Starter not turning over when hot
 

Just bought a 72 - 350hp and PO told me he had been chasing this problem for a while.

I have read a lot on the Forum and goggle search.

My issue is after initial start and bringing the engine up to operating temperature, the starter stops functioning. It has a heat shield and the PO also wrapped the complete starter in heat shield material. The car needs to sit for 4-6 hours before it will start again. Even when everything is cool to the touch, still no starter engagement until 4-6 hours. I can jump the starter solenoid, using a screw driver and touching off the two posts . This solution works every time regardless of temperature and engages the starter immediately. I have also examined all the wires and tighten nuts on all junctions, but I feel if wiring was a problem, than I shouldn't be able to jump the starter solenoid.

I have seen many ideas on issues, but my puzzlement is, why can I jump the solenoid and what does this mean?

Any thoughts appreciated.

Thanks,
Jim

By jump the starter, do you mean engage the solenoid and the starter runs or jump just the motor and that runs.
is the an auto or manual car
bfit

Thanks for the question.

I mean I use a screwdriver to touch off the battery cable post on the solenoid to the other solenoid post on top. When I do this the starter engages and turns over the engine and car starts. Looking for someone that knows how these posts connect and if I need to replace the solenoid/starter or if I need to keep searching for a solution. The PO said he replaced the starter because of the issue (it looks new) but it didn't fix the problem.

Thanks

You have high resistance in the "S" terminal circuit.

I would add a Ford relay and forget about it. Not always a popular mod here but it does work and your car will start. The relay just replaces the screwdriver across the terminals.

Definitely not starter heat soak if you can jump the starter with a screw driver. Could it possibly be a fusible link down by the starter that's broken, but not 'burned up'? Heat could cause the insulation to expand/flex and cause the 'break' to separate just enough to lose connection. Cooled down, it might make contact again.

You have a bad solenoid. Replace it.

Check the starter relay switch on the steering column! The contacts get shorter over time. The switch can be adjusted, or replaced.

VS

The solenoid should be fine, as jumping it with a screw driver is only bypassing the 'key start voltage' - the solenoid is working if the 'screw driver' causes it to engage.

Also check the battery voltage, when the engine will not start ,could be that the alternator is not recharging the battery .

Won't crank when hot
 

VOLTAGE DROPPING your starting circuits is how you will diagnose this problem Jim.

You have two "starting circuits" The primary circuit, with the fat wires, which connects the battery to the solenoid, and the frame connection, engine ground strap, etc.

The secondary circuit is the keyswitch, the wiring to and from the solenoid, whatever feeds the keyswitch with 12V, starting relay (if there is one) etc.

We know the starter itself isn't your problem because you can short it and make it spin.

Either the starter motor isn't getting full battery 12.6+ volts, between the fat red wire and motor, or the solenoid isn't getting sufficient voltage from the ignition circuit, WHEN things are hot.

How do you tell which problem you have?

You put a voltmeter on first the solenoid lug (the big red wire coming from the battery) and attempt to crank the motor.

Of course you need to have the car either jacked, on a rack, in neutral, brake on, wheels chocked, AND have the distributor wire pulled so the engine can't actually start and run you over. That's a rookie mistake!

Put the + lead on the solenoid lug, and ground the other lead to the frame, or a starter bolt, whatever. Or even run a wire back to battery (-)

If you do this WITHOUT cranking the motor, of course you will see battery voltage, that is, the same voltage you would see if you put your meter directly across the battery posts. You have to LOAD the circuit to test it -- by attempting to crank the motor.

If you have a good battery, but see less than 9V when you attempt to crank the motor, you have a voltage drop.

It could be your battery terminals are dirty, corroded. It could be your battery to ground connection (frame) is poor. I got under my car and found a brand installed negative battery strap bolted on top of a freshly painted frame. Ground the paint off, applied a little anti-seize to prevent corrosion, re-bolted the battery strap, and my hot start problem went away.

It could be the nut on the starter lug holding the fat red wire is loose or corroded.

One other thing you've got to inspect / renew: Your frame to motor ground strap. Is it missing? Mine wasn't but I found the connection it was making to the frame was iffy. It needs to be shiny metal against shiny metal. I got my air grinder out, shined up both the lug on the strap AND the place on the frame it bolts to. Likewise where it bolts to the motor may need a refresh.

