Another EFI Fuse Blowing Thread (SOLVED!!) (3 Viewers)

[jchoop]

'97 80 Series LC with 300k. No recent work other than oil top off's

I will try and keep this short. I know there are several other threads pertaining to causes and troubleshooting of the EFI fuse blowing under various conditions. But none matched my exact scenario, so I am hoping my specific conditions might narrow down the list of areas to look in before I start digging around everywhere.

So just to summarize (and I will do my best as my father-in-law has been using truck the last 3 months):
-2 months back, truck started, was moved out of driveway then was turned off. Attempted to turn back on to move back into driveway and would not start. Waited overnight and started fine and had been working normally since.
-Yesterday, was driven to parking lot. when attempting to head home vehicle would not start. He had it towed back to my house. Took a look today and noticed with ignition ON, the CEL was not illuminated. Took a peak in the fuse box and saw EFI 15A fuse was blow.
-Replaced fuse and tried to start. It fired up and ran for maybe 1 second. then died. Fuse blown again.
-Replaced fuse again and put ignition to ON. CEL light illuminated and left it like that for 5 minutes and CEL remained on and Fuse did not blow.
-Attempted to start/crank and as soon as I do CEL goes off and am left with cranking, cranking, cranking but no start. Fuse blew again.
-Confirmed once more with another fuse and exact same as above happened.


The difference here from other threads I have seen is that it consistently only blows the EFI fuse when I attempt to actually start/crank. Does this help narrow down the list of items and wires to inspect for shorting or fraying?

-I am guessing since I can turn ignition on without blowing fuse that that may rule out the fuel pump?
-I did not notice any unusual wire wear or damage to the O2 sensors as I saw that on other threads. I was only able to disconnect the upstream sensor and attempt starting again. Same condition as above. I couldn't attempt the downstream as I have a Slee belly plate in the way of getting a good grip on the stubborn connection. ran out of time during lunch but will try that once I push into the garage.
-I did jiggle some wires in the area of the fuse box with no change. I purchased a new OEM EFI Relay and Fusible link when reading threads before I got a chance to look at it. I have not put either in yet.

Sorry.....idk what happened....I tried but was unable to keep that short and concise

 

[jonheld]

The ECU runs the fuel pump and it does not run with the key in the ON position until you crank the starter.
I would look at fuel pump wiring, especially the fuel pump connector that lives under the truck close to the c-pillar, connector BO1.
You can remove the circuit opening relay which will isolate the fuel pump. Crank the starter and see if the fuse pops again, or probe with a multimeter looking for a short to ground.

 

[jchoop]

The ECU runs the fuel pump and it does not run with the key in the ON position until you crank the starter.
I would look at fuel pump wiring, especially the fuel pump connector that lives under the truck close to the c-pillar, connector BO1.
You can remove the circuit opening relay which will isolate the fuel pump. Crank the starter and see if the fuse pops again, or probe with a multimeter looking for a short to ground.

Thank you for the feedback Jon! I was unware of the functionality. I'll keep the pump wiring in my investigation. Getting it in the garage tonight and will start troubleshooting through. Hopefully I can provide an update tomorrow with what is or isn't the issue.

 

[jchoop]

Apologies for not providing an update on the thread. Been chipping away at tracing electrical lines 30mins at a time during the kid’s naps.

Thank you @jonheld for keeping me on track with the fuel pump wiring. A quick disconnect of the fuel pump relay and crank attempt did in fact yield cranking with no fuse blow. I also inspected that connector I think you were talking about. it was a little dirty, but nothing abnormal.

-I checked continuity and functionality of the relay per Toyota Service Manual. All good there seemingly.

-I checked the Fuel Pump resistor, the resistance there checked out as well.

-However, checking the Relay to pump connector is where I start to get confused and thinking in circles.

-I checked continuity between ground and the power terminal (Red/Green Wire) to the fuel pump on the Fuel Pump Relay connector end of the circuit. Confirmed continuity (Short?)

-There is continuity as well between the Fuel Pump Resistor terminal and ground on the Fuel Pump Relay end of the circuit.

-Those 2 above made me think “OK, there is a short between the Relay and Pump”.

-To confirm, I reconnected the Relay and made my way to the pump and disconnected the connector at the pump. I cranked and the fuse did NOT blow (CEL stayed on during cranking).

-My thought was if the fuse still blows, there is a short in the circuit. If it does not blow, then the problem exists in the pump. (is this correct logic?)

-Having the fuse survive the last test, I performed the Fuel Pump tests in the Service Manual.

-I checked resistance between terminals 5 and 6 of the fuel pump connector. Service manual says 0.2 – 3.0 ohms I believe. I measured right around 1.1 ohms.

-It then says to check functionality by powering 12V across same terminals. Did that and pump turns on.

-So, then I start to get confused and throw all my lack of electrical knowledge at the issue. Because as above there is continuity between those 2 terminals. Those 2 terminals connect to the corresponding terminal in the upstream connector. In that connector, #6 goes to ground and #5 is the power that goes back to the relay. So obviously, when I checked for a short at the relay a couple steps above, that is why there was continuity. When both ends of that circuit are disconnected from pump and relay, I get no continuity between power and ground. I am guessing that is what I was supposed to do from the beginning(?).

-I thought maybe a short existed somewhere within the fuel gauge sender wiring/circuit. So, I jumped the terminal 5 (power) connector on the pump to the corresponding terminal on the connector for the circuit as well as the same for terminal 6 (ground?). So, I am only trying to power only the pump upon cranking. That blew the fuse. For whatever logic, I also disconnected the terminal 5 in that setup and left terminal 6 connected. Tried again and no fuse blow. Reconnected terminal 5 and disconnected terminal 6, that DID blow the fuse.

