps, lets no forget one of everyone's favorite chevy's the Vega. Oil pressure activated electric fuel pump curcuit. Even all of those were removed when the V8 was dropped in.

Just to get back to the original inquiry this is how I did mine, a carbed system.

30 battery
87 fuel pump
85 to oil pressure switch- I also ran a toggle switch inline to ground so I could fill the carb if the car sat for to long.
86 ignition circuit hot in run (pink usually)

It was only activated with the key on, oil pressure switch closed with pressure or intermittent toggle switch held down.

If you need a switch look for a 85 to 87 camaro with a F or 8 vin. if you want the number (if you can't find a Parts person who can use a book) PM me back and I will get you a standard #.

Sam





 
65_Impala 
Very Senior Member
Posts: 4559

Reg: 12-29-02
01-16-20 08:31 PM - Post#2784512    
    In response to Texasray

Whatever, forgot to put EFI in that sentence, even though what I was responding to and the paragraph I wrote was about GM EFI systems.



 
paulo57509 
Senior Member
Posts: 473

Loc: Tracy, CA
Reg: 07-18-00
01-21-20 02:51 PM - Post#2784828    
    In response to george88gta

  • george88gta Said:
  • bobb Said:
for an electric fuel pump i would always run a relay so that oil pressure is needed to turn it on. yes its more complicated but still simple wiring. reason is that in case of emergency, when the engine is off so is the fuel pump, avoiding a bigger emergency.


That's kind of where I was heading. I will admit, that a long time ago ( 6 volt 51 Mercury) I did wire an electric fuel pump to a switched circuit. If I remember correctly, my 1988 GTA ( fuel injected ) used a relay. For the initial turn of the ignition switch, you got a timed pulse to the pump. After that, it relied on the oil pressure to control the relay and activate the pump. Ford also had an impact switch that would cut off the fuel pump.
Bottom line is that it is safer to use a relay.



Yes, your GTA fuel pump used a relay but it didn't run through the oil pressure switch.

Only carb'ed 3rd Gen F-Bodies (VIN G and H) fed 12v to the electric fuel pump through an oil pressure switch (OPS) only after the engine was running and the key returned to the run position.

The fuel pump relay was closed (by passing the OPS) only while the starter was engaged.

There were both electric and mechanical fuel pumps; an odd setup for a carb'ed engine. The only explanation I have is this was done to address vapor lock issues.


1987 IROC-Z 5.0L
1992 Lumina Euro 3.1L
2003 GMC Safari


 
george88gta 
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george88gta
Loc: new york
Reg: 04-23-03
01-22-20 11:24 AM - Post#2784893    
    In response to paulo57509

  • paulo57509 Said:
  • george88gta Said:
  • bobb Said:
for an electric fuel pump i would always run a relay so that oil pressure is needed to turn it on. yes its more complicated but still simple wiring. reason is that in case of emergency, when the engine is off so is the fuel pump, avoiding a bigger emergency.


That's kind of where I was heading. I will admit, that a long time ago ( 6 volt 51 Mercury) I did wire an electric fuel pump to a switched circuit. If I remember correctly, my 1988 GTA ( fuel injected ) used a relay. For the initial turn of the ignition switch, you got a timed pulse to the pump. After that, it relied on the oil pressure to control the relay and activate the pump. Ford also had an impact switch that would cut off the fuel pump.
Bottom line is that it is safer to use a relay.



Yes, your GTA fuel pump used a relay but it didn't run through the oil pressure switch.

Only carb'ed 3rd Gen F-Bodies (VIN G and H) fed 12v to the electric fuel pump through an oil pressure switch (OPS) only after the engine was running and the key returned to the run position.

The fuel pump relay was closed (by passing the OPS) only while the starter was engaged.

There were both electric and mechanical fuel pumps; an odd setup for a carb'ed engine. The only explanation I have is this was done to address vapor lock issues.



Not to start an argument, but my 2002 TransAm has a fuel pump relay and it feeds a 20 amp fused circuit to the fuel pump. Relay looks to be controlled by the PCM.



Edited by george88gta on 01-22-20 11:56 AM. Reason for edit: No reason given.

 
995jim 
Contributor
Posts: 569
995jim
Loc: Ohio
Reg: 12-17-06
01-22-20 05:49 PM - Post#2784910    
    In response to wagonman100

I have a blower on this engine and was always told to wet the rotors before firing up so Im concerned that using oil pressure to dictate fuel being pumped negates my ability to do so. Any thoughts?


Jim

65' Impala SS, 65' Belair, 89' Silverado Z71, 95' Silverado Z71, 12' Silverado, 15' Silverado LTZ, 16' Tahoe LTZ


 
IgnitionMan 
Valued Contributor
Posts: 3810

Reg: 04-15-05
01-26-20 06:36 PM - Post#2785245    
    In response to 995jim

My 1972 Vega car has an all aluminum Rover V8 and 5 speed in it. Used to be carbureted, now, Holley tuned port EFI.

With the carbs from both the inline 4, and the non-EFI Rover, there was a single wire actuator to allow the fuel pump to operate, and stop the pump if the vehicle lost oil pressure, rolled over, was driven into a full-on nuclear event, whatever.

EFI is a Holley (NOT A FITech/SNIPER, THANK HEAVEN), and it still has that single factory fuel pump wire in place and functional with the EFI, now running with the Holley EFI system, NO RELAYS.

The reason there is no relay is multi-fold, systems back then weren't set up to go Top Fuel in them, they were street stock cars, relays were/are more complicated while not being more reliable, and most dealer techs and outside mechanics had NO idea how they worked, how to fix them when they blew the relays out, and other problems.

I have never found a relay to be superior over using the correct circuit parts, wire load specifications.



 
65_Impala 
Very Senior Member
Posts: 4559

Reg: 12-29-02
01-27-20 10:43 AM - Post#2785283    
    In response to 995jim

  • 995jim Said:
I have a blower on this engine and was always told to wet the rotors before firing up so Im concerned that using oil pressure to dictate fuel being pumped negates my ability to do so. Any thoughts?





You didn't provide much info on what you're trying to do or what the application is, but yes it could affect it. You'd want a cranking bypass anyways so you don't have to crank it a bunch with a dry carb before it would start.

Easiest is to wire the pump to the ignition power. Use a relay if you don't believe the ignition circuit can handle the extra load.



Not having a relay between electronics and power parts really isn't a superior setup. If your electronic control box doesn't use an external relay then it either has an internal relay or a high current transistor to switch the fuel pump. In either case, you don't want that component inside the box failing because then you're sending the box back to the manufacturer for repairs which is a much bigger pain than replacing an external relay.




 


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