I've been fighting with an intermittent problem for a while. Sometimes the car would start, other times, it merely clicked. In the process of repairing it, I replaced the battery, made new battery cables from welding cables, and replaced the starter.
The problem only grew worse and I noticed it only had problems when hot. A hot day, say over 95 degrees F would do it, or a drive to the grocery store. Then when it cooled down, it worked like a champ.
Eventually someone suggested the neutral safety switch, or NSS, was the culprit. I was still thinking of a relay somewhere and found multiple references to a starter relay. The only problem was that I didn't have one! After looking over a wiring diagram I came to the conclusion that the starter relay was only for manual transmissions.
So I went after the NSS and this is what I found:
On the right hand side of the transmission, where the shifter connects.
It was ugly and greasy and just plain dirty.
The backside wasn't any better, but I could also see a crack in the epoxy.
Only six screws hold it together and they come out easy.
There is a spot near the breather (blue arrow) where you can pry the two halves apart. it comes apart easily and you need to be careful to avoid losing any important parts. There isn't anything that will come shooting out, but there are two tiny springs that could easily get lost.
Inside the housing, you will find a swing arm with two tiny copper contacts in it. The springs are under those two items. I did notice there was no evidence of outside dirt in the housing, but the wear of 23 years was evident. The grease inside was dry and the contacts had just a slight tarnish on them. I cleaned all that and then used fine sandpaper to clean the contacts.
After I cleaned it all up, I marked the alignment line so I would be able to see it more clearly after it was installed. This line helps you to make sure the switch is reading your gearbox is in neutral/park when it really is in neutral/park.
There is also a round seal for the swing arm shaft. This was in good shape so I left it. There is another one just like it on the contact board half.
All parts clean and shiny and back together they go. The seal between the two halves was in good shape, so I kept it as well. I also put some epoxy on the back over the crack to prevent fluid entry if the crack ever got worse.
The switch is installed now. The red arrow indicates where the alignment line is. This line corresponds to the flat edges of the shift selector when the transmission is in neutral. Tighten the upper bolt, then tighten the castle nut to about 48 inch pounds, then loosen the upper bolt to allow mobility. Make sure you have a nice straight line, then tighten down the upper bolt.
Test it by starting the vehicle in both park and neutral.
This appears to have fixed my problem, though longer term testing is necessary which means I get to go out and drive it, which is why I'm going to all this trouble in the first place.
I'm just a noob with all this, so if a more expert mechanic has anything to say about it, including corrections or best practices, by all means, please speak up. I learned a ton about this problem and the process of fixing it and I hope my experience can help others.
Another interesting thing is that an old problem I've had seems to have gone away. The car tended to shift out of overdrive quite a bit and it was always worse when it was cold. While I was out driving it around, it would go into O/D and stay there until I slowed down or turned it off. I'm wondering if the O/D circuits through the NSS as well and bad contacts were causing those problems.