85 Mustang electrical/starting issue

Never, never disconnect an alternator from the battery with the engine running. The resulting voltage spike can damage the car's electronics including the alternator.

You have a bad connection somewhere; use the voltage drop testing step in the following test path...

No Crank checklist for 5.0 Mustangs

Revised 24 May 2018 to add check for backup lights harness as a no crank problem.

No crank, slow crank and stuck starter solenoid problems have the same root causes – low battery voltage and poor connections. For that reason, they are grouped together.

Use the same initial group of tests to find the root cause of slow crank, no crank and stuck solenoid problems.

Since some of the tests will bypass the safety interlocks, make sure that the car is in neutral and the parking brake is set. Becoming a pancake isn’t part of the repair process…

1.) Will the car start if it is jumped? Then clean battery terminals and check battery for low charge and dead cells. A good battery will measure 12-13 volts at full charge with the ignition switch in the Run position but without the engine running.
A voltmeter placed across the battery terminals should show a minimum of 9.5-10 volts when the ignition switch is turned to the Start position and the starter engages or tries to engage. Less than this will result in a clicking solenoid, or slow cranking (if it cranks at all) or a starter solenoid that sticks and welds the contacts together.

Most auto parts stores will check your battery for free. It does not have to be installed in the car to have it checked; you can carry it with you to the auto parts store.

The battery posts and inside of the battery post terminals should be scraped clean with a knife or battery post cleaner tool. This little trick will fix a surprising number of no start problems.

The clamp on with 2 bolts battery terminal ends are a known problem causer. Any place you see green on a copper wire is corrosion. Corrosion gets in the clamped joint and works its way up the wire under the insulation. Corroded connections do not conduct electricity well. Avoid them like the plague...

If the starter solenoid welds the contacts, then the starter will attempt to run anytime there is power in the battery. The cables and solenoid will get very hot, and may even start smoking. The temporary fix for a welded starter solenoid is to disconnect the battery and smack the back of the solenoid housing a sharp blow with a hammer. This may cause the contacts to unstick and work normally for a while.

A voltmeter is handy if you are familiar with how to use it to find bad connections. Measure the voltage drop across a connection while trying to start the car: more than .25 volts across a connection indicates a problem. The voltage drop tests need to be done while cranking the engine. It's the current flowing through a connection or wire that causes the voltage drop.

See http://assets.fluke.com/appnotes/automotive/beatbook.pdf for help for help troubleshooting voltage drops across connections and components. .



Voltage drops should not exceed the following:
200 mV Wire or cable
300 mV Switch or solenoid
100 mV Ground
0.0V Connections

A voltage drop lower that spec is always acceptable.

2.) Check the battery to engine block ground down near the oil filter, and the ground behind the engine to the firewall. All grounds should be clean and shiny. Use some sandpaper to clean them up.
3.) Jump the big terminals on the starter solenoid next to the battery with a screwdriver - watch out for the sparks! If the engine cranks, the starter and power wiring is good. The starter relay is also known as a starter solenoid.

The rest of the tech note only concerns no crank problems. If your problem was a stuck solenoid, go back to step 1.

4.) Then pull the small push on connector (small red/blue wire) off the starter solenoid (Looks like it is stuck on a screw). Then jump between the screw and the terminal that is connected to the battery. If it cranks, the relay is good and your problem is in the rest of the circuit.
5.) Remember to check the ignition switch, neutral safety switch on auto trans and the clutch safety switch on manual trans cars. If they are good, then you have wiring problems.

Typical start circuit...
Diagram courtesy of Tmoss & Stang&2birds





6.) Pull the starter and take it to AutoZone or Pep Boys and have them test it. Starter fails test, then replace it. If you got this far, the starter is probably bad.

Starter solenoid wiring for 86-91 Mustang









Starter solenoid wiring 92-93 Mustang or earlier Mustang with upgraded high torque mini starter.



Electrical checks for the switches and starter solenoid

Remove the small red/blue wire from the starter solenoid. Use a screwdriver to bridge the connection from the battery positive connection on the starter solenoid to the small screw where the red/blue wire was connected. The starter should crank the engine. If it does not, the starter solenoid is defective or the battery lacks sufficient charge to crank the engine.

If the starter does crank the engine, the problem is in the clutch safety circuit (5 speed) or Neutral Sense Switch (auto trans) or ignition switch.

See the Typical start circuit diagram above for wiring information for troubleshooting.

You will need a voltmeter or test lamp for the rest of the checks. Connect one lead of the voltmeter or test lamp to ground. The other lead will connect to the item under test.
Look for 12 volts on the white/pink wire when the ignition switch is turned to the Start position. Check the ignition switch first.
No 12 volts, replace the ignition switch.

