I have searched but haven't found a reported problem similar to mine.
This is a series 5 vert.
I am having a problem that I believe is with the main relay.
I have tested the battery and just put in a new starter.

When I turn the key on the electrical system is OK.
As soon as I engage the starter the electrical system shuts down.
After a while the electric system works again.

How do you test the main relay.

 

The problem lies in the battery terminals or the ground wire from the battery to the starter not being connected properly. You just replaced the starter. Start there with the battery cables negative wire that connects to the long starter bolt.

 

This problem was the reason that the starter was replaced so its installation was not source of the symptom.

The main ground to the bellhousing was sound.
I don't think that is the problem.

To make sure the problem is not in the wiring,
The car has a leaking master cylinder so the harness is soaked in Dot 4.
I pulled the harness loose at it's connection points and took it apart.

Pulled all of the old tape and loom coversing the wires. No broken or damaged wires other than the oil pressure sender wire is brittle.
All of the connectors appear to be OK.
I cleaned all the oil and dirt off the cables and I will re-wrap and reinstall the harness. It is raining now.

Any other ideas.

 

The headlights will still work if the Main Relay is kaput or even if the Main Fuse is blown.

So what happens when you put the key to start and the electrical system seems to die? I mean do this again, but this time when the electrical system seems to die, turn the headlights on. IF they don't rise and shine, then I go back to the battery being bad or the battery cables/connections.

IF the headlights don't rise and shine, turn the key off. Get a battery out of your other car and temp install it in the failed car. See how things go.

 

I tested this and I get the same result (no power to car) if rather than turning the key I just turn on the headlights. So any significant load shuts the power off.

I will reinstall the harness (if it quits raining) and check the battery terminals.
The battery tested good at the local AutoZone but it sounds like it is the problem. Easy enough to get a replacement.

 

Ok I'm back. Finally got everything back together. I have a installed a new starter, a new battery, new battery terminals. I have fully checked the harness connections between the battery and the starter there are good positive connections and no broken wires.

So the symptom with the new battery is that I have power and everything works in the car. The lights work fine. Volt meter reads 12 volts. When I turn the key to start it I can hear the starer relay switch and the volt meter goes to zero. When I turn the key back the voltage returns to 12 volts.

What should I check next. The car is a '90 vert so it has the security starter relay I don't think that that can be the problem since I hear the starter relay.

 

If you put the car in neutral and go to the front of the engine, can you turn the engine over? As in grabbing the front pulley or alternator belt, and turn the engine over. That's my way of asking "is the engine frozen up?".

 

Hailers, Thanks for your input on this.
It is good to get a second opinion when the problems solution is not clear cut.

I had checked the engine and it is not frozen. Should have posted that info.

After I posted the last message, I determined that the "rebuilt" starter had a bad relay. So back to the parts store.

Lesson learned, assume nothing, bench test the starter regardless of all the stickers and paperwork that indicates it will work out of the box.

I have not had a chance to get the second rebuild starter into the car. I'm pretty sure that it combined with a "marginal" battery this was the real source of the problem.