Starter Solenoid, I have been through 4 and counting! Options

[ctc911ctc]

1974 2.0 - 24Kmiles
All stock

I received the car in late 2018, pulled from a garage in RI. Sat in the garage since 1986/88

The starter in the car was original. We removed near everything in the car, super cleaned and welded, put it all back together, and also bought a new starter. The new starter would not work right - clicked but no turn - so we put the original back in. Ran great.

Then the original started to fail (seemed like the copper wire from solenoid to motor had degenerated so we bought a 3rd. This one would not work so I returned it and got another. It worked great for a month and now this one clicks but the motor does not turn. Had to jump it with a screwdriver. Which though dangerous - works great!

At this point, over 2 years, I have 4 failed starters. One is 46 years old, the others are rebuilt (2) and new 1.

Which way would you go from here?

Another new OEM? (IMG:style_emoticons/default/blink.gif)
Another rebuilt???? (IMG:style_emoticons/default/barf.gif)
Put a relay in parallel with the Solenoid? (IMG:style_emoticons/default/smash.gif)
Get the super fancy Porche High Torque starter? (IMG:style_emoticons/default/first.gif)
Always park on a hill (facing downhill of course). (IMG:style_emoticons/default/lol-2.gif)

[Superhawk996]

OK. Like the troubleshooting aspect of the bypass switch.

12 Gauge wire is a bit small. 12 gauge wire generally OK for 20 Amps. Not sure what the solenoid draws, but, I'll grant that it isn't 20 amps. Plus the length isn't very far - just from B+ to the solenoid. Does this bypass switch work or do you get the same bump/clunk noise but no starter motor & engine rotation? I think from previous post you impled it doesn't work until you completely short from B+ to the starter motor winding terminal (i.e. bypassing the solenoid completely). Does this work every time?

Now here is where it gets interesting. So you have 12v with no load (open circuit). Good.

When you attempt crank you're saying you're dropping to 9 volts. That is a lot of sustained voltage drop! Battery should support continued cranking and not drop to 9 volts continious. Are you sure you've got a good battery?

I just tested two good alternators on bench and I get a 9.75 volt only momentarily when the motor is experiencing a large inrush current. After that it's more like 10.5 volts as the starter motor is runing at free speed.

The sum of voltage drops around a series circuit has to equal the potential across the battery. (Kirchhoff's Voltage Law).

So in my case what we are saying is the motor is dropping 10.5 volts as it runs. (measured from B+ starter termnal to ground). Where is the other 1.5 volts dropped? In the wiring (crappy jumper cables in my case).

The starter motor windings measure 0.1 ohm or slighly less (resolution of my DMM limited) on both starters. How much current are we drawing at inrush? If voltage = 9.75 then by Ohms law we are drawing 9.75 volts/.1 ohm = 97.5 amps at inital motor start (in rush). Not unplausible.

So now back to your problem. Where is your other 3 volts going?

Possibilities:
1) Wiring drop. Still the most likely cause. Ohms law Voltage = current x resistance. More resistance with same current draw = higher voltage drop. If you wiring were running 2x the resistance of my jumper cables that would lead to a 3 volt drop. Now we're back to poor grounds or high resitance wiring adding restance.

If you have a bad ground at body to transmission or battery to body and are trying to draw 100 amps of inrush current we only need 0.03 ohms of resistance to drop 3 volts in your wiring. You need a ZERO ohm battery ground to body ground connection and a ZERO ohm Body to engine ground. When you're potentially drawing hundreds of amps, it doesn't take much resistance to add up to a big voltage drop in the wiring.

Also make sure that you have a clean connection between the motor side wire (other terminal on the solenoid) and that the nut is tight and clean.

2) Starter motor loading is way higher (drawing more current at stall - i.e. can't rotate the engine for some reason). High compression motor that stock starter cant turn? You don't mention this so I'm going to assume this isn't the issue.

Quick check -- if you push / bump the car while in gear and get the current cylinder past top dead center does it then spin the starter motor until the next piston tries to get to TDC on your next starting attempt?

3) Battery is weak and can't support voltage and current under load.

Hope some of this sheds light on why grounds are such a big deal. Good luck! Here to help if I can.

P.S. As I'm thinking about your 3v drop -- I keep referring to ground but you also need a ZERO ohm connection between Battery positive and the B+ terminal on the starter solenoid.