My biggest quarry is this, ONLY when the engine is OFF does the light come on and stay on (but not key off sure)
that hints of wired wrong.?
the switch does not move unless engine is started, only vacuum will do that. (via transfer case valve actions)
the switch does not move when you key off. or stall the engine,
There are no springs in the servo motor at all.(oem stock)
or alien wrong servo motor installed that has springs dead to rights wrong.?<< called jerry rigged.(ww2 slang)
also if hoses are pulled it is possible for sure that the servo motor is stuck half way ! and lies to you. (the switch too, may be intermittent 30 years old)
so shift to 2wd driving dead slow for sure 1 tire up front or both on dirt or ice and not pavement so it can completely UN- dog the part ID 26 and not have
the switch get confused. (1/2 in and out)
I pay no attention to my lamp , because I know if the front is l
ocked or NOT by how my hands feel, if locked in 4wd the steering fights my hands.
for sure on pavement. I need no lamp at all , (and mine is burned out).
part time 4wd is super simple and lets you know fast of front axle is locked, ( sure me singing to the Choir )
I can test the front axle , jacked up. for locked or not. and.
I can use my hand vacuum pump and lock it or unlock it at will. (a the servo motor nipples hoses pulled) at will. easy as pie.
I can cheat it and put yellow to RED hose jump and now in 4wd say stuck in snow.
if the vacuum motor diaphragm is cracked (leak down test fails,same tool ,SOP) it can cause it to fail at random times or is dead.
the same tool allows me to see if the transfer case valve is failing. using the vac gauge of the tool ,easy,
They lamp is not really needed by me, I know when the front end locks. and if the front prop shaftt works.
now what if the servo deep inside the diaphram is 30 year old and stiff as heck. and now acts like spring , it has mind of its own now, SHOT to H3LL
That is all I can dig up. ( keep in mind old cars can have a double failure here not just 1. ) mine had like 100 things wrong.(expected too)