-That has me leaned back toward the pump….but the pump seems to work? So, I go further down my rabbit hole of crack brain ideas.

-So then to take pump further out of the equation, I used my same jumper setup, but instead of powering the pump, I just connected them to another 12v device. I assumed I would either still blow the fuse (short circuit) or power the device while cranking (pump issue). But neither happened in that experiment. It did not blow the fuse but also did not power the device.

-I then instead inserted the multimeter probes into the connector terminals 5 &6 (again, not the pump side connector, but the upstream circuit side connector). I assumed I should have 0v when ignition is in off and on positions. Which was mostly true, except I was getting micro-volt measurements. Then When I cranked it dropped to a solid 0V. Switching to continuity/resistance check , I get zero continuity when ignition in off and on position. But Continuity occurs between those 2 when I start cranking.

-That seems wrong which is why I reach out to all you nice folks. Should I not be getting 12V across my multimeter setup when I start cranking? Makes me think something is wrong with one of the Relays upstream. But the fact that I only blow the fuse when I connect in the actual pump leaves me confused. I am trying to do my due diligence to troubleshoot before just buying new parts and put them in hoping it fixes the issue, but I am at a loss for what is happening. Any help to further steer me in the right direction would be greatly appreciated. Thank you!

 

[baldilocks]

Seems logical to pull the pump/sender unit out for visual inspection of it and the interior of the tank. There have been reports here of bad wiring on that unit.

 

[jchoop]

Seems logical to pull the pump/sender unit out for visual inspection of it and the interior of the tank. There have been reports here of bad wiring on that unit.

Thanks for the suggestion. At this point I had considered just opening it up and putting in an Autozone fuel pump just to easily determine if that is the problem. Would be a good time to visually inspect the wiring. I would love to take a multimeter to those terminals on the connector, but there is no troubleshooting guide that details resistance/open/closed circuit values in the service manual for the sender.

Anyone here know what those should be. Those three wires/terminals all have continuity with the tank ground which I think makes sense and doesn't make sense.

 

[ajax1]

Yes pull the pump and look at the wiring. I recall there is a wire that gets chaffed through and short out. You can get a denso pump from rock auto, was plug and play into my 94. I have part number at home I think still if you need

 

[jchoop]

Yes pull the pump and look at the wiring. I recall there is a wire that gets chaffed through and short out. You can get a denso pump from rock auto, was plug and play into my 94. I have part number at home I think still if you need

Thank you for seconding pulling out the unit for visual inspection. I'll do that before I buy the pump. If it comes up a bust, I'll take that part number. Thanks!

Any chance anyone can still explain why I am not seeing the 12v come across my multimeter when measuring across that connector to the pump?

 

[jonheld]

Any chance anyone can still explain why I am not seeing the 12v come across my multimeter when measuring across that connector to the pump?

If you're measuring at the pump directly, then you have no ground path. Connector BO1 pin 5 (under the body by the C pillar) supplies the ground.

 

[jchoop]

UPDATE: So took advantage of the nice days here in Denver to open up the garage door and pull the pump/sender unit out of the tank.
Thank you @ajax1 , @baldilocks , and @jonheld for getting me to the fuel circuit and checking out that in-tank pump wiring. Sure enough there was a large section wire exposed. See images below.

I am certainly concerned as to HOW this happened. The sections exposed was resting against what I think is vent tube or something, but was facing the opposite way. And it clearly looks melted. I am wondering if it was actually resting on a part of the tank inside or was at some point rubbing against the vertical metal member mounting the pump. Either way, I patched it up with an insulated butt splice as well as re-route the wire to avoid any possible contact. Sorry I did not take a picture of the final configuration...was working quick during my lunch break.

Put it back together and she fired up and is running good again. Thanks all for the help!

A final note and question...It seems like it was probably a really bad idea with this bare wire present to test the pump by directly connecting 12 volts to the connector. I was just following the FSM troubleshooting order. But by doing that prior to pulling and checking for shorts/exposed wire, was there an increase risk of exploding myself? Or with it sealed off in the tank, no oxygen present, and maybe that the low voltage has almost no chance of arcing make this a non-issue?

 

[baldilocks]

Glad you got this resolved.

 

[smritte]

am certainly concerned as to HOW this happened. The sections exposed was resting against what I think is vent tube or something, but was facing the opposite way. And it clearly looks melted.

You should have seen what is called "witness mark's". either shiny or most likely little blast marks. When I see a wire like that, I know its been arcing for quite a while. It just wasn't bad enough to blow the fuse. Just because of the issues I've had with (non Toyota) fuel pumps melting wiring in the tank, I would run a current draw test on the fuel pump.

Or with it sealed off in the tank, no oxygen present, and maybe that the low voltage has almost no chance of arcing make this a non-issue?

No oxygen was your friend. I've pulled pumps out where the wiring had no insulation left. Not even carbon. Bare wires that looked severely over heated. Basically glowing red hot inside the tank. Yours looked like it just abraded on something and was arcing. If the insulation itself looks a bit crispy, I would think about a pump.

 

[manofthewoods]

Best I can tell from the photos...
That sharp edge is a known potential issue as discussed in another mud thread re: fuel pump replacement.
When I did mine I took a section of hose and slit it lengthwise. Then put it over the sharp edge. Then I used safety wire to hold it in place.
I figured that if the wire insulation rubbed against a rounded rubber surface it should be OK.
Super glad you're up and running

 

[Two 3FEs]

Fusible link

where's that EFI Fusible link? got a pic? thnx