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).]

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

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

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.

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.

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).]

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).]

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...)

quote Originally posted by theogre: 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.

Key On does not trigger the fuel pump as it used to. The pump does run once oil pressure has been built. Can I unplug oil pressure unit temporarily, and attempt to run the car without it, to see if the FP relay circuit will run the pump?

quote 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

The ECM DK GRN/WHT has 0v with key off, 12v with key on.

quote 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.)

Wire has 0.2Ω from the ECM pin to the FP relay pin.

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

It is a good ground, 0.3Ω to engine block ground.

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

Not entirely sure what you mean here, sorry.

quote Originally posted by olejoedad: The tan/white (12vdc to pump) should energize when the relay clicks. Bad relay - contacts burnt up.

Neither relay that was in the car(FP and A/C) will click when plugged in but will trigger when i apply 12v directly. So the relay should be good. I got a new one last night, and its the same story.

OP Switch is a backup for iffy relay or ECM failing to turn on the relay.
In your case, Pull the OP sender plug then car will never start.

"When have good Relay then Tan/Wht plug B when work same way as Plug A (If Ignore OP Switch...)"
Means when have a good relay the power to F-pump follows output from ECM.

Last post

quote The ECM DK GRN/WHT has 0v with key off, 12v with key on.

When ECM is unplugged then a "soft" short w/ resistance in that wire to 12v from anywhere and no amps to turn on the relay.
or
When connected to ECM Suspect ECM is "Dead" as for power out to the FP relay while ECM seem to work otherwise.

Cooked wiring often does either or both.

Note: ECM often doesn't set "codes" for problems that can't even start the car. If does set "codes" trying to start is often bogus and mean nothing unless a Scan Tool maybe shows a "dead" sensor. Most times codes set while driving and are old and often there for months to years and mean nothing in your case.

The fuse for amperage is blown on my multimeter. Gonna get a new fuse to check current coming from the ECM to the relay. Spent today trying to get my clutch working. bled the system yesterday and now my clutch is gone, woo

Testing amps w/ shut in Fluke 77 or any other from ECM likely won't help and may give bad data or worse.

I never use a shut meter now because have to "splice" the probes and often just make more problems.

If you're trying to see amps is better to have DC Amp Clamp (Most also read AC Amps.)
Example: https://www.amazon.com/Uni-...itance/dp/B00O1Q2HOQ

Can use that for everything in a car except starter circuit drawing 120+ amps.
I have a couple bigger units that read 200+ amps so I don't have this but very popular on YT and not just maker gave them to everyone.

quote Originally posted by theogre: Testing amps w/ shut in Fluke 77 or any other from ECM likely won't help and may give bad data or worse. I never use a shut meter now because have to "splice" the probes and often just make more problems. If you're trying to see amps is better to have DC Amp Clamp (Most also read AC Amps.) Example: https://www.amazon.com/Uni-...itance/dp/B00O1Q2HOQ Can use that for everything in a car except starter circuit drawing 120+ amps. I have a couple bigger units that read 200+ amps so I don't have this but very popular on YT and not just maker gave them to everyone.

Why would I need to splice anything? Could I not read the current from the prong on the relay harness?

And why would that give bad data?

I know this is a lot of questions, i'm just trying to understand. It seems the most likely point of issue is my ECM right now, is that what you believe it may be?

I think ive isolated the issue to my ECM. Ran seperate wires from the ECM pin to the relay, and a new ground wire. Same issue. Found a thread as old as i am stating the exact same issues im having, and when they replaced their ECM it solved their case. Is an ECM from rockauto reputable or is there someone here on the forum?

No. don't get important/critical parts from RA Ebay etc. More so if parts have a Core Charge.

get at AZ and real parts stores.
AZ, Advance Auto and some others can order online, often w/ better discounts and free shipping, then return core etc to local stores so you don't pay return shipping or have problem like X saying didn't get a core or other BS and not paying.
Plus AZ and some others have better warranty too.

Make sure you careful pull the PROM/MEMCAL in old ECM. You need that for new ECM.

I can't imagine that any ECM you get is just a refurbished ECM. I would just pick up a cheap used ECM and run with it. Have the output driver transistor for the FPR output replaced and then keep your original as a spare.

Got an ECM from autozone, with a 1 year warranty to boot. swapped over my PROM and the other chip(MEMCAL?) and bingo bango the pump primes, and everything is back to usual. Thanks for the help guys, especially ogre

Now that it's running wrap the wires with a new heat resistant material! I had the wire loom that runs from the relay catch fire! and it resulted in a compromised wire just like you did.
Remember that the factiory protective wrap is 30+ years old.

I got this from Amazon.....cheap insurance

A-Team Performance - Heat Shroud Aluminized Sleeving for Ultimate Heat Shield Protection Barrier with Hook and Loop Closure - 1" x 36" (3ft), 500 degrees F direct and 2000 degrees F radiant heat

quote Originally posted by steve308: Now that it's running wrap the wires with a new heat resistant material! I had the wire loom that runs from the relay catch fire! and it resulted in a compromised wire just like you did. Remember that the factiory protective wrap is 30+ years old. I got this from Amazon.....cheap insurance A-Team Performance - Heat Shroud Aluminized Sleeving for Ultimate Heat Shield Protection Barrier with Hook and Loop Closure - 1" x 36" (3ft), 500 degrees F direct and 2000 degrees F radiant heat

excellent idea. i wrapped the exposed wiring with some loom and heat resistant tape, but I'll get some of this stuff and do as much of the harness as i can. ounce of prevention!