A. A 20amp fuse is too little. The Alternator can put out a minimum of 40amps, more if you have upgraded it. Additionally if you add all the fuses together and add the headlights (they usually have a built in circuit breaker in the switch IIRC), that is the potential load your fuse has to handle. Either way you look at it, the voltage potential is too great for a 20amp fuse. Most modern cars use multiple 40 to 60 amp fuses or a single 100 to 150 amp fuse on the main lead.

B. How much have you done to the rest of the harness? Is it new, used and untouched, or used and repaired?

C. I often use a circuit breaker in place of the fuse during diagnostics while I am looking for the problem.


Since it seems you have a short try this.

1. Pull all the fuses from under dash and check to see if the 20amp fuse you are using blows.
1a.If it doesn't blow then replace the fuses 1 at a time until it does. Remove the last fuse before placing the next one in place to prevent erroneous/cumulative current problems. When it blows, that circuit is the problem.

2. If it blows without any fuses installed, disconnect the wires to the alternator and make sure they don't short. Does it still blow?

3. If it still blows check the bolt on connections to the ammeter, a common problem. Does it still blow?

4. If the ammeter is not the problem disconnect the ignition switch wires, another common problem. Does it still blow?

5. Disconnect each switch and connector under the dash one at a time. Pay close attention to the fan switch and wiring. Does it still blow?

6. If everything is disconnected and it's still blowing fuses then it's probably a short in the harness. If you repair the harness it is most likely the alternator/ammeter wiring circuit since it feeds everything in the harness.

I make the following modifications when I rewire a harness.
1. Solder all connections for the Ammeter circuit.
2. Place a fusible link at the bulkhead and the alternator. A runaway alternator will burn up a harness just as fast as a short.
3. Check the Ammeter connections extremely well. NO PLASTIC WASHERS. Ammeters build up heat and require HI-TEMP insulators.
4. Don't run the ammeter wires through the bulkhead connector. Either run continuous long loops of wire through a hole in the firewall to eliminate the connectors OR use bolt on, through the firewall, terminals for a better connection. I have drilled holes through the bulkhead connector and pushed a loop of wire through the hole to eliminate the spade terminals in the bulkhead connector. Not convenient to disconnect that way though.
5. Solder the large wires going to the ignition switch under the column to eliminate the bullet connectors that fail often.

Hope it helps.