Slow crank and low voltage

I have a1994 6.5 diesel blazer new batries(I had the batteries load tested and they read excellent) new alternator I have been noticing my voltage guage reads 9.5 before I start the truck and the starter cranks like it. Immediately after starting the guage goes to 14.5 for a bit and it levels off right at 14 as expected after about 30 seconds. This happens weather the truck has been sitting for 1 min 1 hour or 1 month. I am thinking possible ground issue or possible bad connection at positive starter connection. I'm hoping this a common problem for the volt guage to read low before start and see slow cranking only for the voltage to go back up with the alternator charging. Please save me some time with your knowledge diesel page users.

The low voltage is the glow plugs warming up. When you turn the key wait for the voltage to go up then turn key to start.

Howdy
Welcome to TDP!
Probably the most common issue with the charging/starting on this vintage of truck is CABLES and CONNECTIONS. If you do a search on this forum you will find tons of information about this subject.
The biggest problems are poor cable condition, meaning interior corrosion that cant be seen and the factory end connectors and ground connections.

Do a search and pick the remedy that best suits you. It seems that everyone has a slightly different way to fix this problem and most of the ones posted are effective.

What Snarl said ....

If the truck has the factory type side terminal cables they are notorious for issues after a few years.

Even the crimped ends of the ground cables where they bolt to the block can go sour.

The positive crossover cable that ties the two batteries together is another trouble spot.

The 94 has a long cable on the RH side the connects to the positive terminal and then runs back to the firewall junction block.

This cable provides all the chassis power.

A combination of poor connections within the crimps of all the various battery cables is likely the issue with the slow cranking.

New "OEM" cables are no better than parts house replacements.

The best fix is to remove the side terminal ends and solder on copper eyes.

Corrosion starts deep in the cables and the crimps are usually not soldered.

I have fought this fight before and ended up replacing the cables with fine strand welding cable and soldering on all the eyes.

The small run the feeds the chassis junction block also needs to have an eye soldered on the battery end.

The grounds are every bit as important, both at the battery and the engine block ends.

Make sure to connect the grounds that go from the battery neg terminal to the sheet metal as well.

Once these items have been addressed things should be fine.

seems perfectly normal to me,. 9+ Volts when the glow cycle is on is about right,.
If it's starting ok, i say drive on!

Make sure the grounds are clean at the passenger rear stud, and the ground strap on one of the starter bolts to the frame. I actually run an additional ground from a starter bolt back to the battery.