Hot/Melted EFI Fuse (1 Viewer)

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Oct 17, 2009
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Location
South Central NH
I have a 93 fzj80. I was idling last Sunday in my yard while I was loading up for a day out wheeling. When I came back to the rig after 5-10 minutes or so it was no longer running. It would cranked over but wouldn't start. So staring with the easy stuff I stared going through fuses with my fluke. The EFI fuse measured open. Upon removal the fuse went flying out of my hands. When I picked it up is was noticeably deformed from heat. Long story short I went on mud later that night and came across @scrowley post:

First time my 80 has let me down/towed

following this I was sure I would have had something similar. After I replaced the fuse the rig started just fine. It ran rough at idle for a minute or two but went back to normal fairly quickly. I felt the EFI fuse after several minutes of running and it was hot, also the EFI relay was almost too hot to touch (the round metal one). After reading @scrowley & @landtank 's thread(s)I removed the fuel pump relay, burnished the contacts and tried running it again. No change in running and the fuse and relay are still getting very hot. I purchased one of those harbor freight fuse testers to monitor the current that the EFI fuse was seeing:

30 Amp Automotive Fuse Circuit Tester

I observed when I would turn the Key to the on position but not crank it there was .3 amps on the testers display. When I crank the engine and it starts the current rises quickly to 8 amps then climbs to about 14.9 amps in seconds. Then as the rig idles down to approx 650 RPM's the current on the EFI fuse slowly drops from the 14.9 to about 12 amps after several minutes.

I am going to make a list of all the circuits that the EFI fuse supplies and try to see if I have a poor connection somewhere along the line or if I can disconnect some of these downstream sensors, relays etc. to see if the high current draw drops down. The problem is likely a connection somewhere that has degraded. Unfortunately, I think disconnecting some of these items will cause the vehicle to shut off and prevent me from identifying if they are the source of the high current draw.

FWIW I measured my wife's 80 EFI fuse current at startup like I did on mine mentioned above and observed it to climb to 8 amps and stay there indefinitely +/- a couple tenths of an amp.

Good thing I bought that EWD. It has already saved me a few times lets hope it helps again.
 
you might try removing the circuit opening relay and installing a jumper to manually trigger the fuel pump while ignition is on but truck not running. If the current stays high the fuel pump is likely your problem
 
you might try removing the circuit opening relay and installing a jumper to manually trigger the fuel pump while ignition is on but truck not running. If the current stays high the fuel pump is likely your problem

Good call will try to do tonight if I get home at a reasonable hour. Thank You
 
@bloc I tried your recommendation and if I understood your suggestion correctly I pulled the open circuit relay in the kick panel jumpered pins 2&4 of the relay to make the "switch" and turned the key to the on position. No change I current draw.

I then began to go through some of the connections. I wiggled all applicable relays/connectors. I even tried feeling wires for a temperature rise hoping to trace the culprit. I think this one is going to be a tricky one

I am going.to replace the fusible link this weekend only to eliminate that but I don't believe it will change anything.
 
@bloc I tried your recommendation and if I understood your suggestion correctly I pulled the open circuit relay in the kick panel jumpered pins 2&4 of the relay to make the "switch" and turned the key to the on position. No change I current draw.

I then began to go through some of the connections. I wiggled all applicable relays/connectors. I even tried feeling wires for a temperature rise hoping to trace the culprit. I think this one is going to be a tricky one

I am going.to replace the fusible link this weekend only to eliminate that but I don't believe it will change anything.

Check the wire going from the EFI relay in your under hood fuse box. Take the EFI relay out, and look at the fuse box itself. Many times it will look burnt, or black. The wire running from there to the passenger side, firewall mounted, diagnostic terminal is notoriously undersized and many have to end up replacing it. Good luck
 
@bloc I tried your recommendation and if I understood your suggestion correctly I pulled the open circuit relay in the kick panel jumpered pins 2&4 of the relay to make the "switch" and turned the key to the on position. No change I current draw.

I then began to go through some of the connections. I wiggled all applicable relays/connectors. I even tried feeling wires for a temperature rise hoping to trace the culprit. I think this one is going to be a tricky one

I am going.to replace the fusible link this weekend only to eliminate that but I don't believe it will change anything.

Per the EWD I have it would actually be pins 1&2. You'd be trying to send current from the Y/R wire that is the main ignition switched +12v directly to the R/G output that leads to the fuel pump relay and eventually to the fuel pump.

Note: I hope you didn't attempt to crank or run the engine with that jumper in place. If so I could see the lack of a resistor or coil like the COR normally has potentially damaging the VAF sensor.

The goal is to get your fuel pump to turn on with the key in the RUN position for testing. Normally on a OBDI cruiser the fuel pump only runs in START and/or if there is air flowing through the VAF (airflow meter).

