Another EFI Fuse Blowing Adventure - '94 4Runner (1 Viewer)

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Jul 25, 2016
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SLC, UT
I never thought I'd be willing to take my truck to a mechanic, but I'm about at that point now. Hopefully someone has a crucial piece of insight that I'm missing.

We have a 94 4Runner. 3vze, automatic. Under 200000 miles and in otherwise great condition and until 3 weeks ago reliable and dependable

An hour after leaving for vacation the truck died on the interstate. Checked the fuses, saw that the EFI fuse was blown and popped in the Dome fuse (also 15A). It started and ran for no more than 5 additional minutes. After that any 15A fuse I put in would also blow upon starting (but not when turned to ignition). I did not put in anything higher than a 15A fuse.

Since bringing it home I have gone through about 20 fuses by testing and have spent hours with the wiring diagrams from the FSM.

It does not appear to be due to the most common causes- the O2 sensor wires are not melting on the exhaust, the fuse still blows when the fuel pump is unplugged, and there is no continuity in the fuel pump power supply when the ignition is off. It's not a California truck so there is only one O2 sensor.

Here's a list of what I have checked:
This consistently happens ONLY when the key is turned to START.
I checked for the fuse blowing with different fuses pulled or other components disconnected:

With these actions the EFI fuse WOULD blow when the key was turned to START:
  • Igniter unplugged
  • Ignition coil unplugged
  • distributor unplugged
  • All three above unplugged
  • Alternator unplugged (white and red wires)
  • Alternator unplugged (large white with screw terminal) (NOTE- I'm at work typing this and looking at my notes. I'm not positive of these colors but they are just the alternator ones)
  • All alternator connections unplugged
  • Engine fuse (10A) pulled
  • Fuel pump disconnected
  • Aftermarket alarm disconnected (Rattler brand- which I want to remove but properly)
With these actions the EFI fuse WOULD NOT blow when the key was turned to START:
  • EFI relay pulled
  • AM2 fuse (30A) pulled
  • Ignition fuse (7.5A) pulled
  • AM1 fuse (40A) pulled

Since the O2 sensor is such a common thing I checked the continuity on the four terminals in the plug for the wires that are routed over the trans and into the engine bay on the passenger side. There was continuity with all possible connections, with some showing much less resistance than others. I have a multimeter and I plan on measuring the resistance but I did not have it with me when I did the work.


At this point I am down to just visual examination and making no progress. Much of the dashboard is apart, the steering wheel is resting on the driver's seat and right now I think I have more time in this job than when I changed the transmission out on this truck. So I've pretty much hit a wall.

Anyone who has had experience with this, is there anything you know of that I should check? I'm trying to keep the process as well-defined and as procedural as possible- but since I'm doing this after work and when I can over the course of weeks its likely that I skipped a couple of things.

Thanks in advance for any help!
 

KLF

Frame waxer
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I would start pulling harnesses apart and trying to see if it still blows a fuse. Or, get a multimeter than can read at least 20A in current flow, use that to measure the actual drain across the fuse so you're not burning through fuses.

Disconnect the engine harness from the EFI computer.

Pull ALL the harnesses off the EFI computer.

Keep pulling harnesses apart until it stops. Then you know where the short is.
 
Joined
Jul 25, 2016
Messages
4
Location
SLC, UT
Thanks, KLF. That's pretty much the process I've been using but not with pulling plugs/harnesses from the ECU. I'll give that a shot tonight.

Most of the wiring is still in what looks to be the factory shrouding, and I am hesitant to start pulling those apart just yet. Any experience with a short in a factory-bundled harness? It seems to me that they are tight enough to keep from rubbing.
 

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