Troubleshoot starting problems

Hiyadoin all.

'84 F100 w/6

It has cranked slow, but always started for months.

Before, with extended cranking (still only 5 second crank, but lots of them) I notice battery cable to solenoid heats up, smokes insulation. Truck still cranks and starts.

Newly cold weather, needed extended cranking time, ran out of cranking ability.

Charged battery (10 amp, 10 hours), still very slow cranking, overheating battery to solenoid cable.

I guess my question lies with that overheating pos cable to solenoid - indicate starter failure - solenoid failure? I tend away from battery, just because fresh charge should eliminate it until a little later when battery has time to run down.

Oh, once truck has started and warmed, pretty much cranks alright and starts fine.

Any ideers?

thanx.

Hi, FNA, and welcome to the forum. I'm not familiar with your vehicle (I'm in the UK and we don't have them here) but I'm an electronics engineer so I'll make a few comments anyway, from an electrical point of view.

I suspect that the cable from the battery positive terminal to the starter solenoid has started to corrode and go high-resistance. That cable needs to be extremely low resistance (a tiny fraction of one ohm) because of the huge current (100 amps or more) required by the starter motor - that is why it is such heavy gauge cable, since the greater the cross-sectional area, the lower the resistance, so therefore the heavier the better.

If the cable starts to corrode and go high-resistance, then a voltage will be dropped across it (thereby reducing the voltage available to operate the starter motor) and also it (the cable itself) will dissipate power, or in other words it will heat up when a current flows through it. I won't bore you with the maths, but it only takes a tiny increase in resistance to cause exactly the problem which you have described - an increase in resistance of just 0.1 ohms would be a disaster on that particular cable on any vehicle.

As it happens, I had almost exactly that problem back in November on my diesel Escort - it wasn't the cable from battery to solenoid, but it was the continuation of that cable which runs from the solenoid to the starter itself - it's just about four inches in length but it carries the same massive current, and rainwater had leaked into it and the copper cable had corroded to the point where most of it was little more than dust. Anyway, it finally overheated and burned itself out. I was lucky though - I was able to get away with just replacing that little section with a new length of heavy-duty cable and I was back in business.

So that's the most likely explanation - water ingress into the cable causing it to go high resistance, in which case it will need to be replaced.

The other possibility is that the starter motor itself has started to fail in such a manner that it is drawing excessive current and overloading the cable, thereby causing it to overheat and smoke. However, for it to do that, the current would have to be enormous even compared to the normal 100 amps or so, and I would have expected that the starter motor would simply have failed to operate at all by now.

All of which is a long-winded way of saying that I think your cable is scrap and should be replaced. It's certainly the easier and cheaper option, compared to starter motor replacement.

In case you have access to a multimeter and want to investigate the fault further, I'll make some more comments but I'll put them in a separate post to make it easier to follow.

with a voltage metor attach to the batt. you should not have any more than two volt drop. test your voltage at the starter bolt as well as the batt. post. this way you can test your cable as well as the starter.

The obvious way to test that cable would be to disconnect it at both ends and then measure the resistance across it, but it isn't quite as straightforward as that because any regular meter simply doesn't have either the resolution or the accuracy on its resistance range to measure such extremely low resistances as the resistance of that cable.

Therefore the only meaningful checks that you can make would be voltage checks, whilst the starter is operating. It would be helpful if you knew how to temporarily disable the ignition circuit to prevent the engine from firing whilst you make those checks - I don't know what that procedure would be for your vehicle. However, if you're able to do that then here are the checks which I would make, whilst the starter is cranking:

1. Measure battery terminal voltage
2. Measure voltage at the solenoid end of the battery-to-solenoid cable - should be the same as reading #1. If it's significantly lower, then that means that the cable is high resistance and should be replaced.

If the cable voltage reading is okay, then the next check would be to measure the voltage at the other side of the solenoid but I don't know if your solenoid is the type which is mounted on the body of the starter or not so I can't really go any further than that right now. If it is that type of solenoid then the voltage at the output side should be the same as at the input side, and any significant voltage drop across it would indicate a faulty solenoid (poor internal electrical contacts). I'll leave it that for now though because I do fully expect that you will see a large voltage drop across that battery-to-solenoid cable.

Oh yes, one other thing - I'd unbolt that cable at both ends anyway and clean the connectors and terminals at each end with a wire brush and see if that makes a difference.

Thanx very much for your responses, both of which seem very reasonable.

I now hold hope that it is the cable - the ugliness of which I took to be a symptom of the problem, not a potential cause.

By it's look, I had intended to replace the cable as part of whatever repair I would make - for the money, I think I may start there.

Thanx also for advising to look for voltage drop.

Thanx again for your helpful and promptly made suggestions.

I had to replace the cable and the solenoid; it cranks (and starts) like a champ now.

Any repair I can make standing up instead of on my back on the ground under the truck is a good repair.

Again, thanx for your help.

That's good news, and thanks for letting us know - it's always good to hear what the final resolution was.