1989 Isuzu NPR Diesel 4BD1T (ACC Master Fuse Blowing)

Hello forum!  

One late raining night in January after a really difficult day at work I changed my battery and did the dumbest thing as a mechanic. I hooked up the battery cables backwards.  

Since then I have spent all the free time I can find, trying to resolve the issue. My wife and 2 year old are missing me and this is my handyman tool truck rig that I support myself with.  

The first thing I did was remove the fusible links and replace them with inline fuse holders as the OEM fusible links are obsolete. (Maybe…idk) I figured having replaceable fuses would be better. Original specs from factory service manual on the inline fuses are. Nominal size: 1.0, color: Black, Amps 26. I replaced 1st with amazon 12 gauge waterproof fuse holders and after learning about voltage drop and seeing high resistance in them I changed them again to auto store bought 12 gauge fuse holders (non- waterproof) with 25 amp standard fuses. Now I’m seeing close to 4v drop after the ACC fuse when it blows.  

Then I checked every relay on the bench and every fuse I could find.  I have 2 master fuses wired to the starter.

One is for the glow plug system and one is for the accessories.  

The master fuse that keeps blowing is the accessories circuit.  

When the master fuse blows the lights in the dash cluster flicker momentarily.  

When the master fuse blows the functions of the truck are still normal. Everything appears to be working. All lights, transmission, backup horn, air conditioning, turn signals, etc.  

When I shut down the truck. Then it is dead.  

Now If I change the master ACC fuse I can start it up when cold but when the engine warms up and I restart the truck and give it some gas the ACC master fuse blows.  

Then I changed the starter and the alternator as I was having an over voltage condition. That fixed the over voltage condition but the ACC master fuse is still blowing.  

When I saw the glow plug light on the dash cluster light up brightest I changed the glow plugs, unplugged the thermo switch, tested the glow plug controller per FSM instructions (tested good), changed the glow plug relay and eventually just ran a new 12gauge wire from the relay to the glow plug bus bar. I tested voltage drop with the old glow plug wire and saw a gradual increase in voltage drop up to close to 1.2v. With the new wire I decreased the voltage drop to under .400v.  

I found a temporary solution by disconnecting the glow plug wiring harness from the bus bar and the truck didn’t blow the ACC fuse constantly but I didn’t want to loose the glow plug system as I sometimes need to travel to the mountains for work. Also one day I went to start the truck up to leave a job and the ACC was blown.  

After changing to a new glow plug wire I then instantly began to blow the ACC fuse.  

Thinking in the direction of “what doesn’t work?” I remembered the radio blew (heard a pop) when trying to drive home from being stuck the first time because of the over-voltage condition so I pulled the radio (found a blown 10amp fuse on the back). I tested the wires into the radio and I have good ground and power with tone per my power probe IV.  

The truck started right up again and didn’t blow the ACC fuse so I left it on to warm up and tested voltage drop after the ACC master and saw a voltage drop of .020v… so good…. In the warmed up state. I turned the truck off and on and when I give it a little gas I blow my ACC fuse again. Last I checked when the ACC fuse blows I see close to 4v drop after the ACC master fuse.  

So my problem should lie after the ACC master fuse… but before the main fuse holder block because every fuse in the main fuse block is good. The only abnormalities I see are listed below all others fuses have strong battery power with Power Probe tone. Also I noticed in these circuits that the other circuits had stronger voltage because the alternator was charging and these circuits had lower voltage.  

Fuse block
Engine running  
Battery voltage 14 ish

Fuse 2 (starter)
Top .03
Bottom .03 

Fuse 7(air conditioner)
Ac Turned off
Top .02
Bottom .02
Ac turned on full
Top 12.66 (no tone)
Bottom 12.72 (no tone)
Ac turned on full compressor clutch on
Top 11.45 (no tone)
Bottom 11.45 (no tone)  

Fuse 14 (Headlight RH)
Headlights off
Top .02 (grounded tone)
Bottom .02 (grounded tone)
Headlights on brights and regular 
Top 12.55 (no tone)
Bottom 12.68 (no tone) 

Fuse 15 (Headlight LH)
Headlights off
Top .02 (grounded tone)
Bottom .03 (grounded tone)
Headlights on brights and regular 
Top 12.45 (no tone)
Bottom 12.56 (no tone) 

I’m now thinking maybe I should start disconnecting the engine warming system as I have never had an engine exhaust brake nor have I tested the engine warming function but really I don’t know.  

I want to learn how to fix this but just cannot afford to throw parts and labor at it anymore so now I’m stuck using another vehicle that I have to load and unload my tools from every day.  

I also have the factory service manual and the electrical diagram that I will attempt to attach in this post.  

I searched under the dash for the diodes for about an hour as well but can’t find them yet and will need to begin to remove the dash further to test.  

This is my 3rd rebuilt vehicle and I’m pretty handy but I’m at the point of reaching out to the world with this one.  

Everything I do seems to result in the truck working for a while… then blowing the master ACC fuse.  

Anyone I talk to in my community has no idea what I’m talking about. It’s like I’m speaking another language. 

I just don’t know where to start at this point. I’m open to starting again. I’m open to learning how to restore this beast that I love. I’m open to keeping it simple. It’s just that this problem is invisible and I want to learn how to see.  

Oh this is pre OBD and per the FSM there are flash bleeps in the dash that I can instigate for the Transmission but haven’t got there yet.  

Please help. & Thank you for listening!