Now, let's assume the battery is charged, tests good (there is a difference -- you can have a fully charged, but weak battery!), and your PRIMARY wiring (read: fat battery cables) is all good.

Maybe we should have tested the keyswitch circuit to the solenoid first, eh?

Same deal. You've got the WHITE wire going to the start terminal on the solenoid. Put the + lead on there, or hook it up via alligator clip, whatever.

Give the other end a good ground --- either to the frame, a starter bolt, you can even run a wire all the way back to the battery (-) minus terminal.

Now have a buddy turn the keyswitch and attempt to crank the motor.. If you're not getting 10V or more at the start terminal when your buddy turns the keyswitch, you've got a VOLTAGE DROP in the starting circuit.

It' could be the keyswitch itself, the wiring to and from the keyswitch, even the 12V feeding the keyswitch. You will have to backtrack with your test probe until you find your VOLTAGE DROP -- a corroded connection, a worn switch, whatever. (I don't believe this model has a starter relay, but I could be wrong. If it does it could be burned contacts, etc).

NOW. If you're lazy and stupid like me, you can fix your hot no-start the hard way. Replace the battery and/or starter first. Then the other one. Then clean up your battery terminals and where your negative cable bolts to the frame. Install a new ground strap. And then after you've done all that and it still won't start when hot, you might as well replace the keyswitch & wiring from it because that's the only place left. Almost.

Voltage dropping is how you DIAGNOSE where a problem is with a no-start, instead of "throwing parts" at the problem. Voltage drops are commonly caused by:

Dirty or corroded connections
Loose connections
Cables with multiple strands broken inside the insulation (invisible)
Burned or worn contacts in switches & relays
Use of too thin wire
Motor/frame grounds bolted on top of paint, grease, etc.

Someone's gonna ask me "But won't he need his engine/car to be hot when you do this testing?"

No. If you've got a voltage drop, it's going to be visible (to your voltmeter) even when the vehicle is cold. It's just going to get way worse when everything is hot.

You can TRY a known-good battery instead of buying a fresh one, but if you take yours to AutoZone, etc to get tested, CHARGE IT first. (A tired battery is akin to a swimming pool that's half filled with silt, calcium build-up, sand. You can still fill it to the top with water, but it's only got half the capacity it did, and since the pipes are clogged too, it simply can't deliver as much water (amps) as fast as it used to. Even a brand-new battery will drop to 10V or so while cranking the motor.

Right now I'll bet $20 your problem is either in your starting circuit (keyswitch, wiring, relay...) or in your frame ground (battery (-) to frame connection. Or a ground strap issue: missing, or making poor connection to the frame & motor. Because that's usually what I find when I go after a no-hot crank problem.

So let's go back to VOLTAGE drop again... all wires all have SOME resistance. Even healthy ones. If current is flowing through a wire some voltage drop occurs. V = IR. The Voltage DROP through that wire equals the amount of current flowing through the wire, times the amount of resistance that wire poses.

When we discuss voltage drops, generally what is meant is excessive, unplanned for, accidental voltage drops. EVERY wire, connector, switch induces a LITTLE voltage drop.

In a 100A starting circuit, even an extra .01 OHM or .02 OHM of resistance --- corroded battery terminals, ground strap bolted on paint instead of shiny metal, ditto for your frame ground... can cause your starter to only receive 11V or 10V instead of 12. V = IR. V = 100 * .01 equals a 1.0V voltage DROP

When it gets down to 7 or 8 V it won't crank.

A tiny amount of resistance --- like that added by a ground strap bolted on top of rust, or paint --- will prevent your starter from getting as much voltage as your battery is capable of supplying.

A wire can have every strand but one broken inside the insulation, look fine, and show continuity, but will cause a measureable voltage drop in a circuit. You can't find it using your ohmmeter but detecting it using your voltmeter is child's play.

Your meter can not accurately measure resistances below, say, 5 ohms. But if you turn a circuit on, e.g. CRANK the motor, you can instead measure the Voltage DROP that's occurring, and use that to locate the excess resistance.

(Below 5 ohms how hard you press the test leads into whatever your measuring can vary the resistance reading by several ohms)

Think about it like this. Your battery is 12.6 volts. Turned on, measured at the back of the bulb, your dim headlights are only getting 11.9V. You've got a voltage DROP in your light switch, your dimmer switch, your fuse box your wiring...that all adds up to .7V. (Usually the solution with headlights is to add a relay and fatter wires instead of replacing the switch, the wires, ...) Or you could have a bad ground up around the headlights!

