90 Taurus LX has Pulsing lights

[WBroad]

My 1990 Taurus LX 3.8L EFI 4 door has a pulsing light problem, That is every light will pulse or strobe while the engine is at idle and it will increase as the RPM goes up and you can then not really notice it at driving speeds, Ive replaced the external volage regulator and taken out the alternator and took it into a place and had it checked (they said it was working) ive also ran my grounding wires between the engine and frame and up to the battery, Ive replaced the battery cable ends. The alternator that is in this car is an 100 amp one a friend mentioned he thought that they should have 130 amp but the part stores list both for this car, but at 200 dollars thats alot to spend if its not the fix.

Anything I missed by chance I could really use a hand here

Thanks

[shorod]

Have the battery load tested. The alternator is intended to keep the battery voltage maintained, but is not really intended to drive the car's electrical system.

If your battery is weak, it will not be able to maintain the proper power to the electrical system, and your alternator is not putting out full capacity at idle speed. I suspect a good, new battery will solve your issues.

-Rod

[InstallTech]

Actually, the alternator *is* intended to keep the car's electrical running. The battery is only there to start the car and supply electrical while the car isn't running. My first suggestion would be a faulty alternator, though since you already had that checked, that may not be the case. However...even a "working" alternator may have some issues. An easy way to check to see if your alternator is functioning properly....start the car, then remove the negative battery cable from the battery with the car running. If the car dies, the alternator needs to be replaced.

[WBroad]

Sorry I should have also mentioned that i replaced the Battery after replacing the voltage regulator when that didnt seem to fix my problem.

I should also mention that this car has the cockpit style equipment all digital readouts, heavy draw on power i am sure

[shorod]

Originally Posted by InstallTech

Actually, the alternator *is* intended to keep the car's electrical running. The battery is only there to start the car and supply electrical while the car isn't running. My first suggestion would be a faulty alternator, though since you already had that checked, that may not be the case. However...even a "working" alternator may have some issues. An easy way to check to see if your alternator is functioning properly....start the car, then remove the negative battery cable from the battery with the car running. If the car dies, the alternator needs to be replaced.

With modern cars that are computer controlled, you DO NOT want to remove the battery with the car running and rely on the alternator to run the car. The alternator generates an alternating current - AC (hence the term "alternator") which is then rectified. However, the battery serves as a capacitor to filter the AC ripple to prevent damaging the electronics. Without the battery, all that ripple is going to the PCM and can overdrive the transistors, diodes, FETs, and other sensitive electronic parts.

Disconnecting the battery to test an alternator or generator was common before computerized cars, but is not recommended for modern cars (1981 and newer).

-Rod

[shorod]

Originally Posted by WBroad

Sorry I should have also mentioned that i replaced the Battery after replacing the voltage regulator when that didnt seem to fix my problem.

I should also mention that this car has the cockpit style equipment all digital readouts, heavy draw on power i am sure

Do the lights appear to get brighter than usual when they pulse?

Do you have access to a multimeter? If so, set it to the 20VAC scale and put it across the battery, red lead to Positive (+) of battery and black lead to Negative (-) of battery. If you see a reading, then you probably have an alternator with a bad diode or two (or up to 6). If you don't see a reading, then switch down a scale. If you get down to the lowest VAC scale and the reading is still 0.1 V or less, then a bad rectifier is not the issue.

The alternator is a 3-phase generator. It uses two diodes per phase to rectify the AC voltage to get something resembling DC. I'm not sure if your 1990 Taurus alternator has the diodes built into the voltage regulator or not. If you have a bad diode, your alternator could still be providing sufficient charging power, but have a spike that the tester is not picking up. Depending on the tester, it may just be looking for a DC voltage, but not checking for AC spikes.

-Rod

[WBroad]

OK I pulled out my Vohm meter and did the test near the battery and got a clean signal there, And I pulled out my Haynes manual again and began following the charging system and relized that the output of the alternator is simply dumped into the cars power grid.

So since this type of alternator has an external voltage regulator I did the same test on the output side of the alternator and got these readings.

.08 volts AC and a 50Hz signal which is going right into the power grid unfiltered.

So I got a AC signal in a DC supply and thus I've found my pulse.

Thanks for the help I didnt think to check for an AC signal right off the bat, Gonna see if I can rebuild it or replace it either way i'll get back and let you all know if it fixed it.

Ah yeah the diodes are inside the Alternator and not the regulator what a pain there.

[TomV]

Originally Posted by WBroad

So since this type of alternator has an external voltage regulator I did the same test on the output side of the alternator and got these readings.

.08 volts AC and a 50Hz signal which is going right into the power grid unfiltered.

So I got a AC signal in a DC supply and thus I've found my pulse.

Before you condemn the alternator because of a small AC voltage, I recommend you perform the same test on another vehicle to get a reference AC voltage. As Shorod described earlier, the alternator generates AC and it is rectified to create a DC voltage. However, the voltage will have an AC component and that is normal. Problems occur if the AC value is large. I do not think a voltmeter measurement of 0.08 volts is large. Also, the response of the meter is probably too slow to accurately give you a picture of what type of signal you are measuring. They are calibrated for continuous sine waves. An oscilloscope is required to really see the waveform.

With that said, I do agree with Shorod that the diodes are most probably the culprit. This is in light of the other work you have done to replace parts (regulator and battery). The diodes (rectifiers) may be replaceable for your alternator, but if not then a new/rebuilt one is in order. Diodes can be tested with an ohmmeter. I had a similar problem with a motorcycle years ago with a pulsing headlamp and that resulted in a rectifier bridge replacement.