1978 620 won’t start

[West]

First post on here after using it for most of my knowledge on the 620, my 620 ran great until a guy I was selling it to wanted to test drive it, he said he knew how to drive a manual but I guess he didn’t because he stalled it immediately and after that none of the electronics would work not the horn or the lights only the radio the dash wouldn’t light up, eventually I could cut the key on and the dash lights would come on but if I tried to start it the dash lights would go off and wouldn’t come back on for a while, so I turned the key to the on position and jumped the battery straight to the starter and it cranked right up and ran great, and continued to do for many days, but I went to start it today and only taped the key so it didn’t start but turned over the motor and now it’s having the same issues, I also tried to Hotwire it to see if the ignition switch was the issue but it still didn’t work, and as far as I know I was doing it right, I have a new battery new alternator and starter please leave any help you can 

[datzenmike]

Are all the new parts after this guy drove it??? or before??

 

 

 

Clean both battery terminals and cable clamps and tighten them. Check that the ground cable on the head it clean and tight and the positive cable clean and tight on the starter.

[West]

2 hours ago, datzenmike said:

Are all the new parts after this guy drove it??? or before??

 

 

 

Clean both battery terminals and cable clamps and tighten them. Check that the ground cable on the head it clean and tight and the positive cable clean and tight on the starter.

I got them before the guy drove it, and I’ve already checked  the battery terminals they are clean and tight and the starters positive cables clean and tight

[datzenmike]

5 hours ago, West said:

..... and after that none of the electronics would work not the horn or the lights only the radio the dash wouldn’t light up, eventually I could cut the key on and the dash lights would come on but if I tried to start it the dash lights would go off and wouldn’t come back on for a while, 

 

The dash lights only come on with the light switch, and the horn have nothing to do with the ignition switch. The ignition switch can't cause them to not work. This to me is a bad or weak electrical connection.

 

Next time it's doing that use jumper cables from a good vehicle to your battery posts. If that fixes the problem then your battery is the cause.

 

If not fixed, negative jumper cable to somewhere on the block or head and the positive to the starter post. If this fixes the problem then the battery cables or their connections are the fault.

[EDM620]

A bad ground connection can be the root of many electrical gremlins, as the circuits need to complete the system to function correctly. Look at the other ends of the battery cables, not just at the battery posts.

The vehicle itself is the negative connection, otherwise every circuit would need its own wire to battery and therefore twice the amount of wiring in a vehicle.