I hope I've explained this adequately.

You can VOLTAGE DROP a circuit, ANY circuit, to determine if TOO MANY VOLTS are getting "lost" between the battery and the circuit itself, making the circuit not operate properly (dim headlights) or at all - starter won't crank.

A lot of times having a ten foot test lead you can connect directly to one battery terminal or the other is a big time saver. Your VOLTAGE DROP can be on either the positive OR the ground side of the circuit!!

Another way to rule out voltage drops is DETOUR WIRING. If you run a jumper cable from the battery minus (-) terminal directly to your starter bolt, and it will crank even when hot, you have pretty much confirmed a bad, or dirty ground connection is your problem.

Voltage Dropping is the SINGLE MOST POWERFUL diagnostic skill you can have for locating problems on old cars.

The alternative is throwing parts at problems and hoping you get lucky.

Questions?

I don't expect many people to "get" voltage dropping on a single read-through. But if you commit to learning how to do it and go try it, you're gonna slap your forehead..."This is so gosh danged simple!" and wonder why nobody ever taught you this simple, highly powerful diagnostic technique

BTW, there are dozens of "how to voltage drop" videos on YouTube. I haven't watched any and can't pass judgement on them.

Cheers! .

Permanent Magnet Gear Reduction starter like what came on zillions of '96-up GM light trucks & suv. Years have Proven effective reliable service. They have NO Field Coils to use energy. They're much lighter. Way cheaper. All over amazombie fleabay for around $60 new delivered. helpful search term is PMGR. this is Not a hot rod item, it's an OEM replacement.

His post stating that he can jump the starter and make it crank (even hot) suggests he doesn't need a new starter.

It also SUGGESTS that the primary wiring is OK, and the problem is in the keyswitch, wiring to and from the solenoid, power TO the keyswitch, etc.

Why is it I realize that at 2 am after spending 2 hours writing an essay on voltage dropping the primary side. Oh well, it will help someone else with a hot start problem.

When I VOLTAGE DROP something like a dim headlight, I typically use a 10 foot test lead, connected to one side of the battery (say, negative) and "walk" the test probe back towards the battery along that circuit --- bulb socket, harness connector, dimmer switch, light switch, fusebox, battery post on the positive side.

Let's say the readings are 11.6, 11.7, 12.4, 12.5, 12.6. Obviously the dimmer switch is the problem as it has a .7V drop occuring (12.4 - 11.7)

Remember, you can have a voltage drop (12.6 to 11.6) on the positive side, OR on the negative side (0 to 1.0V...) in what is referred to as a "bad ground". Instead of "ground" being zero volts, a "bad ground" raises it.

Kapeesh?

How to verify the problem

Make a continuity light set up. with a 12v 5w bulb.
Connect one end to the solenoid on starter the other to ground. (Small terminal that actuates the solenoid)
the light should be on when you crank the engine.

If light is on or is a dull glow when the engine is hot and not cracking . then the is a hot joint some where

The fact engine cranks when you jump it , I would think you have a good earth.


You need to work back through the wiring,
The ignitions switch on the column may need adjusting. check this at the bulkhead fitting LHS firewall engine bay side 'purple wire ' you can fit a probe into the rear of the fitting. again you can use the continuity light here .
If the light is dull here there is a high distance ( hot joint ) bad contact . or short some where.
Bfit

A test light provides an easy, VISUAL indication of voltage drop. Turns out that a bulb at 12.0 volts is only about HALF as bright as a bulb at 12.6

I suggest making a test light with a ten foot lead so you can clamp it directly to the battery post even while working under the hood.

Thanks, I'll keep it in mind if I can't figure it out. I would hate to bypass the original in case there is a short, don't want a fire someday.

Thanks,
Jim

Thanks for the reply, I'll be trouble shooting this weekend.

Thanks for the idea, I will have to do some research on this one, not familiar with it.

Thanks, however the battery is keeping a full charge.

Wadenelson,

That was an amazing write up and I really appreciate all the info and time it took you to write it up. I'll be busy this weekend trying your ideas and will post once I figure it out.
Thanks again for your advice.
Jim

Good Idea

Thanks,
Jim