Three warning lights glowing

I have a 1992 Mitsubishi Eclipse with a 1.8L SOHC non-turbo engine. It has 165,000 miles on the clock it lloks good and runs great; however, three warning lights always glow with variable intensity - Low coolant level, Low brake fluid, and Battery charge. The glow is not visible during daytime driving, but is obvious in darkness. The three lights glow brighter as additional electrical load is applied - like AC, blower, headlights, and radio. I have had the alternator checked; it was given a clean bill of health and I have never had a problem with a discharged battery. The engine always starts quickly and the headlights and instrument lights are bright. I checked the wiring diagram and found that each of the three warning lights are on a circuit from the L terminal on the alternator and each warning light has a diode connected to it on the circuit board. I pulled the circuit board and tested each diode - each checked good - voltage is flowing in only one direction. I also checked for loose ground connections in the wiring harness and between the engine and the body. There is continuity between the engine block and the fierwall. There is also a good connection between the negative post of the battery and the firewall via a direct connection. Everything appears to be in order but I still have these three warning lights glowing. I must confess that these lights have been glowing for the past five years without consequence except that it has been bugging me and since I am now retired I have the time to play with the problem. Anybody got any helpfull hints for me other than putting a piece of tape over the lights?

Look for a bad ground in the instrument panel!

While I had the instrument panel apart to check out the three diodes I did do continuity checks on several of the circuit board grounded connections; all checked out. All of the guages (oil pressure, temp and tach) and all other instrument lights function properly so I assume that there is a good ground connection to the circuit board.

Well, all I can tell you is that a bad ground (or grounds) was the cause of a similar situation on my “nightmare from Hell” Volvo many years ago. That was my best shot.

Diodes pass current in only one direction (as any school boy knows); but, what about ac (alternating current)? Lights bulbs (lamps) don’t care if they are getting ac or dc. AC may be slipping past the diodes and lighting the, uh, lights. Put your hand dandy electrical test meter on ac setting and see whacha got.

Do a thorough test of the alternator. Failing alternators have often turned on the BATTERY and BRAKE lights along with others. I have never investigated why. I usually find a marginqal charge rate and rectifier control.

Right you are - the function of diodes is to allow a current flow in only one direction. As I recall there are three sets of diodes in an alternator: 3 for + and three for -. If any of the 6 diodes goes bad it would cause an unbalenced +/- condition that could cause an AC circuit. Now all I have to do is figure out how to test for it. Once the instrument cluster and circuit board is removed to access the instruments and lights there is no longer any electrical power going to it since it’s pulled out of the two ribbon sockets in the back of the dash. I’ll have to backtrack the printed circuit paths on the board to find out which ribbon contacts they go to on the plug-in socket and then test for AC at the socket - this could take a while - removing the instrument cluster is not a fun project.

That did cross my mind also, but since the OP tells us that this situation has been going on for about 5 years, I dismissed this as a possibility.

OldArmy, why do it the hard way? Just take an ac measurement at the alternator.
Someone reported lights burning out too quickly (and, perhaps, too brightly). When they checked the alternator for ac output, they found 25 volts ac !

Thanks hellokit - since these warning lights are connected to the L terminal of the alternatior should I be looking for the AC there where normally I should see only battery voltage or should I be looking for AC at the main alternator output terminal?

Even though the alternator was given ‘a clean bill of health’ it is the cause of the trouble. The warning lights tie to the alternator so they turn on in the ‘test mode’ when the ignition is turned ON and the engine not running. That voltage supplies current to the field of the alternator so when the alternator starts up the field inside it charges up and the charging action starts. Then the L lead has voltage on it coming from the charging action and that turns out the lights since there is a voltage now on both sides of the lights. Something is wrong with one or more of the diodes in the alternator.

The AC voltage that Hellokit was taking about was across the battery while the alternator is running. That is basically the output of the alternator since it is tied directly to the battery through a protection fuse.

Thanks Cougar, I think you hit the nail on the head. The warning lights do dim a bit when the engine speed increases up to about 2000 RPM but never go completely out if there is any kind of additional load on the circuit (like headlights or AC).
In that the voltage that is required to turn the warning lights off comes from the alternator “L” terminal and the main charging current comes from the alternator “B” terminal, do you think there is any danger in continuing to operate in that mode? These lights have been glowing for the past five years but I have never experienced any problems with a discharged battery or other alternator failure symptoms such as dim headlights or dim instrument cluster lights.

Since it has gone that long without any other trouble it seems like you will be ok. You just won’t have the full amount of charge current available to the battery if the demand is real high since there are some bad diodes inside the alternator. The trouble may also just be with one of the diodes in the diode trio for the lamp circuit.

One look at a o’scope pattern of the alternators output (taken from the big wire off the alternator and ground) will show your ripple. A healty alternator has a suprising amount of allowable AC ripple. Open or shorted diodes are very easy to pick out.