This has been happening for a while and since it's just the light I haven't really given it a ton of thought but figured I should at least try to figure this out at some point. 98 with good ol J shifter. It hasn't been truly warm in a while and I've only needed it when cold and it's always been cold where I go wheeling so far. Pretty sure it was coming on fine for the first few weeks I had the truck in late fall and we were still seeing 70s. If I've been driving a while and engage it it comes on right away but if the truck is cold it won't engage till I've driven a bit. I assume it has something to do with the temp of the front diff which is leading me to the ADD position sensor. Think I should pull the sensor and try and clean it up and maybe throw some contact cleaner on it? Seems to me that's the only option. The transfer and ADD are definitely engaging as I'm getting binding on dry pavement. Anyone else ever had this issue? Kind of bugging me now that I've addressed many of the more pressing issues with the truck and I need to track down the smaller gremlins like this. Also have an issue with the ABS that seems to be vaguely related to temp that happened after I changed the second side on my axle seals (I think I might have damaged the wire going to that sensor).

One other strange detail I almost forgot about. If I have it engaged with no light and go back into 2hi the light will come on very dim (so dim I can only see it at night) for a period of time.

Very common bad switch issue. My one is hit and miss on the 4wd light. Most likely, you need to replace that switch to correct thar issue..

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Interesting that I didn't find that issue in a search, I suppose I'll try to pull it out and clean it and if that doesn't fix it I'll see about snagging one at a pick n pull when I go there for some lower control arms soon (figuring it will be less headaches if I grab a set of arms for $40 to swap my whiteline bushings into rather than pulling the arms on my truck and having it potentially on stands for a while if I run into issues).

It's 50 /50 on the used switches. Cleaning may work.

Mine been iffy for at least 5 to 7 years now. As long as the 4wd engages. I am happy.. lol.

I have a limited 97 that doesn't like to come out of 4wd. Gotta love the actuator controlled ones.

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I would suspect the ADD switch or the wiring circuit for it. That switch/circuit is responsible for illuminating the 4wd dash light. Since the front ADD is engaging then the 4wd switch/circuit on the transfer case is functioning properly and powering the ADD control relay to switch vacuum to the ADD actuator.

You can try removing the switch and cleaning it with contact cleaner while depressing the switch multiple times to see if that fixes your issue.

Being it's works sometimes. I thinking it's the switch. As if it were the wiring it probably wouldn't work at all. Granted anything is possible. Toyota was smart to use 2 switches for that indicator circuit. Makes troubleshooting easier.

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That's what I'm thinking, diff warms up, switch warms up and things expand just enough to where a short becomes a contact, maybe because of a dirty contact in the switch. Wouldn't make sense with the wiring as I can' see that getting warm enough to be subject to thermal expansion. Maybe if it's really close to the switch or even in the connector to the switch, I'll make sure to spray that too.