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I have a 1996 Toyota T-100 3.4L V-6 SR5 4WD. It keeps blowing the EFI fuse as I drive. It will sit and run in the driveway just fine, but if I drive for a bit, it will blow. Sometimes if I replace the fuse it will drive for a bit then blow again. Sometimes it will blow right away, but the next day (after I tow it home!) it will start and run fine. Until I drive it. I have replaced the fuel pump a year ago, just had the alternator checked, have wiggled, jiggled, squeezed + pulled on every wire in the harnesses, (yes, under the dash + in the kick panels too) clear back to the fuel tank while the truck was running, and it won't blow the fuse. Get in and drive, and ka-blooey. I just replaced the ignition switch today, and it idled for an hour, then went a half-mile, and blew the fuse. I put a new fuse in, drove a mile and it blew again. Put another in, and it blew immediately, as did the next two. I had it towed home for the FOURTH time. Anyone had this kind of issue with a T-100?

Submitted: 312 days and 6 hours ago.
Category: Toyota
Value: $15
Status: AWAITING CUSTOMER ACTION
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Posted by Rick Crammond 312 days and 6 hours ago.

Answer

Hi, ksankee, and welcome to JustAnswer.

Carefully check for O2 sensor wiring harness melting to exhaust manifold, the most common cause of EFI fuse blowing. The EFI main relay could be bad is another possibility.

There are other components in the electrical chain from the EFI fuse that could be causing problems, but I'd say start with these two most common faults.

Let me know if that advice works, and if not we'll take it up from there.


312 days and 6 hours ago.

Reply

Rick,

Thanks for your reply!

 

I am on duty tomorrow (firefighter) so I won't be able to check your suggestions until Friday. If I need additional info, will that be another "question" + a charge?

 

I will check the O2 sensor circuit and replace the EFI breaker.

PS. This problem has thrown no codes, an O2 sensor "short" wouldn't throw one?

 

Thanks! Jim

Posted by Rick Crammond 312 days and 5 hours ago.

Answer

Hi Jim,
We can keep the question open. Don't push "accept" until we are finished and you are satisfied the question has been completely answered.

The O2 sensor short won't necessarily set off the computer fault. When a hot positive wire such as what goes to the O2 sensor shorts out and blows the EFI fuse, the computer can't tell which of several components and wires could be at fault. Other wires that go to the O2 sensor determine the voltage/resistance seen by the computer, and they don't melt and short as easily.

Cheers,
Rick

Posted by Rick Crammond 312 days and 4 hours ago.

Answer

Hi Jim,

Sorry, I meant EFI Relay to be replaced, not EFI breaker.

312 days and 3 hours ago.

Reply

Rick,

 

Ok, thanks, I will check this stuff and get back to you. Yeah, and I answered with "breaker" too! lol

 

Happy New Year....!

 

Jim

310 days and 9 hours ago.

Reply

Rick,

 

I disconnected the O2 sensor and still blew the fuse.

 

I went and replaced the EFI Relay, blew the fuse.

 

The last time it blew it would at least start the next day. Now it blows the fuse immmediatly.

 

I am on duty 24 hrs again tomorrow, will get to it this Sunday...

 

Thanks!

Posted by Rick Crammond 310 days and 8 hours ago.

Info Request

There must be a bare wire or shorting component in the EFi circuit.

Did you check the wiring all the way to the O2 sensor for rubbing or melting ? Disconnecting the O2 sensor won't stop a possible grounded short circuit in the hot wire to the O2 sensor.

I will be away from the computer for a few hours. I will prepare a diagnostic step-by-step for you on this later. Then we can check the circuits more thoroughly.

310 days and 2 hours ago.

Reply

Rick,

 

Sorry, just got back in...

 

I could only do a "visual" on the O2 wire as it goes up into a harness above the transmission. Looked ok on the outside anyway...I found a manual with an electrical schematic of the engine systems, I am gonna take tomorrow (since I am on duty and can't work on the truck) and look to see if I can trace each one individually. I don't see any other way.

 

BTW, it blows fuses right away now instead of willy-nilly, so at least I can look for a dead short...beats the intermittant thing.

 

I will check my email from the station tomorrow, thanks in advance for your help..

 

Cya! Jim

Accepted Answer

Ok, here is a wiring diagram we can go off of as well
http://tinyurl.com/8matpz

(scroll down to "Fig. 6: Engine wiring-T100 1995-96 3.0L ") The 3.0 L engine electrical for EFI are similar to 3.4L


Yes, You are correct that it's a good thing that at least the EFI fuse is blowing all the time!
You can see by the circuit diagram that several components are connected to the EFI fuse.
The black/yellow wire comes out of the EFI Fuse box splits and goes to the EFI Main Relay and to the ECM.
So, with the EFI Main Relay pulled out, does the EFI fuse blow? If it doesn't blow, the ECM circuit should be ok. If it does blow with relay out, there could be a short between EFI fuse and the Relay. That should isolate one of those two main circuits as the culprit.

Let's say the above test doesn't blow a fuse, but replacing the EFI Relay does. Try removing the Circuit Opening Relay and that will remove the fuel pump from the circuit. If the fuse doesn't blow, there is likely a problem with the fuel pump, the blue power wire to it, or the COR relay itself. Then you could disconnect the fuel pump...which is one of the most likely candidates for excess amperage draw. An amps draw test at the pump could work to test it...but you may need a separate hot wire from the battery to fuel pump to power-up the test!

Besides the Circuit Opening Relay, the EFI fuse circuit feeds other components after the EFI Relay. A white/red wire exits the EFI Relay and goes to the Data Link Connector, Mass Air Flow Sensor, Evap VSV, A/C Idle up, FPU VSV, EGR VSV, and Idle Air Control Valve, and the two oxygen sensors. These are all connected by the white/red wire from the EFI Relay, and a direct short to ground anywhere in this will blow the fuse.

If you have to go this far, disconnecting the above components and checking wiring carefully would be next.

Good luck and let me know how it goes!

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Expert: Rick Crammond
Pos. Feedback: 100.0 %
Accepts: 53
Answered: 1/3/2009

Toyota Mechanic

Self-taught mechanic with over 20 years of experience with various Toyota vehicles.

309 days and 22 hours ago.

Reply

Rick,

 

Great stuff! I will get on this starting Sunday. Meanwhile, Thanks for all your work, you've been very helpful....

 

I am confident this will get me back on the road, and I will get a note to you when I am finished letting you know the culprit.

 

Thanks again!

 

Cya, Jim

Posted by Rick Crammond 309 days and 8 hours ago.

Answer

Thanks for using JustAnswer...and I wish you rapid troubleshooting!
Any question about this let me know.

Cheers,
Rick

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