Hi Parker,

First of all, prayers for you as you face such daunting medical issues, yet continue to soldier on and contribute so much to the knowledge base of this forum!

Secondly, my recollection is that if the voltage sags below about 10.4 volts while cranking, the ECM will not trigger the ignition to fire. So the sag to 9 volts may be the core issue.

Have you checked for corrosion at both ends of the engine ground strap?

My experience with Jags is that if the battery in a Jag is not strong and healthy, jump starting with a parallel battery, jump box or second vehicle with engine running may not start the Jag. Do you have a means of load-testing the battery in the car to determine its true capacity? Some auto parts stores have electronic analyzers that can determine the actual CCA vs rated CCA.

I assume the 250A megafuse must be intact.

Just thinking out loud.

Cheers,

Don

 

I'm back as my computer log in messed up but resolved

And a couple of computer power bricks going out

Had to go to hospital for emergency procedure on myself (dyalisis related ) and father back in hospital

Batteries have been load tested on the old school load bank before , lucky to find one at a automotive parts store in the back room as they weigh a ton

Back to the chronic starter sag down to 9 volts

Gone through a couple of starters focusing on that before and current Bosch remanufactured and Bosch marked starter solenoid installed

Terminal point under rear seat pan 1 foot aft of right heel board fuse box area connections clean , tight , and mega fuse good

Digital meter at that point reads slightly less that 10 volts starter sag and slightly less that 9 volts on the instument cluster

Need to look at terminal post hardware by fully removing post from the right front wheel well as the general basic connection is clean and tight


Had new 1/0 gauge ( original size ) battery cable made up and I installed along with the smaller gauge battery cables to the left and right engine bay fuse boxes

The original 1/0 size cable miked out for size ( 0.375 / 1000 inch ) and is a better conductive very small wire strand and replaced as same

So the positive battery post connection good , protection mega fuse at the battery good and fresh copper section past fuel tank to under seat pan and left and right under hood battery cables

Original battery short negative cable good

No change in starter sag performance

Got 1 cylender to combust without starter fluid but that's it

Large engine ground strap removed and cleaned the surface before a few years back but rechecking along with starter solenoid battery cable connections

Recharged battery and motor oil bore wash conditioned. For a regroup

New Battery working with for past couple weeks ( not load tested ) charge state 12.50 volts but it does charge fully with battery post terminals removed

and have 2 batteries to work with

Gotta get running again to place in garage to avoid driveway prison

Editing

 

If you see any green corrosion on the engine ground strap where the eyelet terminals are crimped on, it is worth either replacing the strap or adding a second one in parallel. Auto parts stores carry suitable prefab cables in their battery departments.

 

Thanks Don ,

Original woven short ground strap looks visually OK back in the past observances , may have degraded further since last ran engine the Christmas before last

Starter sag was present then but started and alternator bearing went out

I'll recheck the alternator connections and possibly isolate the alternator B+ cable ( heavy leather glove pocket ) for test run

In the past I have tried using both battery jumper cable wires as a supplemental engine block / starter ground path and will try again as I may not have been on a valid block ground point

Aware the valve cover is electrically isolated but may have errored back then few years back

I was probably right there using the starter mount bolts or close

Took original short ground strap off and cleaned strap ends and frame surface back then years back

Rechecking my work

 

So what happens if you connect the spare charged battery directly to the starter. Bypass everything between the battery and the starter.

 

Checked for installation of the large ground strap

Battery cable supplemental ground path again from starter mount bolt to car frame as steering rack mount bollt

No help

Vee ,

starter has been spinning fine albeit 9 volts so low RPM

I see what your saying as having 2 isolated systems , starter , rest of car

I did the right fuse box on 1 battery and rest of car on trunk battery to somewhat ensure ECU has 1st power voltage , it started back then last year

I guess by running the engine it reconditioned the bore wash and car would start with all in original factory configuration , that is when I burned up the alternator front bearing

But I was probably on the threshold of voltage badness , I'm asuming if I got a couple of combustions it would unload the starter instantaneously for rest of car voltage recovery

2nd power as the large ECU control relay clicks but must swap yet

I'll have to ponder how to jumper just the starter solenoid / starter with independent battery

I think right there at the right front wheel terminal post ( starter cable ) would satisfy every need on the starter circuit and unload the trunk battery for rest of car

Alway jumperd the fuel pump relay and always can hear pump

fried CKPS couple months ago and replaced last week

I need to do rest of the fuse boxes fuse check ( ECU control relay power source , looking for the ECU control relay specific fuse )

# 11 / 20 amp right engine bay fuse box hot at all times

 

2nd battery on starter cable no help

Ground clamp on exhaust stud on block

No help but instrument cluster improved to 9.5 volts and starter rotation improved up to 200 RPM , 300 normal

50 amp starter position on trunk battery charger during start tries

Starter fluid no help but got more combustions , just not enough to go forward as sustained engine running

 

Ya gotta find the voltage drop, no?

