Weak starting, battery full

[MilehighGS]

After swapping motors on my 07 GT, when I try to start the engine it turns over as if the battery is weak. If I keep the key turned, it all of the sudden will crank normal and fire up. All connections are good.
I will mention that I added a DOB denso alternator and relocated the battery to the trunk recently, but the starting issue happened before the additions.
Anyone familiar with this scenario?

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[Juice]

Two items stick out.
Engine swap, did you get all the grounds tight. Block ground strap?
Trunk mounted battery. Voltage drop from the lengthened cable, and possibly a lose connection with that.
Sounds like it is something you did, time to do some voltage drop tests.

 

[DieHarder]

Two items stick out.
Engine swap, did you get all the grounds tight. Block ground strap?
Trunk mounted battery. Voltage drop from the lengthened cable, and possibly a lose connection with that.
Sounds like it is something you did, time to do some voltage drop tests.

Agree with above. Sounds like you have resistance/weak connections to ground. Since the issue began before the additions it's likely your grounds needed to be cleaned up or improved (read modified) before the swap. Using a VOM perform some voltage drop tests from each major ground to a body ground (at each end). If you see voltage indicated on the meter try disassembling/cleaning/assembling/retesting the cable. If it doesn't improve you may have to replace that part of the battery/ground cables or jumper around them (see below).

I would also do some resistance checks on the major battery/ground lines from end to end and from each end to ground. Those should ring out at less than an ohm.

The other change you mentioned is adding a DOB denso alternator. When I added mine I found my voltage output was low (13v) compared to the customary ~14.7v we normally expect. I didn't know it at the time but that was an indication of resistance in the lines/metal connections and/or bad grounds. If you find the voltage output low (measure across the battery) when the car is running you might want to so some voltage drops from the alternator case back to ground. I ended up adding ground lines from from the alternator mounting bolt and from the alternator case to the mounting and ran them back to the main ground on the passenger strut tower. That's finally what solved my issues (in addition to replacing my battery cable assy). I also think the block ground strap is another weak area but I haven't changed mine yet since I jumpered around it.

If you're interested in reading about my saga here's a link: https://www.s197forum.com/threads/f...he-all-important-grounds.139544/#post-2517191

 

[DieHarder]

After swapping motors on my 07 GT, when I try to start the engine it turns over as if the battery is weak. If I keep the key turned, it all of the sudden will crank normal and fire up. All connections are good.
I will mention that I added a DOB denso alternator and relocated the battery to the trunk recently, but the starting issue happened before the additions.
Anyone familiar with this scenario?

Sent from my SM-G975U using Tapatalk

After more thought I would recommend a closer look at the starter cables (both pos/neg) and starter itself. Remove/clean/reassemble/test. If symptoms are still the same try troubleshooting the starting circuit. Section 303-06 describes the starting system. On pg 4 is a symptom chart that lists a symptom similar to yours and Pinpoint tests you can perform that may be of help.

Section 303-06 Starting System: https://iihs.net/fsm/?d=204
Pinpoint Test C: Engine Cranks Slowly - https://iihs.net/fsm/?d=206&f=Starting System.pdf&p=5

 

[Juice]

I have seen a tight starter connection have enough resistance that it would not even click. The nut securing the positive cable was tight. Losened and retorqued nut, fixed. No signs of corrosion.

 

[nfrizell]

Before going too far. Test the battery under load. Auto parts stores should be able to do this for you. You can show good voltage with very little capacity.

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[Juice]

Before going too far. Test the battery under load. Auto parts stores should be able to do this for you. You can show good voltage with very little capacity.

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Or do it yourself.
Put dvom on battery, crank starter.
Battery voltage should not drop below 9.6v while cranking.
If its lower, bad battery or low charge. If it stays above 11v and cranks slow, it is a connection/high resistance issue with the battery cables or a bad starter.
I have a gear reduction high torque starter on my 91. Draws way less current than the stock starter. Starts easy even when the battery is low.