I know many people have issues with parts of the main Y/R circuit that powers so much on these rigs via the EFI-MAIN relay/fuse.. but I'm having trouble seeing it draw that kind of amperage from a simple resistance issue. Plus, if it were anything else on that circuit it shouldn't matter much whether the engine is running. The ignition coil and injectors are driven through a different circuit than the EFI-M. If it were a short to ground.. the fuse should pop. A failing fuel pump can pull too much amperage, and what I recommended above was just a simple test to isolate and trick the pump into running to test the current draw vs with it disabled.

If there was no increase in current.. it's something else.
 
@bloc my mistake. I am not the greatest electrical trouble shooter and clearly I have a lot to learn. I did not crank nor start the engine with this jumper in. I will jumper pins 1 & 2 tonight and report back. Thank you for the guidance and support.

@White Stripe I will do just that as well and report back thank you as well.
 
@bloc not sure what the typical draw on the circuit from the fuel pump is but when I jumpered 1 & 2 pins on the ocr and turned the key to the on position the current went to 7 amps. Not sure if that coupled with the draw from the remaining circuits is get me to these higher currents

@White Stripe pulling iac connector did not affect the current draw. Thanks you at least that's on more thing eliminated.
 
I recently just bought a land cruiser and unfortunately it started having the same issue with the efi fuse popping. I used a tool which finds shorts in wires by sending a radio frequency down the line. I was able to locate the short in the wiring harness right next to the egr. Worth taking a look at if you still haven't found the issue
 
@HPFstrawn what tool are you describing? Thanks
It is the Power Probe ECT2000. I borrowed it from one of my friends who was a mechanic. One end sends a frequency down the suspect circuit then you take the remote and move along that circuit and it points you to the direction of the short.
 
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@HPFstrawn I am not sure it is a short. I would think a short would cause a blowing fuse condition. I think it must be a poor connection creating increased resistance?My reasoning is I=V/R. Current is I, voltage is v, resistance is r. The current is 12-15 amps vs typical 8 amps on the circuit. Voltage is 12-14v so mathematically resistance is higher than normal somewhere?
 
@HPFstrawn I am not sure it is a short. I would think a short would cause a blowing fuse condition. I think it must be a poor connection creating increased resistance?My reasoning is I=V/R. Current is I, voltage is v, resistance is r. The current is 12-15 amps vs typical 8 amps on the circuit. Voltage is 12-14v so mathematically resistance is higher than normal somewhere?
I think you got it flipped around. Your current increased meaning your resistance got smaller which is what a short does. If you plug in the data you have gathered into ohms law which you provided you will see that your wife's truck shows 1.5 ohms of resistance for the circuit where your truck shows .8 ohms roughly. I do find it interesting that your fuse does not instantly blow. If you measured 15 amps without the fuse blowing I would say you need to find some better fuses and for the time being put in a 10 amp to protect the circuit further till you can locate the issue.
 
@HPFstrawn you are correct! After I posted this I actually did the math properly and found that depending what current I use the resistance is now .8-1.0 ohm whereas my wife's rig is like you said 1.5 ohms. It is strange that the current draw spikes at 15 amps at initial start-up then settles to 12-12.5 after a little while. I think placing a 10 amp fuse will just blow the fuse but I can try that in the mean time. I will try to measure my wife's fuel pump current by jumping the open circuit relay like I did on mine. 7 amps seems okay but couple that with the additional load of all other associated circuits and it seems kinda high. I would think the fuse on the circuit is roughly double the static load rating for that circuit. I will report back. Thanks for the help.
 
Finally had a couple hours to mess with this issue. Slowly going through the circuits disconnecting where I could. I think I made have had some success. I disconnected one of the o2 sensors and wallah....the current draw at start up only goes up to 8.4 amps. The EFI relay isn't getting nearly as warm. I need to run it a bit before I can declare victory. 2 of the O2 sensor terminals measure 1.2 Ohms. The other O2 sensor on the same 2 terminals measure 13.1 ohms. I am guessing that is my short? Should I get o2 simulatorss? I don't need to worry about emissions here.
 
The o2 sensor reading 1.2 ohms is the issue. The o2 sensors should read 11-16 ohms. Are you sure you measured the resistance on the right terminals? The right ones should be the two terminals right next to the 3 tabs on the connector
 
@HPFstrawn I am pretty sure I measured the right terminals based on your description. Those are the only terminals that measured any on the o2 sensor. There were two black wires, a white, and a blue. No other configuration of measurements displayed any on my fluke 87v. Meaning the display read OL on all other terminals. I think it does make sense for the short like condition we were suspecting.
 
Might as well put my issue in here too. My EFI fuse also started blowing. I used the short circuit detector tool which located a compromised wire next to the EGR valve. Problem solved right? Wrong. EFI fuse popped again. But this time the short is random. I'll start trying to track it down and the short will disappear. That makes me think this is a wire that's moving around and sometimes shorting out. Which makes this much harder to track down. I think at this point i might as well just yank the whole efi harness out and inspect the whole thing. Or even just replace it.
 

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