Fuel pump relay potential issues (Page 1/3)
A_Lonely_Potato JUN 27, 05:03 PM
Hey all, back again. A couple days ago when i tried to atart my car, i noticed that the fuel pump did mot prime with key on. The car still started, but died after about a second of running. Looking at other posts here, it seemed my issue is with the relay or wiring from there to the pump. Once i have built oil pressure from cranking it starts right up, and has no issues running. Checking my relay, i swapped the connector to the A/C relay, with no change. (unknown if the A/C relay is actually any good, my cars A/C is not plumbed together. Gonna pick another relay up today)

2 things ive noted so far. Firstly, my wiring harness seems to have rested on my exhaust, exploding a wire. that wire has continuity to a few of the prongs on my FP relay connector. Gonna splice that hopefully today.

Secondly, some of the prongs have continuity to each other and i dont know if that is correct. The orange/black wires have 1.3Ω to the tan/white wire, and both of those are ~11KΩ to the green/white wire. Looking at the wiring diagram, this doesnt really make sense to me that they would be connected, so i figured i should ask here.

EDIT:
For visualization, here are the resistances "mapped"


The car isn't running, so the oil pressure sensor should be open, so I assume I must have a short somewhere, because there's no other way for ORN/BLK(prong D) to be connector to TAN/WHT(prong B)

[This message has been edited by A_Lonely_Potato (edited 06-27-2022).]

theogre JUN 27, 06:33 PM
Digital meter?
Does meter has a "Rel" button and was the meter "Zeroed" to see low Ω?

If not then "1.3" Ω includes probe wire and other meter resistance often pushing 1Ω or more w/ cheap or old meters.

This is Why all Analog meters have "Zero" knob/wheel for Ω ranges and now many Digital meters have "Rel" button.

Even if you Zeroed the meter, 1.3Ω across relay or switch contacts is often normal because of dirt etc but when cheap to replace then replace anyway.

This won't cause no power to the pump but may be part of low volts... After fixing cooked wiring...
See my Cave, Electric Motors
greenturnedblue JUN 27, 07:42 PM
I would also unplug the oil pressure sender, and recheck continuity between prongs D and B. then you can tell if the short is in the wiring or the sender itself
A_Lonely_Potato JUN 27, 09:17 PM

quote
Originally posted by theogre:

Digital meter?
Does meter has a "Rel" button and was the meter "Zeroed" to see low Ω?

If not then "1.3" Ω includes probe wire and other meter resistance often pushing 1Ω or more w/ cheap or old meters.

This is Why all Analog meters have "Zero" knob/wheel for Ω ranges and now many Digital meters have "Rel" button.

Even if you Zeroed the meter, 1.3Ω across relay or switch contacts is often normal because of dirt etc but when cheap to replace then replace anyway.

This won't cause no power to the pump but may be part of low volts... After fixing cooked wiring...
See my Cave, Electric Motors



Yes a digital meter; a fluke. The D prong has 12v going to it with key off/on, so i know power is getting to the relay at least The D to B pins should be an open, not 1.3Ω. I unplugged the oil pressure sender as greenturnedblue suggested, the resistances are the same regardless of the oil pressure sender being connected. I spliced in new wires on the harness with no change. Replaced the relay to no effect as well.
theogre JUN 28, 10:18 AM
Wait the relay Plug empty shows 1.3 Ω and 11k?
Miss read was relay itself had low Ω etc...

A D should never be but Open. Pull Pump Fuse to make sure car doesn't have a sneak path to read 11k. Sim to See my Cave, Sneak Path using ECM and other wires/fuses.

Often you have to pull 1 or more fuses even when the I--key is off. Even then may not solve a sneak path in a power or ground wire.
Could try unplug C502 going to fuel tank below bulkhead "seal" thu fire wall. Or unplug C203 below ECM.

⚠️ you did read "ECM Solid State Do Not Measure Resistance?" Any wires to ECM w/ Ω meter attach to it can to weird things or worse damage the ECM. Like 11KΩ between A & D on relay plug.

OP switch and maybe pump fuse needs unplugged before testing sockets. Then should see Open circuit for sure w/ i-Key Off.
Engine Off Should see Open w/ OP switch in too but pull it to make sure isn't a problem.

After you fix burn wires, W/ I-key On and Relay In should see 12v on dk grn/wht and anywhere on tan/wht wire when ECM turns on the relay.
But If dk grn/wht is shorted can fry the ECM "Drivers" while ECM seems to work otherwise.

Fluke is good meter but most of the Best ones still don't have Rel to Zero the Ω. Only some recent Fluke models have this.
Use a fix Ω range not auto. Push Range button, often several times, until see OL. Ω
short out the probes, read Ω and remember to minus the amount for all final Ω. (My old Fluke runs ~ 0.5 doing same. Cheap units can be 1Ω or more just shorting the probes.)

