Large voltage Drop

[jpaz]

I added some extra grounds and it was a great improvement, no more weird electrical problems. I put a ground strap from back of engine to the firewall and a heavy 10 gauge ground wire from front of block to front clip.

 

[351c1971]

Absolutely, I think we are on the same page here. I am using HD after market cables and one cable is routed directly from the frame bolt to a bolt on the front of the block.
You have to be aware that they sometimes call motor mounts "Motor Vibration Insulators" for a reason.

 

[Don C]

I used a 4-gauge cable from the battery to the bottom screw on the regulator, then a 4-gauge cable from the regulator screw to the engine block. I also ran a ground (flat braided cable) from the block to the firewall. I believe a firewall ground gives a better chassis ground than the one on the fender apron.

 

[351c1971]

That should be good. I have seem many stock firewall to block grounds in Ford and non Ford applications.

 

[mrgmhale]

I like the idea of making some solid, additional grounds to ensure the engine and its mounted components (Alternator, Starter, sending units) are properly grounded to the rest of the vehicle. HemiKiller, jpaz, 351c1971, and some other folks all make some good suggestions re: how you can beef up the grounding to both the engine block and to the body/unibody. In our case, for both of our 73 Mustangs, I have been able to maintain a good connection to the unibody and the engine using the battery cable with the mid-point ground tab that HemiKiller shows on his first post on this thread. But, I also took the additional time to check for voltage drop during starter motor cranking to verify I have no excessive voltage drop between the negative cable and engine.

Were I to have seen excessive voltage drop (more than 0.2v) I likely would have fabricated a separate ground strap between the engine and unibody to ensure I have a good connection. There are several ways to accomplish that, but the suggestion HemiKiller gives in a subsequent post in this thread, where he has a red arrow pointing to his grounded connection in his 2nd post in this thread. I also like how tpj71mach attaches his negative cable ground tab to one of the mounting bolts for the voltage regulator (factory approach). That is how I ran the negative cables for both of our 73 Mustangs, and I made certain the mounting bolts for the voltage regulator that connects to the ground cable interim tab is tightened properly.

If there has been any repainting of the engine bay you may need to take the time to scrape off some paint to ensure a good connection to the unibody electrically, as paint acts like an insulator. I know that sounds like a real PITA to do, and it is. But, if you are having a voltage drop between the engine and the unibody, and you have the interim ground tab connected to the unibody, I suspect the mount to the voltage regular bolt for said tab could very well be the location of the bad connection to ground. Also, the mounting bolts for the voltage regulator run through the inner fender using machine bolts, not just large diameter sheet metal screws. I have found that getting a wrench on the nut for the mounting bolts for the regular can be a real PITA. It is possible someone, before you, only tightened up the mounting bolts/nuts up just enough to hold things in place physically, with no regard or consideration for the need to have a solid tightening in an effort to ensure a good electrical ground connection.

Okay, I think I have beat that dying horse about enough. I look forward to you posting what you eventually had to do to correct this problem.

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[Ja.mc44]

Factory ground wire went from Battery(-) to fender apron to engine block. If you don't have it grounded directly to the engine block, that's your issue.

I've never been a fan of the factory setup, as it relies on the lower screw for the voltage regulator to ground the chassis.


View attachment 71969

Ahhh I do not have it grounded directly to the engine block!
As of now I have the Battery (-) grounded --> Frame rail --> starter, and the engine grounded from the water/cylinder head pump bolt to the opposite frame rail. So they are both grounded to chassis but as separate circuits. Would this still not be applicable?
None of this front end is stock so i'm piecing the thing together. I'll give it a shot running the negative straight to the block.
Thank you for all the replies guys!

Any suggestions on best spot to mount on engine?

 

[351c1971]

I like the idea of making some solid, additional grounds to ensure the engine and its mounted components (Alternator, Starter, sending units) are properly grounded to the rest of the vehicle. HemiKiller, jpaz, 351c1971, and some other folks all make some good suggestions re: how you can beef up the grounding to both the engine block and to the body/unibody. In our case, for both of our 73 Mustangs, I have been able to maintain a good connection to the unibody and the engine using the battery cable with the mid-point ground tab that HemiKiller shows on his first post on this thread. But, I also took the additional time to check for voltage drop during starter motor cranking to verify I have no excessive voltage drop between the negative cable and engine.

Were I to have seen excessive voltage drop (more than 0.2v) I likely would have fabricated a separate ground strap between the engine and unibody to ensure I have a good connection. There are several ways to accomplish that, but the suggestion HemiKiller gives in a subsequent post in this thread, where he has a red arrow pointing to his grounded connection in his 2nd post in this thread. I also like how tpj71mach attaches his negative cable ground tab to one of the mounting bolts for the voltage regulator (factory approach). That is how I ran the negative cables for both of our 73 Mustangs, and I made certain the mounting bolts for the voltage regulator that connects to the ground cable interim tab is tightened properly.