Leave the voltmeter positive lead connected to the positive battery terminal and move the voltmeter negative lead back through the circuit toward the battery. Continue to test each connection while cranking the engine until a noticeable decrease in voltage drop is found?

I know that's easier said than done...

 

Swapped the 2 ECU relays no help

Confirmed correct large ECU control relay socket with wire colors

 

Vee . Thanks

Editing

Ya gotta find the voltage drop, no?

I'm thinking the major voltage drop is from battery to the under rear seat mega fuse post , shade less then10 volts at that point , OK until starter sag

Battery protection fuse terminals tight and positive battery clamp good and not stretched , replaced old stretched positive terminal clamp last yeat

But that 90 degree bus bar from the clamp metal casting to the mega fuse terminal post in question on the equivalent of a cable crimp area , battery jumper at the mega fuse last terminal post being thought of

After replacing battery cable at that section past fuel tank and the 2 for the left and right engine bay fuse boxes ( green inside cable terminal crimps ) the observation of 10 volts at starter sag the same at the under rear seat pan post ( different then the heelboard area )

I understand you have the XJS chassis, but it is hidden undrer the rear seat pan after removal

So that very thick cable was replaced as new copper welders cable built up item as full replacement

The voltage drop can't be from restrictions fwd of that point as it would still remain around battery voltage ( with consideration of voltage / current / resistance relationship )

So that points to the cable voltage being drained somewhere

Leave the voltmeter positive lead connected to the positive battery terminal and move the voltmeter negative lead back through the circuit toward the battery. Continue to test each connection while cranking the engine until a noticeable decrease in voltage drop is found?

I know that's easier said than done...

 

When my brother gets back will take the new battery up for load testing

 

I'll have to look to see if starter cable is not pinched and shorting as it goes over rear of engine

That was tinkerd with on rear of engine drop for starter replacements

but that would be a constant draining short and dead battery overnight as is on the hot at all times side of the starter solenoid relay

But it does start me thinking of the starter solenoid relay shorting during command to close on the external to the electric motor pigtail to the starter brushes

I have tinkerd inside the starter electric motor and may have errored inside the motor

 

the positive post clamp is one continuous casting to the mega fuse terminal post , so no crimp

will bypass the mega fuse in case it is counterfeit material.( improper manufacturing )

 

Eliminated mega fuse at positive battery post

same starter sag , no help

 

Checked starter battery cables from below and starter cable run over the back of engine

starter pigtail to starter brushes have a insulating shrink wrap as should

If the alternator is shorting out there would be a overnight drain but will remove alternator B + post anyway

Laptop charging brick going out so will probably be intermit off line

 

both batteries load test OK

Large A8 battery no help in the past

 

At this point you gotta bypass it all and get 12v directly to the starter. That’ll surely reveal whether or not there’s something going wrong with the starter….which seems to be where all signs are pointing at this point!

 

Put the 2nd battery on the starter cable right front wheel well terminal post

Ground point on engine block manifold studs

but failed to remove terminal connection to rest of car , a little bit of improvement but no joy

 

isolated rest of car to run on trunk battery

Jumper cable with 2nd battery on starter cable that was lifted and insulated from car frame with a shop towel

Jumper cables correct polarity , ground on manifold studs

Only get a click with key in last start position

Gotta dialysis to maintain my Doctor's order to keep pure , counts against you on transplant index

Maybe not enough current through the jumper cable clamp contacting the starter battery cable end

I could get testing purpose 2nd battery correct terminals as the battery will sit to the side of engine , A/C hoses removed already

 

I can take apart a spare X300 starter in the garage to see if there is something on the inside of the starter I errored with before removing currently installed starter

Yep , gotta spare as I went through 4 starter assemblies so I'm good at installing them

I should at least get a participation trophy