This is on a 1988 F150 XLT Lariat with the straight 6 (4.9 L, 300 engine), 4-speed transmission. Primary problem is that the starter is very slow to start the engine even when jumping off another vehicle. It is now impossible to start off the battery, just makes a very rapid clicking noise somewhere in the vicinity of the starter relay. The battery, battery-relay cable, the relay itself, and starter (twice) have been replaced. Still doing the same thing. Also used a new cable temporarily to go from the relay to the starter. The ground cable is good. This problem was at first intermittent and then over several weeks got progressively worse. I was able to get the truck started today and let the engine run to fully charge the battery. Noticed that the dome light would not turn off. Believe that this model has the switch at the door hinge (don't see any wires between the door and frame and the door has nothing electrical in it). Pressing the switch caused the dome light to dim slightly. In addition, the tachometer jumps when pressing the brake pedal. I've never noticed this before and believe it's related to this whole mess. Hopefully someone has had some experience with this. Have the Haynes manual and guess will start looking through the electrical diagrams.

 

Isn't rapid clicking a symptom of low battery power at the starter relay? There's enough to cause the relay to close, which causes the starter to "suck" lots of amps from the starting circuit, which depletes the supply at the starter relay so much it opens. And then, since there's no longer the "heavy" current draw, the whole cycle starts all over again.

You didn't mention your alternator in all this. Are you sure it is charging your battery properly?

And, if the dome light is staying on, it will gradually deplete your battery. The inside lights on my truck are controlled by a door switch at the latch, not the hinge. Isn't there also a switch on the dome light itself you can move to manually turn it off and keep it from coming on? My truck, though, has other things that come on too when a door is opened that are NOT manually controllable - yours may have these too. If so, about all you can do is remove bulbs or fuses until you can fix the door switch problem.

A slow start while jumping might be nothing more than the gauge of the jumper cables. If your battery is close to "dead" then all the starting current has to come through the jumper cables. If they aren't at least 6 gauge (lower numbers are better), you can't pull enough current through them to power the starter properly.

I haven't any thoughts about the tachometer at all.

- Jack