Electrical?

SHADYLADY

MEMBER
1991 MERCURY COUGAR
80,000 MILES

Purchased 1991 Mercury Cougar LS with3.8 V-6. Previous owner states car sat for 4 years. Replaced battery with new and car started. Notice gauges are wacked out. Showing car was hot and had full fuel tank (only put in 5 gals) etc. Alternator tested fine. All wiring harnesses under hood new, in steering column new. Replaced electronic and key lock ignitions. New fuses and fuse box. New head light switch. All lights fine. Problem is when brakes are pushed the car dies. Try to roll down power window car dies. Open door to where dome lite comes on, car dies. Turn signal makes dome lite blink like signal indicator light. Appears to be parasitic draw but cannot pinpoint where. Any suggestions?

3 Replies

CARADIODOC

MECHANIC
33,908 POSTS

How was the generator tested? It is common to just measure the battery voltage with the engine running. It must be between 13.75 and 14.75 volts but there's more to the story. There's at least six diodes in it and if one is defective that will reduce the generator's maximum output current to exactly one third of its rated value. That is not enough to meet the demands of the electrical system under all conditions. The battery will have to make up the difference until it runs down, and many systems will do strange things when the voltage drops. The generator needs to be professionally load-tested for maximum full-load output current and "ripple" voltage.

Also be sure the belt is tight.

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Wednesday, May 29th, 2013 AT 9:16 PM

SHADYLADY

MEMBER
2 POSTS

Alternator was taken to Autozone and tested on their machine. Tested fine. Belt is tight. As stated before all the parts that were replaced before I purchased it. New battery etc. Ground wires checked. Car still dies when you push the brake pedal, turn on turn signals, attempt to roll down a power window etc. It is a parasitic draw but how to tell from where. Any suggestions?

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Monday, June 3rd, 2013 AT 12:31 PM

CARADIODOC

MECHANIC
33,908 POSTS

I'm not a fan of removing parts for testing. Generators need to be tested for full-load output current and "ripple" voltage. That will identify one bad diode of the six. With one bad diode the generator will only develop a maximum of exactly one third of its design rating and that's not enough to meet the demands of the electrical system under all conditions. The battery will have to make up the difference until it runs down. The test benches at auto parts stores aren't strong enough to run a generator at full load. They can only measure the output voltage, (electrical pressure). It has to be on the car to measure maximum current, (electrical flow). In most cases when the maximum current is low due to a bad diode the output voltage will still be near perfect, so you can get a false "good" result on a test bench.

All the symptoms you mentioned point to the generator being unable to deliver the required current. Have it tested on the car for full-load output current and ripple voltage before we go looking for some other cause that may not exist.

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Tuesday, June 4th, 2013 AT 12:31 AM