1976 FJ40 - Engine Fuse Blowing (1 Viewer)

[TimOrmondBeach]

1976 FJ40

I have reviewed other "engine fuse" issue threads on this site, but they are slightly different than my issue. When I turn my key to accessory mode, the engine fuse blows immediately. I can still start the engine and it seems to run fine. I have limited experience with auto electronics so I am looking for some guidance.

I understand that the following are on the engine fuse circuit:

• Regulator
• VSV (my vehicle seems to be desmogged so I do not have one)
• Fuel cutoff solenoid (I have a Weber38 so I don't think I have one)
• Emission computer
• Cooling fan controller (no carb cooler fan installed - no relay or motor)
• Maintenance interval detector (my MID is connected to the wiring harness, but not being used as I have a bypass speedometer cable)

With the fuse removed, I put an ohmmeter on the engine fuse circuit to ground (it reads .03 ohms). I have unplugged the items on the circuit that I think I have with no changes (if I did it right). Based on other IH8MUD issues, I assume that it is a short somewhere but have no idea where to look.

What is this connection for? I do not see it anywhere in my wiring harness references (see image).

 

[45Dougal]

Nothing should happen when you turn the key to ACC - the Engine fuse is on the next position (IGN).

You've tried unplugging the regulator and no change if I understand?

How does the harness look where it passes through the bulkhead just below the regulator?
If the grommet is missing then the sharp edge can cut the insulation.

 

[TimOrmondBeach]

Nothing should happen when you turn the key to ACC - the Engine fuse is on the next position (IGN).

You've tried unplugging the regulator and no change if I understand?

How does the harness look where it passes through the bulkhead just below the regulator?
If the grommet is missing then the sharp edge can cut the insulation.

@45Dougal - Thank you for that correction. When you say unplug the regulator - are you saying disconnect it and then turn the key to IGN?

I reconnected the old regulator (working) to see if it still blew the fuse, and it did.

I have recently cleaned, inspected, and rewrapped as much as the wiring harness that I can.

 

[45Dougal]

Yes exactly.
Unplug the regulator and everything else that is connected to that fuse and turn on to IGN.
If it still blows then could be a wiring short - other places would be behind the fuse box, or among the bent metal tags that secure the harness under the dash, but it sounds/ looks like your harness is pretty tidy.
There is also the condenser connected to that line according to my diagram - another possible culprit.
Do you have an aftermarket warning light for charging, or other aftermarket stuff like stereo etc connected to that fuse??

 

[TimOrmondBeach]

Yes exactly.
Unplug the regulator and everything else that is connected to that fuse and turn on to IGN.
If it still blows then could be a wiring short - other places would be behind the fuse box, or among the bent metal tags that secure the harness under the dash, but it sounds/ looks like your harness is pretty tidy.
There is also the condenser connected to that line according to my diagram - another possible culprit.
Do you have an aftermarket warning light for charging, or other aftermarket stuff like stereo etc connected to that fuse??

I disconnected the regulator, computer, and maintenance interval detector and it still showed about 11.78 volts across the engine fuse terminals.

I connected the condenser near the igniter but that was not connected when I bought it so I don't think that is the issue.

I do not have anything else connected to that fuse that I know of.

What is the connector (between fuse box and computer) in the picture of my original post?

 

[45Dougal]

Sorry I don't know what that connector is for.

I think you have another condenser at the regulator - have you tried disconnecting that? (See pic).

Measuring voltage across the fuse is unreliable ... it doesn't tell you whether you have a dead short to ground, it only says that something is still connected (a normal load from connected equipment is way smaller than the resistance of your meter).

If you've ruled out possible causes by only testing voltage across the fuse then might be worth revisiting.

The 0.03 ohms you initially measured is very small and suggests a very good connection. I assume this was between the fuse output and ground?