No PCM Ground to Fuel Pump Relay

[kdenmcfish]

Vehicle is a 1992 Ford Ranger XLT 4x4 2.9L V6

Long story short, my fuel pump isn't receiving power. Been working on trying to get this truck started for hours pretty much every day, but am new to this and have not dabbled much with vehicle electrical systems. I have been running through lots of tests and have read countless forums but haven't been able to find something which is exactly outlying my case. Hoping somebody can point me in the right direction.

Inertia Switch:

1. This is fully functional; however, with the fuel pump relay installed I am only getting ≈7V to the inertia switch via circuit 238.

Fuel Pump Relay Findings:

1. Tried replacing the relay for a quick fix, but still only getting 7V to the inertia switch, so I tested the plug:
   • Circuit 175 (BK/Y): Constant 12V
   • Circuit 361 (R): 12V with KOEO
   • Circuit 238 (DG/Y): I ran a jumper wire from BK/Y to DG/Y and the fuel pump ran continuously. In addition, I am getting 12V to the inertia switch with this jumper in place.
   • Circuit 926 (LB/O): 0.00V no matter key position.

Code Reader Findings:

1. KOEO:
   • 87 O: Primary fuel pump circuit failure
   • 95 C: Fuel pump secondary circuit fault
2. KOER:
   • None?

At this point, I know that the pump, inertia switch, and all wiring up to the relay should be good. My understanding is that I am not getting a momentary ground from the PCM to the relay, hence the pump is not getting power for its initial priming.

Apparently, this could be as simple as a bad chassis ground to the PCM, fingers crossed as I will be looking at this later today. If not, I plan to test the LB/O wire for continuity; after which, I will either diagnose the wire or back probe PCM connection 22 where the LB/O wire is terminated. My understanding is that I should see 12V at PCM #22 until I turn the key, upon which it should drop to 0V for 1-2 seconds, before returning to 12V. Is this the correct path forward at this point?

Thanks in advance!

 

[franklin2]

Yes, the 12v on the LB/O wire comes from the red wire, circuit 361. The LB/O wire is just floating, pretend it's just hanging out in the air. So the 12v runs right through the relay coil and the meter reads 12v. When the computer commands the pump to run, it grounds the LB/O wire. So current starts flowing from circuit 361, through the relay coil, and then to ground inside the PCM.

The PCM should initially ground the relay to prime the pump like you said. And then it waits to see signals from the distributor, that the engine is turning. If it never see's any signals from the dist, then it assumes the engine is not turning and it will not turn the fuel pump on after that initial prime sequence.

I leave the PCM as a last resort, but they do go bad once in awhile. And for your info, you can ground the LB/O wire over at the test port and make the fuel pump run.

Does the truck run if you make the fuel pump run and start the engine?

Here's a diagram

 

[kdenmcfish]

Thanks for the response!

Last night (with jumper from BK/Y and DG/Y at relay plug) I was able to get the pump running and it did start (after about 10 tries, fuel pressure test forthcoming after I figure this out). It cranks, the engine will sound like it wants to start up (idling at like 400-500 RPM), then it dies. Eventually, the RPMs will surge to 1200 and it will idle normal.

Today I exposed the EEC, testing both nearby grounds along with two in the engine bay for continuity. 0.00 ohms all around.

I also tested the DG/Y and LB/O for continuity, no issues here.

Lastly, I back probed LB/O at the EEC and tested for voltage and observed 0.00V with the ignition on. If I am not mistaken, I should have been reading battery voltage here? This was much more than 1-2 seconds after turning the ignition key.

 

[franklin2]

12v when the fuel pump is not running. 0v when the fuel pump is running(computer has grounded this wire). So it might be a interesting test to put your meter on the red 361 wire and see if you have 12v there at all times. Key on, key off, cycled on and off, just like you have been testing with the LB/O wire. The 361 Red wire will cycle on and off (12v) when the key is cycled, since it's fed from the EECIV relay.

Your relay socket is in perfect shape correct?

Would be interesting to find out the problem may actually be the EECIV relay.

 

[bcschief]

astly, I back probed LB/O at the EEC and tested for voltage and observed 0.00V with the ignition on. If I am not mistaken, I should have been reading battery voltage here? This was much more than 1-2 seconds after turning the ignition key.

