91 explorer starting issues

[firecam421]

the problem is the car will not start or even turn the crank while turning the key but if you leave the key in the on posititon you are able to jump it from the solenoid on the fender and it works every time it also works with the key once in a while but very rare i've replaced the solenoid, the ignition switch and the ignition key lock cylinder and the only change was after i put in the ignition switch it worked for about a day any info would be greatly appreciated thank you

[Selectron]

That sounds like a problem with the feed to the coil terminal of the fender-mounted starter relay (that's the solenoid which you have already replaced), and since you've already also replaced the ignition switch, the problem is likely to be the transmission safety switch. For a manual transmission, that switch is shown on my diagram as the Clutch Triple Function Switch (only allows cranking when clutch is disengaged) or for the automatic transmission it's shown as Starter/Backup Lamp Switch (only allows cranking when in Park or Neutral).

Your fender-mounted starter relay should have three terminals - two large and one small - when the ignition switch is turned to the Start position, you should be able to measure 12V on the small terminal with a multimeter, or verify its presence with a 12V test lamp - you'll probably find the 12V is either low or missing.

This is the path from the ignition switch to the coil terminal of the relay:

Manual Transmission:
From the ignition switch Start contacts, a red/light blue wire leads to the Clutch Triple Function Switch - the colour of wire on which it arrives at the switch isn't specified so it may emerge from a splice somewhere along the way on a different colour. Then the output from the switch is a pink wire leading to a connector, from where it emerges on a red/light blue wire, which connects to the coil terminal on the relay.

Automatic Transmission:
From the ignition switch Start contacts, a red/light blue wire leads to the Clutch Triple Function Switch Jumper (note that's only a jumper - not a switch, because the automatic isn't fitted with one). Then a pink wire runs from the jumper to the Starter/Backup Lamp Switch, from where it emerges on a red/light blue wire leading to a connector, from where it emerges still on a red/light blue wire, which connects to the coil terminal on the relay.

Something along that path - probably the transmission safety switch - is likely to be open-circuit, although it could also be a loose or corroded connector.

If you don't have a wiring diagram then go to this thread, follow the instructions in the first post and select your vehicle. The starter circuit is listed in the Wiring Diagrams section, and the item shown on the diagram as the 'Starter Relay' is your fender-mounted relay, and your fault lays between the ignition switch and the red/light blue feed into the coil of that relay.

If the voltage is indeed low or missing, then tracing the fault by voltage measurements would be awkward because of course you only get that voltage when the key is held at the Start position. It's much easier to switch a multimeter to its resistance range and find the problem by checking for continuity along the path (with the ignition switched off), and probably easiest to start at the relay coil terminal because it will be readily accessible, and follow the path backwards from there.