I'm having an electrical problem with my 1979. My headlights significantly dim and become brighter depending on the motors RPMs. I checked the battery and the alternator and both are good. I replaced these two parts and the regulator just 6 months ago. I'm thinking its a electrical ground problem.

Currently my negative battery terminal grounds directly to the engine block and the car runs fine, but the light act funny. Where does it bolt to on your pinto???

 So I tried to ground the negative battery terminal to the body and the car wont even turn over which has me thinking that perhaps the motor itself is not properly grounded to the chassis.

Any thoughts before I have to pay a real mechanic to fix what must be a very simple problem. Maybe I need a ground cable between the motor and the frame? IDK   

I have not as yet added a cable between the motor and body. I need to go to the parts store today. My battery positive goes to the solenoid and the negative to the engine block. That's it! where does the negative battery terminal on your pinto ground to? Please tell me the specific bolt location. Thank yhou 

i think he said he took the ground cable away from the block and put it on the body .. therefore it would not turn over .. you can put a ground wire from the engine to the body anywhere you wish .. it seems you dont have one now which may indicate the engine has been out of the car once or so .. many times that little cable is left off by mechanics in a hurry to make a buck ...but i doubt that is yor problem ..

 Thats what it sounds like to me too.

 A bad ground from the battery to the headlights would increase resistance in that circuit and create a voltage drop in relationship to the current drawn through it. I dont think it would cause the headlights to dim and flare with engine rpm.

 Check the voltage at the battery with a decent dmm set on 20 volts range and get somepne to raise and lower the engine speed or do it yourself with the throttle cable, and see if the voltage on  the battery changes too. Or read on the actual battery connectors and at the battery posts while engine speed is raised and lowered.

 I think its most likely a source voltage variance.

Proper ground circuit (stock): Batt to block, block to chassis.

Improved: Batt to block, batt to body; block to chassis (2x).

What rpm is this dimming occurring? If at idle, that's normal, if you're still running halogen lights. My brand new 2002 Ranger did that, back in the day.

If you don't have the starter grounded, it won't spin. That's why when you test a starter out of a vehicle, you have to connect the ground cable to one ear. That's why, when you changed the batt ground to the body, it wouldn't do anything. Electricity makes a loop. Break the loop, it doesn't go anywhere. Ground the motor to the chassis (2x really only needed if you're adding a system or some other high-amp draw, but it never hurts).

Only if you have a diode out in alt..   Fred   

Only if you have a diode out in alt..   Fred   

If properly grounded, then Fred nailed it. It's a simple system. If it dims off idle (at higher rpm), and is grounded properly and all other connections are correct, then go back to the alt/voltage regulator.

Ymmv.

There may well have been something wrong with the regulator, BUT, all alternators on earth must supply more voltage than the 12 volts of the battery. If they have same volts as battery then they are not charging. I look for like 13-13.5 on these, 14 is not that high. As example the Focus charges at 14-14.5, if below 14 the alt has popped a diode and long term will run the battery down.

What they said on extra grounds, you can never have too much ground. Should be a minimum of battery to chassis and battery to block always.

 13.8 is the nominal charging voltage for most `12 volt` systems. Whenever I build regulated 12v supplies I always try to get right at 13.8

 If the voltage from the alternator doesnt go over 12.36 or something like that it wont charge the battery at least not well. NiCad and NIMH batteries are charged with a constant current. Lead acid usually charges in 2 parts with higher current then a voltage leveling. Some chargers change states.

 The alternator output is controlled by the regulator by controlling current through the field winding. The regulator is a little different than a series regulator in normal power supply but in some ways not a lot.