4WD dash indicator light on '77 with TH350

[mw67rs]

I have recently acquired a '77 Chevy truck. It needs a new engine so I have purchased a crate engine for it. I was driving the truck to test out various systems to see what needs attention. This is my first 4wd truck so I don't have much experience in the 4wd system. My truck has full time 4wd. When I shift the transfer case into Lock the dash indicator light comes on some of the time but not every time. I think the lock is engaging but cannot say for sure.

Does anyone know how this indicator light is triggered? Is there a switch in the transfer case or linkage somewhere that triggers this light? If so, where could I find it?

Also, any way to tell if the Lock is engaged even if the light is not illuminated?

Thanks for any advice.

[ray_mcavoy]

Originally Posted by mw67rs

I have recently acquired a '77 Chevy truck. It needs a new engine so I have purchased a crate engine for it. I was driving the truck to test out various systems to see what needs attention. This is my first 4wd truck so I don't have much experience in the 4wd system. My truck has full time 4wd. When I shift the transfer case into Lock the dash indicator light comes on some of the time but not every time. I think the lock is engaging but cannot say for sure.

Does anyone know how this indicator light is triggered? Is there a switch in the transfer case or linkage somewhere that triggers this light? If so, where could I find it?

The light is activated by a switch mounted on the transfer case shifter mechanism. The arrow in this pic is pointing to the switch:



It's a plunger type switch that gets pushed in (to close the contacts) when shifted to high or low lock. Since yours only works sometimes it probably needs to be adjusted. Or the shifter mechanism might need to be cleaned & lubricated so it fully engages the lock position.

Originally Posted by mw67rs

Also, any way to tell if the Lock is engaged even if the light is not illuminated?

It's kind of hard to describe, but you should be able to feel a difference in the way the truck drives when the inter-axle differential in the transfer case is locked out (shifter in high or low lock). It'll be most noticeable when making a turn. Unless you're driving on a really slippery surface, you should be able to feel it sort of "binding up" a little and not wanting to go around a turn as sharp as when it's unlocked.

[enaberif]

Just be forwarned not all NP203s came with this switch. Mine did not.

[ray_mcavoy]

Originally Posted by enaberif

Just be forwarned not all NP203s came with this switch. Mine did not.

You're right ... The 4 Wheel Lock indicator light was added part way through the 1976 model year when the instrument clusters were revised to include some extra indicator lights. So the earlier year NP203 shifters won't have the switch.

But mw67rs's truck must have the switch because he said the light has worked a few times.

[enaberif]

Originally Posted by ray_mcavoy

You're right ... The 4 Wheel Lock indicator light was added part way through the 1976 model year when the instrument clusters were revised to include some extra indicator lights. So the earlier year NP203 shifters won't have the switch.

But mw67rs's truck must have the switch because he said the light has worked a few times.

Yeah going to say the shift mechanism needs a cleaning and the levers adjusted.

[mw67rs]

Thanks for the replies. We exactly what I was looking for.

I plan to pull the transfer case out and will clean and lubricate the linkage.

Does the transfer case need to be drained before it is removed? Will is spill fluid when removing if it is not drained?

And if it does need to be drained, any tips on doing so? I see a fill plug but not drain plug.

Thank for all the help.

[enaberif]

Always a good idea to drain. For the NP203 when your looking at the back the large circle on the right side undo the two bottom bolts and it'll drain.

[mw67rs]

Thanks. I will do that. Is NP203 the name for the transfer case?

[enaberif]

Originally Posted by mw67rs

Thanks. I will do that. Is NP203 the name for the transfer case?

Yup.

[rich weyand]

Why are you taking it out? It weighs 250 pounds and is a major PITA.

To clean the xfer case shifter in place: Put a large flat pan under the shifter linkage to catch all the crap. Remove the shifter knob, trim ring, and boot from the inside of the truck. Hose down the shifter linkage with PBBlaster or Silikroil or WD-40, while moving the linkage back and forth through all the positions. Use about half the can. Let sit overnight. Hose the whole thing down again with the other half of the can, again moving the linkage back and forth through all positions. It should end up shifting silky smooth. Now pour about half a cup of engine oil over the shifter and let it drain into your pan. Replace boot, trim ring, and shifter knob and you are good to go.

[mw67rs]

I am replacing the engine and transmission and from what I understand the transfer case must come out when the transmission comes out.

[rich weyand]

Yup, the transmission and transfer case are most easily dropped as an assembly.

If you find you need a new 4WD lock indicator lamp switch, you will not find it listed that way. The same part number was used as the throttle position sensor switch for the cowl induction door motor on the 1969 Camaros. That you will find listed as a repro part, along with the clip it mounts on.