Electrical Starter Solenoid Keeps Breaking. Urgent Help Needed.

Very common problem- your solenoid is welding in the open position, usually associated with bad grounds, a faulty ignition switch, and aftermarket solenoids. Sometimes you can tap on the solenoid with a screwdriver and it will release the plunger but once it is welded together it's toast.

Your starter keeps engaging even after you turn the key off?

The reason the wires are getting hot and smoking is because the solenoid is still engaging the starter.

First, make sure the mounting points on the solenoid where it attaches to the fender are clean and shiny. Second, only use a FOMOCO starter solenoid- the aftermarket ones are garbage. Check all of your ground wires. All of the ground connections need to be intqct, tight ,free from rust, and connected to a good ground.

A way to test the ignition switch is with the engine cranking and the key not in the start position, pull the wire off the S terminal on the solenoid. If it stops cranking then the ignition switch on the column is bad. Otherwise you will need to check the other items above.

I solved this issue on one of my mustangs by running an extra groundwire from the negative battery cable to one of the solenoid mounting bolts.

I had this issue badly with my fastback. It would go through solenoids pretty fast and weld them shut, almost lighting the car on fire on multiple occasions. I fixed it with a new battery, new high torque mini starter, and a new solenoid all in one shot. I had an old battery and my old starter was drawing too much current as well, so a combination of issues.

If your running the stock 60-78amp alternator; you will have dimming lights at idle & lower voltage readings. If your running under-drive pulleys with the facxtory alternator; it only makes the situation worse. That's why many such as I have converted over to the 3G 130 amp alternators.... The barely adequate alternator is just one of many design weaknesses we inherit with our beloved foxes..... We won't delve into the ignition modules nor brakes at this time.....
I experienced the same issue about 2 years ago. I find it odd since I had already upgraded everything imaginable; additional grounds, 3G alt., gear reduction starter, larger dash one cables everywhere, 1200 CCA battery, etcetera. Basically one to three starts on replacement solenoids before the contacts welded together & I'm racing to pull off the positive cable..... It was only resolved when I went to Autozone to buy their best premium solenoid versus the old factory unit & Napa replacements.... This makes me think that the parts store variety of starter solenoid quality is going south IMHO.....

You have to have 13.8-14.2 volts at 1000 RPM or higher to properly charge the battery.

No Crank checklist for 5.0 Mustangs

Revised 24-Oct-2013 to update voltage drop figures.

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. Doing a voltage drop test without putting a full load on the circuit under test is useless: it won't find the problem.

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.

damn, clean up some of those connections. I hate crimped connections, they always tend to loosen up after time. Either solder them or get some shrink wrap tubing.

The third picture of the battery terminal with the 2 bolt clamp was probably the source of your problem...

Anything that was melted or overheated last time around you should replace, even battery cables that look ok. Once they get fried it's hard to judge how safe they are or how much additional resistance they are inducing into the circuit while starting