Even most current Fluke 77 and others don't have Rel because adding just that button can/would affect Thousands to Millions test docs and procedures for many Govrmnts and big business word wide. Remembers Huge amount of Fluke product is sold under Contract to Big Business and Govrnmts and can't change/add/delete features w/o big problems. Current version of 77 looks and works near exact same as First version made in the 80's for this reason.
A_Lonely_Potato JUN 28, 12:29 PM

quote
Originally posted by theogre:

Wait the relay Plug empty shows 1.3 Ω and 11k?
Miss read was relay itself had low Ω etc...



Yes haha, bit of a kerfuffle.


quote
⚠️ you did read "ECM Solid State Do Not Measure Resistance?" Any wires to ECM w/ Ω meter attach to it can to weird things or worse damage the ECM. Like 11KΩ between A & D on relay plug.



With IGN off, shouldnt it be an open to the ECM from pin A?


quote
OP switch and maybe pump fuse needs unplugged before testing sockets. Then should see Open circuit for sure w/ i-Key Off.
Engine Off Should see Open w/ OP switch in too but pull it to make sure isn't a problem.




quote
A D should never be but Open. Pull Pump Fuse to make sure car doesn't have a sneak path to read 11k...



With FP fuse pulled, and OP switch unplugged, pin D to pin B on the FP relay harness is still 1.3Ω


quote
Fluke is good meter but most of the Best ones still don't have Rel to Zero the Ω. Only some recent Fluke models have this.
Use a fix Ω range not auto. Push Range button, often several times, until see OL. Ω
short out the probes, read Ω and remember to minus the amount for all final Ω. (My old Fluke runs ~ 0.5 doing same. Cheap units can be 1Ω or more just shorting the probes.)



The multimeter measures only 0.1Ω on fixed range with the probes just touching each other. Funny you mention Fluke 77 because thats what it is.


quote
After you fix burn wires, W/ I-key On and Relay In should see 12v on dk grn/wht and anywhere on tan/wht wire when ECM turns on the relay.
But If dk grn/wht is shorted can fry the ECM "Drivers" while ECM seems to work otherwise.



Burnt wires have been spliced. The relay does not have an audible click or one that can be felt when holding it when the key is turned to ON.


quote
Could try unplug C502 going to fuel tank below bulkhead "seal" thu fire wall. Or unplug C203 below ECM.



I cant reach the C502 until i get the car jacked up. Its in a bad position right now for that so ill have to put it back together to turn it around so i can get it lifted lol. Not a big deal but annoying nonetheless.

[This message has been edited by A_Lonely_Potato (edited 06-28-2022).]

A_Lonely_Potato JUN 28, 02:24 PM
Wow....i really dont want to admit this but...im apparently incompetent. After measuring these pins so many times, i had them mixed up in my head. I am reading 1.3Ω from TAN/WHT(pin B) to BLK(pin C). I dont understand how i had that so mixed up so consistently...

Now im really lost lol

[This message has been edited by A_Lonely_Potato (edited 06-28-2022).]

A_Lonely_Potato JUN 28, 03:10 PM
Ok, making corrections here.
ORN/BLK has constant 12V with battery connected.
GRN/WHT has 12V with key ON
Tested my relays with 12V and they function.
Jumped ORN/BLK to TAN/WHT and the fuel pump runs.

Still nothing from the pump with key ON when the relay is plugged in.

Had my dad come out and make sure i was telling myself the right things lol.
olejoedad JUN 28, 04:02 PM
The tan/white (12vdc to pump) should energize when the relay clicks.
Bad relay - contacts burnt up.

[This message has been edited by olejoedad (edited 06-28-2022).]

theogre JUN 28, 04:12 PM
ECM Dk Grn/Wht plug A runs the pump w/ 12v to relay coil when:
Turn Key On but not start will run ~ 2 sec. Turn Key Off and wait a few sec then will repeat forever. (Side note: If you work on fuel pump plumbing that allows you to purge air out before you try to start.)
Turn Key On then Start will stay On.
Turn Key On but not start will run ~ 2 sec but leave Key On for whatever time then start then ECM will turn pump On again and stay on when running.

Basically IF that wire has power w/o engine running then Something is wrong w/ ECM or the wire.
Seeing 12v is ½ of issue... If ECM has "blown" driver part(s) for the Relay... may read 12v but no amps to actually turn the coil On. Most if not all ECM "Switches" shown above are MOSFET that die outright, iffy support parts or "just" crack solder joint that are "dead" or doesn't handle power to whatever. Dead MOSFET can fail short or open but short may not be a "dead short" w/ 0Ω

Unplug ECM. Check wire for volts, should be 0 now. Now find other end of Dk Grn/Wht. What pin depends on year and engine. and check Ω of the wire. should be near 0. (Likely have help and/or make 1 long probe and read meter thru glass.)

Blk or Blk/Wht (Depending on year/engine) on plug C is Ground for Relay Coil.

When have good Relay then Tan/Wht plug B when work same way as Plug A (If Ignore OP Switch...)