Fuel pump help

[Adamj272001]

Have a 1988 k5 blazer and I’m not getting power to my fuel pump. I bypassed it from the relay and it turns on but nothing when I turn the key. I’ve changed the relay and oil pressure sensor and grounds are good.
Other places I’ve looked said could be the ECM-b fuse and there in lies the problem. I can not find it anywhere. I thought it was on the firewall on passenger side but I’m worried cause the previous owner did some shoddy wiring and it’s a mess. Currently cleaning it all up but if anyone could tell me where it’s supposed to be I’d appreciate it.

 

[Wes Harden]

ecm B should be in the cab fuse block. The computer doesn't power the fuel until the engine starts, it only supplies a 2 second prime when the key is cycled to the on position.
Have some turn the key on while you listen near the tank.

Does engine run with by pass powered ? If it does start it then remove the by pass and see if it continues to run.

 

[Adamj272001]

ecm B should be in the cab fuse block. The computer doesn't power the fuel until the engine starts, it only supplies a 2 second prime when the key is cycled to the on position.
Have some turn the key on while you listen near the tank.

Does engine run with by pass powered ? If it does start it then remove the by pass and see if it continues to run.

Thank you for responding,
When I turn the key on I get no sound that the pump is priming. Only when I use the bypass. After bypassing and getting fuel it will turn over for a split second and then shut off.
I had an intermittent fuel delivery problem in the past where it would start and run fine but then not start for a day. Grounds were a mess so I cleaned those and replaced the oil pressure sensor behind distributor. Was running fine since then.
My concern now is the ECM. The previous owner had a makeshift fuse holder under the clutch pedal. I must have stepped on it , smelled something burn for a quick second and found the “fuse holder” had arced. Traced the wires and it comes off the back of the fuse block through the in line fuse holder mentioned above then To the ignition switch that I had to change after all of this, then to the ECM. Now I’m not getting any fuel . Not sure what else to do.

 

[Wes Harden]

Do all the fuses in the factory fuse block have power?

Tring to figure out if they were feeding the fuse block or a circuit on the ignition switch.


The fuse links at the starter and engine side of booster should be examined. A short may have damaged 1

 

[Adamj272001]

I’ll have to check each fuse again but I think I had power to all the fuses except 17 which is ecm, crank circuit and oil pressure switch.
Middle column on bottom. 3 amp fuse. Have to check that wiring when I get home from work


Doubt this helps with what they were trying to do with the ignition but it was swapped from a automatic to a manual transmission prior to me getting it.

 

[Wes Harden]

Cool that it is a stick, how many gears? Only difference between them is the neutral safety switch, and I am assuming the 700r4 controls.
I can't think how either of those would effect fuel pump.
We'll get this sorted, might take a bit of back and forth.

The ecm uses the ignition control module fir it's signal to turn the pump on, aside from the key on prime

 

[Adamj272001]

Cool that it is a stick, how many gears? Only difference between them is the neutral safety switch, and I am assuming the 700r4 controls.
I can't think how either of those would effect fuel pump.
We'll get this sorted, might take a bit of back and forth.

The ecm uses the ignition control module fir it's signal to turn the pump on, aside from the key on prime

I’ll probably thank you 500 more times for the help but really appreciate it.
Transmission is a SM465 4 speed. Really enjoying it when it runs.
I was thinking about changing out the ignition control module but I didn’t want to throw parts at it either.
But it’s cheep enough and in stock at parts store near me. Just need the time to do it

 

[rampage]

I’ll have to check each fuse again but I think I had power to all the fuses except 17 which is ecm, crank circuit and oil pressure switch.

I think that’s your answer right there. If you don’t have power to the ECM it won’t start. Replace that fuse with the appropriate size before doing anything else.

 

[Adamj272001]

So I checked the fuses and got voltage to the crank fuse and all other fuses.

Now I’ve been going all of the problems prior to this happening.
I work overnights so my brain isn’t always functioning so I forgot to put the entire past of the truck.
but prior to this I had the intermittent fuel delivery problem . It would start and run no problem but every so often it would just crank and get no fuel. The first time I noticed this it had started fine at my home. I drove 5 minutes to a gas station, filled it up and crank no start. Checked what I could, banged in the tank and it started but barely.

As I said before I checked to make sure it wasn’t the pump, cleaned all grounds and changed the oil pressure sensor behind distributor and that seemed to fix the problem. Drove fine for a while then recently I went to go to work and had crank no start. - no fuel again. Swapped out Oil pressure sensor again and
It eventually started again but it was running extremely rough. Checked all vacuum lines which were fine. It eventually evened out and drove fine then the in line fuse blew and got a no crank no start until I changed the ignition switch and fuse holder. Now there’s no fuel

Prior to this it always took a while to start maybe 10-15 seconds of cranking before It turned over. Also when I’d drive it for long periods of time 45-60 minutes to my work it would take some time to start again. Few hours-Would get a no crank no start if I tried to start it immediately after shutting it off
Changed the starter since I figured the headers and 100+ degree weather were boiling the starter solenoid which lead me to this point. Never got a chance to check if it was actually the starter.

