For several years I've been living with the dashboard lights staying on when I started by '92 Isuzu pickup (2.3L). Not only would the dashboard lights stay on, but the internal blower fan would not run, and the windshield wipers would move very slowly. I learned pretty quickly that I could get the lights to go out (and everything to start working normally) if I rev'd the engine to high RPMs, or in the course of normal driving let the RPMs climb pretty high.
Of course, when this first started, I tried to figure out what was wrong. I tested the voltage on the alternator immediately after starting the truck (with the dash lights on), and it was about 12 V, which isn't enough. However, after rev'ing the engine and getting the dash lights to go out, it was 14.5 V like normal. The conclusion....bad alternator. I took the alternator back to the shop where I'd gotten it, and since it had a lifetime warranty they gave me another one. Both units were bench tested, and both tested perfectly. I installed the new alternator....EXACT SAME PROBLEM. The conclusion....not a bad alternator.
I didn't know what else it could be, but I did some research online and found at isuzuforums.com that some others had run into this same problem. One guy fixed his, he swears, by replacing the internal fuse box. Really?...not me! Another found that he had a bad connector going from the alternator back to the battery. I had long since removed that inline connector because it would get dirty and prevent the battery from charging correctly, but I decided to revisit the permanent connection assembly I had put in place there. I bought a heavy duty straight thru connector, restripped the wires, and redid the connections...wrapping it up nice and tidy with heat protection and tape. Still no change....so that wasn't bad.
I was at the end of my abilities and figured I had some bizarre electrical issue, so I just lived with it for the next 5 years, until just recently. It became apparent over time that it was getting more and more difficult to get the lights to go out, and last month it became impossible. All the old tricks that used to work could no longer satisfy whatever condition was necessary to get the dash lights to go out...and hence, the internal blow motor for the heater didn't work. Winter was approaching and I'd already had to experience driving the truck while it was cold outside, and I decided that it was time for this to be fixed.
I did a quick tune up (not sure when the last one of those was done) just to make sure that wasn't it (it wasn't), and took the truck to my mechanic. After some research and work, he determined that this truck has an interesting little electrical system in which the alternator sends an "I'm charging" signal to a relay, and that relay is responsible for allowing the rest of the electrical systems in the truck to start up (including the heater blower and dash light systems). He replaced the relay, but that didn't fix it (naturally). He did some more testing, including grounding the relay, and as soon as he did that, the dash lights went out. The conclusion....the relay is not getting the correct "I'm charging" signal from the alternator.
He did some additional research, including reading postings on some professional mechanics forums, and found that it was pretty common for replacement alternators to not correctly handle the sending of this signal to the relay, requiring the installation of several different alternators before finding one that worked. Before he started looking for a wiring issue leading from the alternator to the relay, he bought a new alternator from O'Reilly Auto Parts (he's had better luck with these than from Autozone or Advance), and it worked. The conclusion...all alternators are not created equal.
So, to recap...
Problem: 1992 Isuzu 2.3L pickup was leaving the dashboard idiots lights on after the engine was started
Cause: The replacement (reman'd) alternator (2 of them) was not sending the correct "I'm charging" signal to the relay that allows the rest of the "optional" electrical systems to turn on
Solution: Install a good alternator that handles this signal correctly (may take a few).
My replacement alternator was about $150, total cost of repair was $250 with diagnostics, labor, and the new relay
