Greetings,

My problem is that my 91 RS TBI is acting like it has a bad alternator. When I start the car, everything is fine. Once I've driven around for a little while, I notice that when I'm idling or sitting in park, my voltage gauge drops below the 13V mark. When this happens, the windows move slowly, the dome and instrument lights dim, headlights dim, and even my radio reception drops a bit. Everything is fine when the motor has some revs on it though. I notice the headlights especially when I pull into my garage at night. I'll tap the gas to roll into the garage, and for that brief moment of throttle, the headlights will brighten up. It also seems that this issue is affecting engine performance. When I first start the car, throttle response is fine, but after running for a little bit, there is a very noticeable lag when hitting the gas. Maybe the ignition system and/or fuel delivery system is lagging behind as a result of this issue?

I purchased a new alternator and battery early this year and, in trying to diagnose this problem, I recently exchanged them under warranty for replacements, despite both testing fine. I've checked all the connections around the battery and the alternator. Everything looks good, including the grounds. I did some testing with my multimeter, and I do see a drop in voltage when reading at the battery and the alternator. I show 12.5 V give or take, compared to 13-14 V just after a cold start or when RPM's are above idle.

The car always starts, although it's a bit slow to turn if it's been sitting for more than a day. I drive a very short distance to work every day and don't give the car much time to charge the battery, but I guess it's getting a somewhat sufficient charge. I'm an amateur when it comes to cars, and I've done my best to try to figure out the issue, but I'm at a loss. Any suggestions would be greatly appreciated, and if I need to clarify on anything, please let me know. Thanks a lot!

(By the way, this forum rocks!)

 

Next time you get a chance, completely disconnect the battery and alternator and clean up the wire terminals with a wire brush. Also at the starter and engine block as well.

Then, put the battery back together and the wire back on the alternator. Turn the key on and leave the engine stopped. Check the voltage at the alternator connector (with the RED wire in it), on both wires. Both wires should be at or near battery voltage. If not, there is a common issue on the brown wire due to the fact that it's spliced into the harness for the radiator fan relay.

 

I went though the steps you suggested. I cleaned the terminals really well and tested the voltage with the battery on and engine off. The battery was showing 12.06 V and the alternator was showing 11.96 V. Only a difference of 0.1 V.

Could you elaborate on this issue with the brown wire? Maybe it would be good to mention that a previous owner, for whatever reason, hot wired the fan to the accessory fuse, so the fan runs all the time.

 

The problem with those relays (air conditioning, coolant fan, fuel pump) is that the wiring isn't the greatest... GM did put stuff in the bottom of the connector to help keep water and such out of the terminals, but the wiring still tends to get hot and deteriorate. And, since the alternator is attached to the coolant fan relay, the wiring on the regulator's voltage sense circuit is affected, reducing or possibly cutting off all charging of the battery.

With good connections at the alternator and battery, a GM CS130 alternator SHOULD be putting out between 14 and 15 volts (usually around 14.7 is the calibrated output).

Since you mentioned that the PO has modified the wiring to the coolant fan, the relay would be the first place I would look at for this issue. Start with the relay connector itself. You can either get one from a junkyard car or go the cheap route and get one from a car security system installer (for, say, power door locks) and replace the connector. There is also a fuse in the fuse panel, marked C/H FAN. If that fuse is blown, I would definitely look at the relay next.

 

I don't even have a fan relay in the car, so I'm not sure how the wiring is done up at the moment. However, my battery is indeed getting charged. My issue is that the car seems like its not getting enough power at idle and everything electric bogs down.

 

Well, you see, that's the problem. Yes, the battery is charging, but it's not enough with a high load at low engine speeds. Because there is possibly a problem with the alternator's voltage sense circuit.

Check the battery voltage with the engine running. My guess would be somewhere in the low-to-mid 13V range (or, worst-case, high-12s). As I said before, you should be at almost 15V.

 

Strange how that's tied into the fan wiring. Last time I tested voltage, I recall getting something above 14 V. I believe that was after a cold start though, which is when everything seems to work best. I'll give it another reading when I get home from work. I'll also take a look to see what's what with the fan relay wiring. I've been wanting to wire that back up like its supposed to be, I just haven't gotten around to it. I don't like how the fan runs all the time, even when just the battery is turned on.

 

As temp increases, resistance in the wire increases. More resistance = less voltage available to the voltage regulator.

IDK why GM did what they did. Almost seems like the CS130 voltage sense was an afterthought...