Started, quit, and the mystery began...

4Runner

Well-known member
May 24, 2007
461
27
Boise Idaho
You should really look at the wiring between the from the fuel pump forward. Do you have a constant 12v for a few seconds at the pump when you are cranking I would guess that if you hot wired the fuel pump and then you tried to start it, you could rule out anything else but the fuel pump wiring. Your new pump and relay are probably fine despite what’s going on. I know it’s frustrating but hang in there. You’ll get it and be smarter for it.
 

Spark6

Active member
Oct 31, 2020
27
3
Pittsburgh, PA
Excellent point, and well taken, 4Runner, thank you! Yes, cooler heads have prevailed, and once I had wifely assistance (after she finished work!) I was able to do a little more digging. The new relay is giving a solid (though subtle) "click" upon selection of ignition Pos. II, which at least means that that particular piece of hardware is functioning properly. I agree that the pump is likely good as well. I've spent the last few hours peeling back the layers on the RAVE documentation, and am starting to understand it. I checked the C407 plug in the left rear wheel well, and didn't see any indication of blackening or corrosion, but I'm suspecting that as I check continuity along the harness I'll find a break... off to buy a multimeter tomorrow. I'll report back.
 

WaltNYC

Well-known member
Mar 3, 2010
520
26
NYC
If you have a LR Discovery you will need that multimeter eventually anyway. Add an OBD dongle and a real time app (OBD Fusion, Torque, etc) and you'll solve a good portion of any trouble for <$50.
 

Spark6

Active member
Oct 31, 2020
27
3
Pittsburgh, PA
If you have a LR Discovery you will need that multimeter eventually anyway. Add an OBD dongle and a real time app (OBD Fusion, Torque, etc) and you'll solve a good portion of any trouble for <$50.
It’s true, I’d just hoped it wouldn’t be quite so soon!

Okay. I have continuity in the harness from plug C407 to plug C434. (Basically, the pigtail from the fuel pump to the main harness, connecting in the Left Rear Wheelwell.) Of course, that would have been the best place to NOT have continuity, because it is relatively modular and easy to replace. So, that means that, if I’m missing continuity, then it’s from plug C407 (wheelwell) to plug C277, which is buried somewhere behind the fuse box to the left of the steering wheel. The ETM helpfully shows the fuse box pulled out of the panel with sufficient wire leads for everything to remain plugged in... it doesn’t look like I have that much slack. So before I start tearing out the fuse box... does anyone have any other ideas?

The reason I ask is that, in my mind, at least, a harness that runs inside the car seems like the least likely candidate for a break/wear damage, and I worry that I’m more likely to damage something that works than find something that doesn’t digging through the panel.
 
Last edited:

Gordo

Well-known member
If it was mine I’d run a hot wire to the fuel pump and try that to be sure that is the issue. My D1 is a 96 with early evap but there are only three wires going to the pump. White with purple stripe is power. If so I’d ask Will Tillery to send me the fuel pump relay/multi relay next to the ecu. I assume u tried to bypass the inertia switch already.
 

discostew

Well-known member
Sep 14, 2010
5,868
323
Northern Illinois
If it was mine I’d run a hot wire to the fuel pump and try that to be sure that is the issue. My D1 is a 96 with early evap but there are only three wires going to the pump. White with purple stripe is power. If so I’d ask Will Tillery to send me the fuel pump relay/multi relay next to the ecu. I assume u tried to bypass the inertia switch already.
Why not just use a voltmeter and check power and ground right at the pump? If it's plugged in and you have battery voltage across those 2 wires it's a pump.
 
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Spark6

Active member
Oct 31, 2020
27
3
Pittsburgh, PA
Update: went to work with the multimeter. Mostly reinforces things I already kinda know - there’s no power getting to the pump... However, the grounds are good. On a wing and a prayer, I ran a hot wire to the pump from the battery and grounded it to the proper pin in plug C434. Little bugger whirred away happily. Fingers crossed, had the wife crank the motor over. The engine caught without hesitation and ran smoothly. What a wonderful sound it is! Okay. Pump works. Fuel pump relay clicks when power applied, and the inertia switch is brand new... So I’m left feeling that there MUST be a damaged wire or connector somewhere along the run to the pump. So the hunt continues. Next step, uncover plug C277. Anyone have any experience pulling the under-dash fuse box?
 

