Electrical issues when in gear

I don't what understand what you are trying to prove by measuring the negative terminal for voltage. If you are verifying the ground is good you should be measuring resistance in ohms.

Tom, it is a voltage drop test. Doing voltage ground to ground will show a loss of voltage through a cable under load. It is the correct way to test it. If you take the volt meter and place one lead end on the battery terminal and the other at the end of the cable you will see the amount of voltage loss through the cable. Anything over .5 volts needs to be looked at.

There is a link I posted in my following post on this procedure. The issue with a resistance test is the circuit must be isolated to get accurate readings.
 
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When in drive electrical issues show up. Headlights go out. Dash lights go out. Blinker lights don't work and nothing showing on dash. Brake lights don't work or any body lights. Blower on heat/air slows way down. If in park all these things work but start to go out if I turn on the blower or the lights. Any kind of load affects it.


I've tested battery its good

Tested alternator-it's good

Cleaned grounds by each headlight.

Cleaned ground on fire wall that attaches via strap to hood

Cleaned ground by driver side door

Cleaned ground by passenger door

Tried to clean ground on motor near filter but couldn't do it well as it has an e Tex socket. Two piece. Backed out a little and brushed. Used a light tester and engine doesn’t cause light to illuminate.

Don’t see another ground


Online suggest replacing

MFS that controls lights and blinker

Headlight switch

both ordered.

Anything I’m missing that I should try.
Here is a quick write up on voltage drop tests, it can be done on circuits without disconnecting and isolating the circuit. https://www.alldata.com/sites/default/files/file-attachments/voltage_drop__test_122718.pdf
 
I have never heard of this in my extensive experiences in electrical trouble shooting over 62 years. I will try it, thank you.

Your welcome.
This is really the way to test circuits under load. I had more vehicles towed in with new starters that don’t start then when I do the voltage drop on the cable I get 10-12 volts. Another place this procedure works great is testing voltage drop through relays
 
Was working on a 48 VDC forklift one day that would work just fine for about 100 yards. It had two 400 amp fuses that supplied all the power to propel it. After it drove about the 100 yards it lost all power and shut down. The fuses visually looked good. Resistant read good, but when you applied power one of the fuses showed open when power was applied. Read 48 volts when you put your meter leads across the fuse. Bottom line was when the fuse heated up from current running through it, it separated. Thus sometimes you gotta read voltage to confirm continuity.
 
Was working on a 48 VDC forklift one day that would work just fine for about 100 yards. It had two 400 amp fuses that supplied all the power to propel it. After it drove about the 100 yards it lost all power and shut down. The fuses visually looked good. Resistant read good, but when you applied power one of the fuses showed open when power was applied. Read 48 volts when you put your meter leads across the fuse. Bottom line was when the fuse heated up from current running through it, it separated. Thus sometimes you gotta read voltage to confirm continuity.
The instructor at the training center had a battery cable that would check good with an ohm meter but you could touch it to battery terminals and it wouldn’t short.
 
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