Electrical problem... fuse box in engine bay has power, one in car does not

[natedog437]

just bought a 95' 240sx with 125k miles and it cranks up fine, idles fine, head lights work.... but NOTHING else works on the inside of the car. The windows, guages, odometer, blinkers, wipers, sunroof, A/C fan, heater, PDL, etc.... do not work at all. They will randomly come on out of the blue for a few seconds... which is suspect, because that means that the fuse is not bad. I replaced the ground wire to the battery, terminals on the battery, the battery itself, and checked all the fuses. The fuse box under the hood by the battery has power, but upon checking the fuse box inside the car by the clutch pedal, there is no power getting to any of those fuses. At one point, everything came on and it all worked fine, but a few minutes later, it went back off and stayed off.

Anyone have any idea what might be the problem here? Thanks

[slipnfall]

Do you have a factory service manual? It lists all the body/chassis ground locations. Check those that are easy to get to, don't go all out. In the S13. there's a SMJ(super-multi-junction) under the D/S dash, above the fuse box. It's a huge connector, with a bolt(in the middle) that pulls the two mating parts together. Unscrew it, pull them apart, and put it back together. Make sure it's screwed down as far as it will go. In a nutshell, this connects the engine bay harness to the instrument harness.

About the only way you're going to find the loose connection(my guess) without going out of your mind following schematics, is the highly technical 'wiggle test'. Seriously, put the key on ACC, and just start wiggling wires/harnesses around. This is about the easiest way to isolate the problem.

Also this may sound silly, but make sure you are measuring from a *good* ground when crawling around under the dash. When diagnosing some electrical problems, I made the mistake of assuming that grounds were there that should have been. You can do this to double check individual grounds: run a long, fused wire(doesn't have to be thick or pretty, just temporary) from the positive battery terminal inside the car(just run it through the open door, again, not permanent). Using a volt-meter, measure from this known +V, to what should be grounds under the dash. If you get 12V, you know that point is a good ground.

HTH, good luck. Electrical problems can be a PITA.-Slip

[natedog437]

I'm thinking it's the alternator and here is why... A couple of days ago when I was enitially test driving the car and it was doing it and then it started to sputter and then completely die! I rolled it and popped the clutch, then drove it to meet my friend for him to look at it. He turned his 240 off and we dropped his battery in my car and everything worked fine. We parked it and then I put the other battery back in and nothing! Like I said, I put a new battery in it, and it didn't fix the problem. I got home from getting the new battery with it hooked up on the car, turned it off and did a voltage check. I'm getting no more than 11.88 outta the thing and while the car is runing it seems to slowly drop in voltage. I just my car off with another car that was reading 13.9 while running and left mine connected and tested mine. It was still around 11.40 and dropping really slowly. I tried the wiggle thing, but I may try it in the dash like you said.

[slipnfall]

Oh, well after hearing that, replace your alternator first! It's surely not outputting enough voltage. Just be 100% sure to connect all the grounds/wires back up to the alternator!!

-Slip

[natedog437]

Well I just replaced my alternator and I saw the lights on the dash come on while the key was turning over and then when the car was cranked...Nothing!! I'm getting reversed polarity reading coming from the alternator and I'm thinking that it blew. The batter was outta the car the whole time and yes everything was hooked up correctly. What the hell is going on with my wiring?

[slipnfall]

Reversed polarity? Must have your multi-meter leads reversed. I don't think it's possible to hook the alt up backwards due to the different connectors.

::edit:: you said the battery was out the entire time(??) Did you remove it after you got it started?

[natedog437]

The battery was out before I got started. It just became like the whole thing was sending out ground after it blew! that is what I meant by that.