john89gt

Car (1989 GT stock) Question 1-My oil pressure gets lower the longer I run my car. I changed the oil sending unit. It has been like this for sometime, so I don't think it is in the motor. Starts out cold at about 60lbs, then when warmed up it goes to about 20 lbs.


Question 2- When i turn the a/c blower motor on high the voltage drops on the volt guage. I have changed the switch, blower motor, alternator, and the battery over the course of sometime just trying correct this. It has still continued. Thanks in advance for any help!!!

67mustang302

Oil pressure drops as oil warms up and thins out, that's normal. You should have at least 10lbs at idle and at least 10lbs per 1,000rpm(ie, 20lbs and 2,000, 30lbs at 3,000 etc).

Welcome to the world of crappy Mustang wiring. The electrical systems on the Classics are even worse. Voltage drops aren't uncommon to see, depending on where the voltage is being measured from. If in your case it's measured from somewhere in the harness for the passenger cabin, then it will show a voltage drop when accessories are running. Get a voltmeter and test the voltage across the battery with the car running and the a/c and lights on, and from battery (+) to chassis ground. It could be something as simple as a corroded chassis ground connection, which will cause voltage drops everywhere.

john89gt

Thanks for the input. I have checked the alternator voltage and the battery voltage. Its been awhile, I don't remember the exact numbers. I've owned the car since I was 18 so there is no cuts and patches. I also have solid motor mounts and have replaced the ground from battery several times. So I just keep driving her thinking she will just quit one day and i'll know what it is. Needless to say thats been since about '98 !

MustangBradley

20 lbs of oil pressure is not a big problem. Your motor is just well broken in. haha After 21 years, some oil pressure drop is expected.

What amperage alternator did you replace the stock one with? Do you run an underdrive pulley on the alternator? I don't remember what the stock output is but it is common for an electrical system to go into discharge at idle with the a/c on max. Especially if you have an underdrive pulley. If you used a stock amperage alternator to replace your old one, and you don't use an underdrive pulley, the simple fix is a higher rated alternator.

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