Timing chains usually do not need to be changed until an engine needs to be overhauled. You may need to change it if it makes a weird noise, other than that you should be fine.
its a belt and my dealer couldn't tell me when it needs to be changed, so off to another dealer who said its at 100,000miles but get the tension units changed.
Normally an engine this old that needs a new timing chain, also needs an overhaul. I cannot tell you in this limited space how to change the chain and gears. I suggest you purchase a Haynes, or Chilton's repair manual.
That's the 2.6 Mitsubishi right? It has a chain...
You will have no symptoms that it needs to be changed. Also looking at it will not tell you whether it needs to be changed or not as it may look good and be ready to break. For this reason timing belts are required to be changed at a set mileage or years interval. Referred to your owners manual as to the maintenance schedule on the timing belt. It varies with manufacturer from 60,000 miles 110,000 miles.
One way to tell if the timing chain is broken on your Nissan Altima is to see if it will start. If it starts, it is not the timing chain. You can also remove the distributor cap, have someone step on the gas while you watch the engine. If the rotor moves, it is not the timing chain.
I am almost certain that the timing chain should last throught the life of the Explorer.
Highly doubtful, the engine has a timing chain and the ignition relies on a crank sensor for timing. But if the chain did jump you could probably tell by doing a compression test.
You did not list Make, Model, Year, or Engine Size. So all I can tell you is that the vast majority of vehicles with a timing chain have a self tensioner that keeps the tension tight. If the chain it loose or noisy then it is more than likely worn out or the tensioner needs replacing or both.
It uses a timing chain
You can tell a timing chain is bad when your engine makes a sharp washboard-like sound that increases in intensity when the engine's RPMs increase.
The chain will become noisy and you may experience poor performance.
You cannot tell by looking at it. For this reason it must be changed at a set time period of miles or years. Without knowing the make, model, engine, & year I cannot tell you when it should be changed. Warning: If the engine is an interference engine and the belt breaks you will have internal engine damage.