Volts drop when heater is on...

I had my heater on for about 5 minutes tonight. I looked down at my battery gauge and saw that it was almost in the red. I immediately shut the heater off, and sure enough, went right back up to 14 in the middle.

It has done the same with AC, although AC generally blows a fuse, so I've got my fan clutch switch unplugged. Heater/AC works but doesn't blow cold air. I had assumed that I was only getting the voltage drop with AC, I had a bad fuse that didn't blow when it should, which is why the AC turned on in the first place. Now tonight I see that the heater is pulling too much voltage.

Anythings I can check for to see if it's something simple? It's going into the shop Thursday to have the AC worked on and recharged w/ Duracool. I'd like my bill to be as little as possible, and I know hunting down electrical problems isn't going to help keep it low.

Thanks

It's a 1990 4.0 XJ

If you havent already, you should up-grade your charging system so that you can get access to the amperage from your alternator. The positive cable from your battery has a small cable connected to your relay. This cable should be changed to a 4 gauge (same as the positive cable that connects to the starter). The wire that connects the alternator to the relay is a fusable wire set to burn out if there is a serge.It is usually a very low amperage (probably between 40 and 60 amps), which is not consistant with the amperage from your alternator (most of us have 100 to 200 amp alternators). At this connection, I ran a 4 gauge cable to a 100 amp marine type fuse (or blade type fuse),because I have a 100 amp alternator. Then I ran the fuse to the relay, also with 4 gauge cable. With my 100 amp alternator, I can turn on loads of gadgets without a drop in power to the headlights (when they are on). The ground on XJs is a braided wire that connects the body to the engine. This should also be changed to a 4 gauge, and even that is a limited effort at getting the DC back to the battery. You should ground the negative terminal of the battery to the body using a 4 gauge cable connected to the closest torx screw on your radiator bracket. Sand off any paint on the body for good contact. The marine fuse amperage should match the amperage of your alternator. I picked this information up from another Jeep site.They recommend using lower gauge cable (read "bigger" cable) if you have a higher than 150 amp alternator. My charging system runs great; no complaints when the radio (with two 200 watt amplifiers) is on, along with the high-beams, bush lights, and someone wants the air-conditioner turned on.

I've already done the cable upgrade. This is not a problem with my charging system. It has something do to do with the heater.

The simple answer is, don't believe the gauge. If you want to know what your charging system is up to measure voltage at the battery. The gauge shares its circuitry with the heater fan, and the voltage drop is probably in the circuit that feeds the gauge, not the charging system. I had the same scare with my 87.

There are two places that you need to check. One is the blower switch connector and the other is the resistor pack connector. My blower switch fried, smoke and all, and this is what I had to fix. Took me a while but everything is working again.

There is a reason for this. As switches and relays get old they get corrosion and dirt on the contacts and contact points. This will create a voltage drop and HEAT. As you go "down-line" the more connection points you have the greater the voltage drop. You could have a bad, or going bad, switch or even a dirty relay contact. So this is what happened to me after my voltage gauge started reading low.

BLOWER SWITCH CONNECTOR



and RESISTOR PACK CONNECTOR



Now this might not be your problem. The voltage gauge, as I mentioned before, is down wind in the electrical path in the Jeeps and will read off as the Jeep gets older. This might not happen to everyone, but I have seen enough posts on here to come to this conclusion. Even after I fixed my problem my voltage gauge still only shows around 12.5V or so. There is one way to make sure that it isn't your charging system, you can probe the battery with the engine running and you should see right around 14V. If not then you need to get your alt or battery checked. I haven't found out exactly why mine is only showing 12V at the gauge.


Alex

That is exactly what my plug looked like. JIM.