My '91 s 10 ecm b fuse keeps periodically blowing.I checked the entire engine wiring harness and found nothing. what should I do next?

the first place I would start is the harness to the transmission and the TCM and look for crushed wires , water in connections or wire cut through where they go into the transmmission
check the TCM next for an internal short

Did you check the crank sensor wire, it may be hitting the hot exhaust. does the wire going to fuel pump have a problem and is shorting out. ECM 1 ..... then there are 5 components that this fuse feeds
1-crank sensor
2- all injectors
3- ECM
4- electronic ignition control module
5- ignition coil - check for a bad wire in these areas. Hope this helps.

Using a smaller fuse should make the problem happen more often.
A mechanic would unplug the pump harness at the tank and check the circuit for high resistance or a short to ground.
If nothing is found in the harness, I would suspect the pump in the tank.

If the fuse KEEPS blowing it is because someone keeps replacing the fuse without diagnosing and repairing the reason the fuse is blowing.

Please review this article: Where's The Fuse?

Anyway, you have a dead short to ground in your brake lamp circuit that needs to be fixed before your wiring harnesses light up and smoke your entire vehicle.

hi, i have seen this problem before, i am a trained ase mechanic who works in a shop weekly. in my case, the suburban was a 91, it started by blowing the fuse randomly, then as it got further along, the ecm b fuse would blow consistantly as you describe. I found the problem to be in the wiring harness between the fuse block and the electronic control module on the pink/black power wire. It had become hot next to the radio and the insulation had melted into a direct ground to the ecm. then as time passed, it melted six other wires till it hit battery power. this is when it shorted the fuse all the time with key in or out. You should start by checking behind the glove box at the wiring harness going into the ecm, if you can get a schematic for the wiring, it would be better advised. See if you can find the Pink and black stripe wires(there maybe several of them as they power everything from egr solenoid to oxygen sensor possibly) and look for melting of the insulation. If you do find this to be the case, technically you should replace the whole harness, but it is possible the cut and splice in new wires. If any other wires are affected, you should replace these also. Check the whole length of the affected wires, they may have shorted all the way up or down. If this sounds like a big deal, it is! so, good luck, hope this helps.

Hi, this could be caused by a short circuit in the ECM or it more commonly found a short circuit in the alternator. Disconnect the battery wire from the alternator, make sure that the wire does not touch any part of the body or engine. Replace the fuse and see if it still blows the fuse. If the problem is solved, replace the alternator, if nor you will need to test the wires for a short circuit.

Check your o2 sensor harness. The harness if not properly secured will rub on the axle to a point where it rubs through the insulation of the wire & shorts on the axle causing the ECU fuse (20A) to blow. Repair the harness & replace the fuse & bada bing! You should be in business! Hope that works. IF NOT follow this K.I.S.S.

Keep It Super Simple! It never fails!

When you go to wide open throttle, the only thing that happens is your a/c cut out is activated by the throttle position sensor. I owuld check there, but it is very rare that the TPS would cause a fuse to blow.
You may have an engine wire harness that has the wiing insulation decomposing causing wires to short against each other. Peel open the plastic convolute and look for cracks in the insulation. Here is info on a Ford program to repair if less than 100,000 miles on your car.

October, 1999
To:
All Ford and Lincoln-Mercury Dealers
Subject:
Owner Notification Program 99M03 - Certain 1995-1997 Contour and Mystique vehicles - Under Hood Wire Harness - Additional Coverage.
PROGRAM TERMS:
This program provides extended coverage on certain under hood wire harnesses through 10 years or 100,000 miles which ever occurs first, from the original warranty start date. AFFECTED VEHICLES
Certain 1995 - 1997 Contour and Mystique vehicles built from Job #1 1995 through Model Year End 1997. REASON FOR RECALL
Under ambient factors of high temperature and high humidity, certain 1995 - 1997 Contour and Mystique vehicles may be susceptible to the wiring insulation becoming brittle and cracking on certain engine compartment wiring harnesses. These harnesses deliver electrical power to the engine and its cooling and control systems, the climate control, and other power distribution systems. If wire harness insulation cracking is found during the additional warranty period, the under hood wire harnesses are to be replaced at no charge to the customer. SERVICE ACTION
Additional Warranty Coverage
Effective immediately, Ford is providing additional coverage for the replacement of the under hood wire harnesses on these vehicles for a period of 10 years or 100,000 miles, which ever occurs first, from the original warranty start date. This coverage exceeds the provisions of the original component warranty coverage on the affected vehicles and is automatically transferred to subsequent owners at no charge. This additional coverage does not include normal service diagnostic or repair charges unrelated to wire harness brittleness and cracking. After this additional coverage has expired, wire harness replacements will be at the expense of the vehicle owner.

This is almost always caused by a wire in the engine harness being rubbed through and shorting to the engine, check the harness and look in places where the harness moves allot. I did this kind of work for 30 years and this is the most common issue for the ECM fuse to blow.