The PCM grounds this circuit to run the fuel pumps, you would not see power there. Sounds like the fuel pump is bad but your pressure test will confirm it.

 

[Terrys87]

Electrical is a weak point of mine, and really out of my league. There is a ground wire coming of the computer in the passenger kick panel. I can only suggest that it is clean and not corroded and getting a good ground. Might check all other grounds. I am interested in finding out what the fix is when you get it figured out.

 

[franklin2]

The PCM grounds this circuit to run the fuel pumps, you would not see power there. Sounds like the fuel pump is bad but your pressure test will confirm it.

Just to let you know, you will see 12v there if the computer is not grounding the circuit.

This is good to know for troubleshooting purposes. If you take a light bulb, hook one side of the light to 12+, and the other side of the bulb is not hooked to anything, if you hook your meter to the battery - and touch the other lead of the meter to the side of the bulb that is not connected, you will get 12v there. As soon as you ground the unconnected side of the bulb, the bulb will light and of course you then will read zero volts on the meter.

 

[kdenmcfish]

A few updates here as it seems maybe some others are following this as well.

Yesterday I went ahead and ran a jumper on the fuel pump relay to get the pump running; and after pressure testing, 0 psi KOEO and 0 psi while cranking (truck would run for a few seconds, then die. 0 psi nonetheless.)

Before digging into EEC replacement, I checked the EEC relay as suggested above and observed 22 ohms at the ground. Based on the wiring diagram, this runs to G101.

Planning to go clean the hell out of this ground to hopefully get this addressed. Would be a huge win!

More to come!

 

[kdenmcfish]

Update time!

The cleaning of G101 did fix my resistance problem at the EEC relay, and in turn, my voltage issue at the fuel pump relay, inertia switch, and fuel pump. However, it did NOT fix my hard start problem.

I took a week off of this project to keep myself sane, but I’m back with new findings and curious what you all think of them.

At this point, I have pretty much narrowed this down to a fuel pump issue. Today I drove the truck to the store to see how it would do, and it was not great!

First and foremost, it just about always takes 10-15 tries to get the truck to idle without stalling. At first, it will only crank. Eventually, it will stutter and then stall. After that, it will run roughly at around 500 rpm for 5-10 seconds, and then stall. But eventually, it will start at 500, and then surge up to 1200 rpm, eventually settling in at 750 rpm and idling fine. Seemingly based on engine temp (air/fuel mixture at temp and whatnot).

Once I reach this point, I can lean slowly onto the gas and the RPMs rise accordingly, making the truck actually operable. However, if I give it any more than a hair of throttle, it will stutter at 1000 RPM (as I found out on a hill with traffic behind me today). At this point, I was thinking fuel pump or FPR. I pulled the vacuum line from the FPR, no leaks.

Hooked up 3 different gauges in the last couple days, all read 0 PSI KOEO and KOER. The most that I have seen from the Schrader valve to date is 2-3 inches of fuel come out, certainly not a spray, more like a lousy water fountain.

HOWEVER! Today at the grocery store I couldn’t get the truck to start to come home. I tested the Schrader valve and was greeted with a full column of high pressure air, no fuel. I relieved all of the pressure, turned the key, and had the truck starting in 4-5 tries. I quickly tested the Schrader again and got a nice spray of fuel, what I imagine 40 PSI would look like. So, I drove home and hooked up a gauge… 0 PSI. Wtf?

The Schrader valve situation makes no sense to me; however, my pump is obviously getting air from somewhere. Did I mention my tank leaks when I fill it completely at the gas station?

Think I will be dropping a new fuel pump into this thing to see if I can get lucky.

 

[kdenmcfish]

In case anybody is reading this in the future and has found the thread relevant so far, the new fuel pump solved this issue pretty much immediately. I did notice that my fuel tank filler neck was completely severed near the tank inlet (solving another issue I have had with leaking fuel at the pump); however, I am not sure if I mentioned that above or not.

Still getting 0 PSI at the Schrader valve, but at this point I do not care and will figure this out later. The truck starts right up, and I now have a consistent idle with immediate and consistent throttle response.

 

[franklin2]

Stay on the edge of your seat. You put a new fuel pump in and it fixed it....great. But do not be surprised if you start having the same problems again within a year. The the parts they sell now are junk. I just heard in the news yesterday Ford is recalling 850,000 new cars and trucks for the same problem, faulty fuel pumps.