What should the voltage be ?

when the glow plugs cycle voltage should be the same on both heavy terminals and the same as battery voltage on both batterys.

Ok I'll check it out

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I can just use an ohm meter ?

if it is set to DC volts-sure,ohms won't tell you anything.

How hard is it to change the back crank shaft seal ? I understand you have to take off th oil pan and it is a 2 peice seal but how hard is it to change ?

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Is there a good break down of th 6.5 whole engine I could find somewhere?

Unless you have an 82 6.2 you will have a 1 piece rear main seal.

The oil pan does not have to be dropped to change it,I use something along the lines of about 3 drywall or wood screws about 1 1/2" long spaced evenly around the seal near the outer edge when doing 1 in the vehicle-just scew the screws in evenly to push the seal away from its locater.

To install there is a special kent moore tool-I don't have that so I pack the back side of the seal with bearing grease to hold the spring in place and find a ring the same size as the outer edge of the seal,place it over the crank centered in its locater and the crank and gently and evenly drive it into place.

So the tranny and clutch would have to come off right . And while I have that Off I mite as well change the clutch, and do you know where I could find a manual for this whole truck engine and tranny?

Yes trany clutch and flywheel need romoved-get flywheel resurfaced if changing clutch.

ebay is the place to look for gm manuals.

Ok well i still haven't fugured out this glow plug code p0380 glow plug/ heater circuit A , I changed glow plugs and celiniod , what's this intake heater thing tho ?

There is a fuel heater in the filter housing,it could be that it isn't working or even plugged in?

Since there is a fuel temp function it is likely a problem with the heater.

you have to remove the filter housing to replace the heater and it is mounted in te bottom of the housing with a plastic nut similar to the filter in the top.
a new O-ring for the heater and also get an O-ring for the water in fuel sensor while you are at it.

The fuel heater is so seldom a problem besides leaking thet it didn't occur to me to consider it from the start.