The next step will require you to push the clutch pedal to the floor (5 speed) or put the transmission in neutral (auto trans) while the ignition switch is turned to the Start position.
Good 12 volts, check the clutch safety switch (5 speed) or Neutral Sense Switch (auto trans) for good 12 volts on both sides of the switches. No 12 volts on both sides of the switch and the switches are defective or out of adjustment. Check the wiring for bad connections while you are at it.



The next check is courtesy of Killing Hours.
For manual shift transmissions only: check to see that the backup light switch harness is intact and in good condition. There is a wire in the harness that loops 2 pins coming from the body side of the harness. This is the bypass for the auto transmission Neutral Safety Switch. If the connector is damaged, oil soaked or missing you will have a no crank condition.

Backup lights harness for 5 Speed transmission showing the bypass loop for the Auto Trans NSS
Courtesy of Killing Hours

I had a similar problem of what you are describing, if i had to guess, I'd say it was the starter. But don't guess, do the troubleshooting. Just sounds like "heatsoak" on the starter...

85Punisher said:

I have had my high torque mini starter rebuilt and no change. So I bought a new one from Powermaster 9607 and installed that, no change.

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Ok. But if you let the car sit, maybe for a few hours it will start no problem? And it retains a full charge on battery?

85Punisher said:

That’s correct. It only takes until it cools down. So like 30 minutes. Then it will fire right up. Yes somewhere between 12.55 v and 12.75 v. I’ve tried a few tests adding in a 2nd battery wiring it up in Parallel.

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That is strange so there is resistance being created after the car has been hot and running. Did you follow up on any of jrichkers post yet? Also im not totally knowledgable about electronics but I believe ground wire should match size of positive wire for your battery connections.

my old ford ranger does this actually. I know full well its fuel injectors but I don't want to pay the labor to take off the intake plenum.

85Punisher said:

Yea I’m working on upgrading the cable to all match. Figured I’d bump it all up to 0GA while I’m at it. And yes I’ve gone through most all of his troubleshooting. The whole voltage drop part is confusing to me tho. But I e eliminated all major working parts down to the cables or connections.

Click to expand...

the first paragraph in the "No Crank Checklist says "No crank, slow crank and stuck starter solenoid problems have the same root causes – low battery voltage and poor connections. For that reason, they are grouped together.

Voltage drop is measured between wire and component when there is normal load on the circuit. For a starter circuit, that means you try to crank the engine while the meter test probes are connected.

For more help on voltage drop testing, including YouTube videos, see
https://www.google.com/search?sourc...20.1700.0..0i131k1j0i22i10i30k1.0.qE3J80GND-0

Keep working on cars and you will learn that Google and YouTube are valuable resources when you need some further explanation of fundamental principles.

85Punisher said:

Hey guys/gals,
I have an 85 Mustang GT w/sbf 306 built up. I just redropped the distributed back in and set timing to 16 deg. I recently did a wire tuck into the fender well and upgraded my charging system w/SVE 3g alternator and cable. Relocated the battery to the trunk w/2g wire coming back to the solenoid. Motor runs great at cold startup.

I’m having a weird issue starting my car. With a fully charged Optima battery reading 12.7v it starts the engine no problem when cold. I let it run for 20-30 mins, shut it off, and then it won’t restart after hot. Sounds like drained battery. It cranks and tries to turn over but there’s just not enough juice or something. Battery tests 12.7v when off still, when running it shows around 14.2 depending on what accessories are on. At the alternator when running it shows about 14.7v. I disconnected the negative battery while running and the alt holds the charge and stays running. I even had the batt load tested and it’s good. I tested my ground and that’s good. While cranking the voltage drops about 1v cold. When hot the voltage drops 2v. What am I missing here? Seems too easy!

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It looks very, very much like a problem in the starter, I had a similar situation. There is a part in the starter that electricians call "brushes". From time to time it is necessary to replace them, as they wear out and fall into disrepair, because of this the contacts are disturbed and the starter simply stops working. Also, when there is a problem with the starter brushes, the car suddenly starts badly. The engine ceases to emit a smooth calm roar. Instead, it makes a suspiciously high-pitched noise. All you need to do is drive up to an electrician and he will fix the problem very quickly, replacing the brushes will cost you very little and is done fairly quickly.

danielmeyers said:

It looks very, very much like a problem in the starter, I had a similar situation. There is a part in the starter that electricians call "brushes". From time to time it is necessary to replace them, as they wear out and fall into disrepair, because of this the contacts are disturbed and the starter simply stops working. Also, when there is a problem with the starter brushes, the car suddenly starts badly. The engine ceases to emit a smooth calm roar. Instead, it makes a suspiciously high-pitched noise. All you need to do is drive up to an electrician and he will fix the problem very quickly, replacing the brushes will cost you very little and is done fairly quickly.

Click to expand...

@danielmeyers
You can check on the date of the last post (2018) to see if you are responding to an old post. The Similar Threads at the bottom of a thread have lead many of us into answering zombie threads.
Using the search feature is a good way to find info before making a new post, so of that’s how you found this, you are off to a good start.