Spark6

Active member
Oct 31, 2020
27
3
Pittsburgh, PA
So, the fight continues. Back from work, back to the Disco. Raining today, which slowed things up. Did some comparison troubleshooting... both Fuel Pump Relays “click,” (it’s faint, but they’re not shunting a ton of power?) when the key is turned to position II. Starting to wonder if I needed to buy the new relay in the first place. Also verified I have voltage TO the relay - exactly equal to voltage at the battery. Great! And I have continuity from the relay to the plug on the firewall. However, I don’t have voltage after the relay. It’s a bit barbaric, but I carved some of the wire insulation back from the down-stream side of the inertia switch (which I’ve verified is closed!) and had the wife switch on the ignition. Relay clicked, and the multimeter showed “0.01V” for about 5sec, then went back to 0. Tried with both relays, same result. Soooooo. Either I have two bad relays, which are clicking but not actuating, or I have a major voltage leak between where it exits from the relay, and where it exits the inertia switch. I don’t really want to go piercing the insulation in a million places, but my plan for tomorrow is to test for volts immediately at the relay outlet, then start moving down... I’ll be the first one to admit, I’m an ape with a multimeter. I’m using the negative terminal on the battery as my ground to test for voltage - as I figure that rules out a bad ground as I test.
 

Spark6

Active member
Oct 31, 2020
27
3
Pittsburgh, PA
Okay. Continuing my diagnostics from last night, wifey and I went out to the Disco this morning for some more multimeter work. Both relays are putting out "0.01V" immediately after the relay. I'm starting to think I got a bad relay... But thought I'd check first for a short that might have killed the new one... Resistance between the relay plug (C1029) and plug C1028 is between 0.0 and 0.1ohm... doesn't seem like there is a short there. How about the whole run from C1029 all the way back to the wheel well at C407? 0.1-0.2ohm. Considering that is going through the inertia switch, and a couple plugs, it doesn't seem like there is a short along that run either. I've also read that a short in the fuel pump ground can cause a Fuel Pump Relay to die, so while I've got this thing out, lets check that. (Nevermind that I've already functionally checked it by using it to successfully power the pump off of direct battery power.) Resistance to from plug C434 (at the pump) to ground E400 (in the driver's side D-pillar) is between 0.0-0.1ohm. All of this is making me question my diagnosis of a wiring fault. I have continuity and low resistance along the entire circuit. I have voltage at the supply side of the relay. I have 0.01V at the exit of both relays upon activation, which I CAN hear clicking when ignition position II is selected. Did I just get a bad replacement relay? Am I missing something?
 

Spark6

Active member
Oct 31, 2020
27
3
Pittsburgh, PA
Further depth: according to the RAVE ETM, the fuel pump relay is actuated by power from fuse F3 in satellite fuse box 1, left of the steering wheel. Furthermore, in the AEL Discos, the fuel pump relay is housed within in a multifunction relay which has, no surprise, some additional functions. So sometime this afternoon, while mashing potatoes, it occurred to me that perhaps the “click” I’m hearing at power application was not in fact the FPR energizing. To test this, I dragged wifey back downstairs for a voltage test on post 2 of plug C1030, which according to RAVE is the power-side of the FPR actuator circuit. Aaaaand I find I have good voltage. So, now I have good voltage at the actuator, good voltage at the supply, and no voltage at the exit of the FPR. To my limited understanding, this feels like a pretty clear-cut case for a bad relay. Except for the glaring fact that I just put in a brand new FPR. Leaving me either to continue to believe that I had a bad replacement part, OR, that somehow something in the wiring harness shorted it out. But I’m at a loss as to how to figure out the culprit. At this point, I have to imagine that all y’all are sick of hearing me go on about this, and I can promise you that my wife is! Ha. But perhaps, if nothing else, this will ring a bell for someone down the road. Or just help me organize my thoughts for the next move. Thanks anyway!
 

Spark6

Active member
Oct 31, 2020
27
3
Pittsburgh, PA
You could have a bad relay base??
Hmmm. By that, do you mean is the plug going into the relay (C1029) bad? That’s an interesting thought. I’ve checked continuity through that run several times now, but I’ll try to test the plug specifically tomorrow. I don’t really want to buy and install another new relay at $148 until I’m sure that’s the problem. I have this harebrained idea to try wiring a fuel pump relay in next to the multi-function relay, and simply hijack the wires that would otherwise address the FPR function of the MFR, and see what happens. All y’all with pre-AEL trucks and your $17 relays have it made!