If there has been any repainting of the engine bay you may need to take the time to scrape off some paint to ensure a good connection to the unibody electrically, as paint acts like an insulator. I know that sounds like a real PITA to do, and it is. But, if you are having a voltage drop between the engine and the unibody, and you have the interim ground tab connected to the unibody, I suspect the mount to the voltage regular bolt for said tab could very well be the location of the bad connection to ground. Also, the mounting bolts for the voltage regulator run through the inner fender using machine bolts, not just large diameter sheet metal screws. I have found that getting a wrench on the nut for the mounting bolts for the regular can be a real PITA. It is possible someone, before you, only tightened up the mounting bolts/nuts up just enough to hold things in place physically, with no regard or consideration for the need to have a solid tightening in an effort to ensure a good electrical ground connection.

Okay, I think I have beat that dying horse about enough. I look forward to you posting what you eventually had to do to correct this problem.

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[351c1971]

The bolt on the front of the block that supports the cable from the solenoid to the starter might be a good candidate.

 

[Cleveland Crush]

Ahhh I do not have it grounded directly to the engine block!
As of now I have the Battery (-) grounded --> Frame rail --> starter, and the engine grounded from the water/cylinder head pump bolt to the opposite frame rail. So they are both grounded to chassis but as separate circuits. Would this still not be applicable?
None of this front end is stock so i'm piecing the thing together. I'll give it a shot running the negative straight to the block.
Thank you for all the replies guys!

Any suggestions on best spot to mount on engine?

Well ja.mc44, that's why we are all here for; to help in some way!!!

 

[Ja.mc44]

So this is what I'm running. Top black is Bat(-), Red goes to engine block, bottom black goes to starter mount. The common bolt is unpainted frame rail. I also took jump cable hooked it to negative on bat and to bolt on engine block to make sure there's a direct ground from bat to engine. Even with this I have a voltage drop of 14V --> 8V when cranking. Does this sound like Internal starter short like Bobby mentions?

I should mention it is a remanufactured starter of same design as original.

 

[Bobby]

There is a small tab on the block just below the alternator that is threaded to 5/16 I believe that is the intended spot for the block ground. You'll see it in the pic just below the red alternator wire....the blue tab from the block and the ground is attached.

 

[Vicus]

There is a small tab on the block just below the alternator that is threaded to 5/16 I believe that is the intended spot for the block ground. You'll see it in the pic just below the red alternator wire....the blue tab from the block and the ground is attached.

That's totally correct!

 

[Ja.mc44]

There is a small tab on the block just below the alternator that is threaded to 5/16 I believe that is the intended spot for the block ground. You'll see it in the pic just below the red alternator wire....the blue tab from the block and the ground is attached.

This is just the simple diagram of what I have set to the car right now.
What I tried today:

1. I pulled starter and bench tested it, solenoid activated and held constant 12V. Obviously not under load so I will have it load tested this week at auto parts store.

2. I took the battery off of our old 67 GTO with a 400. It has similar CCA and gave that battery a shot, same deal, barely turns starter. So can't be the battery.

3. I pulled the wire off the solenoid that goes to the starter, bolted it directly to battery positive to send all power directly to the starter when I connect the negative lead. Even when doing this, wiring straight to starter and bypassing solenoid, the starter barely turns. Drops from 12 to 8.
- What is weird, is that when I attach my jump-starter to the battery and crank again it spins fine for about 5secs then loses momentum real quick and when you try and turn it over again, even with the jump it turns real slow unless you wait 5 mins. As if it has to build charge capacitance in order to turn starter again.

It is a new engine... maybe because the cylinder walls are dry causing alot of friction. idk. Im at a loss but thank you for the replies boys.

 

[Cleveland Crush]

ja-mc44 it appears your grounds are good (per the high tech Bobby diagram ) you mentioned new engine with high friction maybe the issue. Pull the plugs and hand turn the engine; you're going to get a "feel" for the internal resistance. After the initial turning resistance it should turn quite smoothly. If its turning smoothly by hand, start digging deeper, such as new starter (get it load tested first). Just my thoughts.
You'll get it figured out.

 

[351c1971]

If the motor has been sitting for a new time or with a new motor, put 2 squirts of 2 cycle oil in each cylinder, then 2 turns by hand and before replacing the spark plugs.

 

[71ProjectJunk]

What I would do is to try to see where the voltage drop is coming from, trace it all the way down from the battery to the starter, piece by piece. Here is a good video on how to do that:

 

[351c1971]

This spec below calls for a voltage drop to 9 volts so you are not that far off. Perhaps there is excessive resistance in the circuit. You might try measuring the resistance of the + circuit and then the - circuit using a digital ohm meter with the battery and starter disconnected.

"The cranking voltage of a car battery is the amount of power that’s available to start your engine. That power has to be high enough to overcome things like compression and friction in the engine so it can turn over and start running on its own. A typical cranking voltage for a 12-volt car battery is around 9 volts."

thepowerfacts.com/how-does-cranking-voltage-compared-with-battery-voltage/

Also is there any chance the paint on the block plate was not scraped off there the starter frame contacts it?