 

[stealthytom]

all good comments, just something else to think about is fuel filter if you havent replaced it lately.

try turning to on and waiting a second to let the line pressurize then crank it over
if it starts up normally at this point then consider a new fuel filter

this wouldn't rule out above electrical issues but fuel filter is cheap

 

[MilehighGS]

I really appreciate the responses!!!! I will have some testing to do today. It is very odd indeed. I will post my (hopefully) fixes today. Again, thank you!

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[crjackson]

Keep us updated on your progress, I’d like to hear what your final solution is.

 

[Pentalab]

Before going too far. Test the battery under load. Auto parts stores should be able to do this for you. You can show good voltage with very little capacity.

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Be extremely careful with this. I read the local Ford dealer the riot act, and told em to stop doing their damned ..'battery test'. Put a clamp on DC ammeter on the cable feeding the starter. It doesn't draw any 200-500 amps, no way. On idle, my 2010 reads 14.75 vdc across the battery terminals, with eng running. With eng off, the next morning, battery typ reads 12.4 vdc. With any battery in the trunk, ur gonna need some HD wire to the eng bay, or else you will get a big V drop. This is no place for puny 4 ga wire.

There is no less than a dozen methods of 'testing' a battery. Ford dealer lays a dead short across the battery terminals, then uses a clamp on dc ammeter to see how much ..'cold cranking amps' they can suck out of a battery...for 20 seconds. If you really wanted to 'cold test' a battery, you would remove it from the car, stick it in the deep freezer (typ -8 deg F) for 3 days, then re-install it in the car..and see if it starts.

What else that makes life a little bit easier for a battery, in colder climates is 0W-20, or 0W-30, or 0W-40, made by Mobil-1. 100% synthetic.

 

[Juice]

A 'full' load test is not that harmful. That max current test only takes a second to do. If the test "killed" the battery, that battery was on it's way out anyway.
This test is also unreliable if done on a battery that is not fully charged. Here a good battery will fail the test, not because it is bad, but because the charge is low. If that battery can only put out 100Amps, and the starter needs 100Amps, but has a startup surge of 250Amps, what do you think will happen? (say nothing of is that 100Amps beind delivered at 12volts or 5volts?) Amps alone dont mean shit, Volts alone not enough info.

There is no "one" definitive test when it comes to electrical circuit diagnostics. Atleast two of three items (volts, resistance, and current) must be tested and third calculated to properly diagnose an electrical issue. Observe Ohm's law.

Resistance/continuity can be good under no load, but not under load. Voltage can be good on a bad battery when there is no load, then drop like a rock from just a dome light coming on. This is why I suggested measuring the battery voltage AT the battery while cranking. Testing it this way does more than just check voltage at the battery. And because the voltage was measured WITH a load, you can predict where to test next to narrow down the issue.

 

[oldtexasdog]

I had a simular problem and after a lot of hair pulling found that I had a intermittent cell in the battery that sometimes tested good then would go bad. New battery fixed it. I had it tested several times and it tested good then one time finally not.

 

[MilehighGS]

I followed Dieharder's link grounding the Alt, alt bracket and then ran another ground from block to chassis. Checked connections, and replaced the battery.........spun better when cranked (faster).
The cranking is more of a "stuttering".....hard to describe, but this is best way to describe it. I even replaced my ignition switch since electrical parts can do weird things.........I currently have the battery grounded in one of the exhaust bolt holes. Is there a better ground I should use???

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[DieHarder]

I followed Dieharder's link grounding the Alt, alt bracket and then ran another ground from block to chassis. Checked connections, and replaced the battery.........spun better when cranked (faster).

The cranking is more of a "stuttering".....hard to describe, but this is best way to describe it. I even replaced my ignition switch since electrical parts can do weird things.........I currently have the battery grounded in one of the exhaust bolt holes. Is there a better ground I should use???

