I get a check engine light on my 1990 f150 every once and a while, I used a code reader and it says overvoltage condition. The voltmeter reads 16-18 volts? SO, here's the question: would this cause my check engine light to come on?

 

Yes, it will. Not good for the computer either.

 

With the engine running you should see something like 14-14.5 max across the battery. Any more does nothing n your truck any good.

In order to see if your voltmeter is reading close to correct shut the truck off and read the battery voltage. If it is somewhere between 12.3 and 12.8 the meter is probably reading correctly

 

Everything is reading as you said above, Except the gauge which hovers aroun 17-18. I keep getting a intermittent check engine light and the code scanner says something about charge voltage being above 17.5 volts. Any ideas?

 

Go get high-power stereo to use up those extra volts =).

Always funny watching the little ricer guy's headlights go dark each time the bass hits....

 

Your alternator voltage regulator is failing. 17 volts is too high.
Most voltage regulator is built-in the alternator and you just have to replace the alternator. However, some voltage regulator is separate from the alternator and can be replaced on its own.

Have the alternator checked at Autozone or Advance Autoparts for Free.

 

so, even though I'm reading 14 at the cables the reg could still be putting out 17-18?

 

Now.. that you are getting 14 volts across the battery while running then it might be just the voltmeter on the dashboard that might be misreading and so the Computer thinks it's over voltage.

 

The computer doesn't care how the guage reads, there's no input to the computer from the guage. Get the regulator tested to see if it's bad. Be sure to check all electrical connections as well.

 

If you are reading 14-14.5 across the battery, measured with a accurate voltmeter, while the motor is running you do not have a problem with the alternator/regulator.

Your problem is most likely with the computer. The only two ways you can have a higher voltage right at the alternator output than you have across the battery are:

1) if you have and extremely, and I do mean extremely, bad connection between the alternator output and the battery positive terminal. If you can get to it try reading the alternator voltage output right at the alternator output while the engine is running.

2) The other possibility is you have a really bad ground connection from the alternator ground to chassis ground. The alternator voltage regulation is relative to it's own ground so if there is for instance a 3 volt voltage drop from the alternator ground to the chassis ground your alternator will generate 14-14.5 volts plus 3 volts relative to the chassis ground which is pretty much the same ground your computer and dash voltmeter use. If this is happening you will get 17 volt or so level at you gauge and computer input relative to their ground input. This has nothing to due with the alternator functions except the connection/connections from the alternator are poor.

Good luck.

 

Remember that the alternator connections in that vintage truck are known to start fires, and could be dirty or corroded. I would start there. If the regulator does need to be replaced, even the "Internal" regulators are very easy to change without removing the alternator from the truck. One should also replace the brushes at the same time. They are right there.

 

correct. as the few last posts stated, corosion is the number 1
enemy and cause of most components working improperly or failing prematurely. I would clean ALL contact points first to make sure that is not a possibility before dropping $$$ on a alternator or voltage regulator. if it still the lproblem, then take it in for inspection.