Now I’m not sure other than the ICM or wiring

 

[Wes Harden]

Well extended crank does point to the fp relay not working, and the oil pressure switch is doing the duty.
Since the whole fuse box has power the fuse links are working. They could still be compromised under heavy load.
The fuse that broke was added to power something, we need to figure out what and why.
The no crank I believe is a separate issue. Some of the causes marginal connections of battery cables,internal corrosion of battery cables, neutral safety switch condition/ adjustment. The new ignition switch is in correct adjustment?
Banging on the tank and successfully getting the pump running,and the poor idle quality after a no start episode are sign that the pump itself may be on its way out.
These are general take aways from the history you gave.
Can elaborate on some if you want.

 

[Adamj272001]

New I forgot something…fuel relay is brand new

With regards to the in line fuse and the no crank I got after the in line fuse blew I traced the wire and it goes from the back of the fuse block( yellow wire) Up to the ignition switch

not sure why the duel fuses. It looks like they cut the yellow wire. Spliced extra wire to each end that was cut and then added the inline fuse. What was there wasn’t even a fuse holder. They just added terminal ends that fit a fuse to the end of each spliced wire, electrical taped them together and then put a fuse in it. So if my truck is in flames when I get home I won’t be surprised

One more problem I forgot is that the oil pressure gauge has been pegged to 60 psi since I’ve had it and does not drop. I put in a new gauge and it did the same thing. I thought this years square body had duel pressure switches but I can not find one near the oil filter anywhere. Haven’t looked for very long though either
Adding the new oil sensor behind the distributor did nothing to fix gauge.

As far as the ignition switch I can adjust it tonight and see if that changes anything. When I initially put it in nothing happened when I turned the key. After adjustment it turned over at least.

With the fuel pump I didn’t think it was a problem since it always ran when I hooked a wire from the relay to the battery. But like you said banging on it is a sign of a problem.
Going to also change out all of the battery terminals cause they are old.

Also last night I found that there was a piece cut out above the fuel tank so I have easy access to the pump and the grounds. Missed that the last time I was checking grounds so I’ll clean those up as well and check wires.

 

[rampage]

Just so you know, the gas tank is ground for the pump and the tank is electrically isolated from the frame by the felt pads on the straps on the tank. Try cleaning off a bare metal spot on the frame and tank and use a connect a temporary wire between them. You can use something like clothes pins to hold the wire in place. A good spot on the tank is the lip on the corner where it’s folded upwards since you’ll be removing the coating on the tank and it won’t matter if that gets rusty.

 

[dyeager535]

Just so you know, the gas tank is ground for the pump and the tank is electrically isolated from the frame by the felt pads on the straps on the tank.

Unless I'm misunderstanding, there is a ground from tank/sending unit to frame. At least on my K5 TBI Tank/sending units.

 

[rampage]

Unless I'm misunderstanding, there is a ground from tank/sending unit to frame. At least on my K5 TBI Tank/sending units.

Yes there is. But like I said, the tank is electrically isolated due to the anti-squeak pads so it needs that ground wire to the frame. I’m just having him check to make sure that ground wire isn’t the problem since it can be making a bad connection under terminal to the frame or it could be broken somewhere inside the insulation - both are common problems. Guess I should have explained the reasoning for doing that in my previous post.

 

[dyeager535]

Yes there is. But like I said, the tank is electrically isolated due to the anti-squeak pads so it needs that ground wire to the frame. I’m just having him check to make sure that ground wire isn’t the problem...

Nope, makes perfect sense! Ill use the multimeter to ensure continuity between the tank and frame when I'm doing anything around the tank, that ground is usually covered in dirt and sees a lot of water, so I'm always questioning whether it's still in good shape.

 

[Adamj272001]

Hope everyone had a good holiday season. Post isn’t abandoned just had some issues the past few months.
Gone over all wires at pump grounds etc still having fuel issue though.
I’m not sure if I posted this before but I shot wires at relay when key is on getting 2.1 volts. Also nothing at wire that primes the fuel pump when key is on and when I use the jumper wire the pump will run but truck will not run for more than a few seconds.
Thanks for the help with this

 

[Wes Harden]

where is the 2.1 v ? pump will not run key on engine off. This yellow wire with the hokey fuse is that the one from fuse block to ign swt ? @10ga diameter ? If so remove that fuse, butt connect the wire, use with shrink tube to insulate. No fuse needed in that wire.

 

[Adamj272001]

Here’s where I might be getting confused. I thought I read that with the key in the ACC position I should have 12 volts at (A) orange wire that goes to the relay. The schematic I have shows it coming directly off the battery through the oil pressure switch to the relay. And then (D)
Green and white wire goes to the ecm.

Going to solder that wire and remove the extra fuse this weekend. Plus check a few more things

I don’t want to add anything else to this scenario yet but I noticed something when we jumprered the fuel pump from the relay. Ran. For two seconds but roughly. So with my son pooring fuel into the bowl I was able to get it to run long enough but horribly like the timing was off. I haven’t messed with the distributor and it ran smoothly before all of this. Doesn’t the computer control the timing as well?

 

[Blue85]

So with my son pooring fuel into the bowl I was able to get it to run long enough but horribly like the timing was off.

There is no fuel bowl on a TBI, so pouring fuel down the the throttle body is going to give very random air/fuel ratios. You could even hydrolock a cylinder.

 

[Wes Harden]

Having 12v on (A) orange wore is possible, oil switch needs pressure to close. Oil switch is normally open.
Is (D) at the oil switch or the fuel pump relay. Gussing relay, if so this is how the ecm activities the relay. Bu providing a ground to the control side.
Could be running rough be cause fuel isn't metered and possibly fouled spark plugs.