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Sounds like you got it covered.

Other areas/items I would look closely at is the starter lines (loosen/clean lugs & posts/reassemble) and starter itself. While you have the lines apart test resistance end to end (should be less than a 10th of an ohm) and inspect for any copper oxidation. If the starter cables look like they're oxidized/green and you note any appreciable resistance you might want to replace the main battery harness (it consists of the main battery to starter cables and grounds to the strut tower and block). It's still available (~$70 - $100) and significantly improved the performance of my charging system (increased output of my alternator about 1.5v) which told me they were in fact slowly degrading due to oxidation.

Since your symptoms improved it's important to test/recheck your work to see if anything else needs additional work. With the engine running put your VOM on Volts DC and with one end on the main body ground (passenger strut tower ground) take the other end and test various ground points on the engine block and alternator. I would also test points on the engine itself (i.e. neg on block to alt ground on the back/body of the alternator). If you see voltage indicated on the meter at any of those test points that means there are resistance differences that must be grounded so they're at the same ground potential as the main ground on the strut tower. In my case I found voltage drops on the engine itself (alt body to engine manifold) and from the engine block to the main ground on strut tower. Adding a ground that tied those together and running a new ground back to the strut tower ground solved my grounding issues.

Of note the alternator output across the battery (should be close to ~14.7vdc). Low output/high resistance/voltage drops are good indicators of grounding issues.

 

[Pentalab]

A small amount of Permatex dielectric grease on the battery terminals and else where, would go a long way to keeping water / moisture / oxidation out of all connections.

 

[DieHarder]

I followed Dieharder's link grounding the Alt, alt bracket and then ran another ground from block to chassis. Checked connections, and replaced the battery.........spun better when cranked (faster).
The cranking is more of a "stuttering".....hard to describe, but this is best way to describe it. I even replaced my ignition switch since electrical parts can do weird things.........I currently have the battery grounded in one of the exhaust bolt holes. Is there a better ground I should use???

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Might want to move the exhaust manifold ground to another point on the block that's not so hot (unless you're using straight metal braid). Since you're still experiencing stuttering and you've grounded the system properly I'd suspect the starter itself and/or battery harness which includes cabling that runs down to the starter/block. That's really all that's left now.

 

[Pentalab]

Might want to move the exhaust manifold ground to another point on the block that's not so hot (unless you're using straight metal braid). Since you're still experiencing stuttering and you've grounded the system properly I'd suspect the starter itself and/or battery harness which includes cabling that runs down to the starter/block. That's really all that's left now.

What left is the batter itself. Can the car now be jump started ?

 

[MilehighGS]

Thinking back, I replaced my alt with a DOB Denso unit.........powdercoated black. Decided to put my OE alternator on, turned they key.........car fired right up! Shut it off/on 3 times total and it started without a hitch. Kinda pisses me off since this alternator is a brand new Denso that is no longer made.........I'm sure the powdercoat is keeping it from grounding correctly.

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[DieHarder]

Thinking back, I replaced my alt with a DOB Denso unit.........powdercoated black. Decided to put my OE alternator on, turned they key.........car fired right up! Shut it off/on 3 times total and it started without a hitch. Kinda pisses me off since this alternator is a brand new Denso that is no longer made.........I'm sure the powdercoat is keeping it from grounding correctly.

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What left is the batter itself. Can the car now be jump started ?

You are right about DOB Denso units having powdercoat that's difficult to ground. However, there are 3 ground points on the back. To check if you're still having grounding issues (with the car running) recheck for voltage drops from a ground on the back or body of the alternator to the engine block and to the ground on the passenger strut tower. You did say things improved after you added the additional grounds...

After I ran a 4 gauge from the passenger strut tower to a mounting bolt for the Denso and another smaller wire from the mounting bolt to one of the grounds on the back of the Denso I finally got a good ground reference and 14.5v output. Works great now. I'm assuming battery good and fully charged.