Grounded Positive Cable

92 XJSC/LT1: It never ends. Went to start the car but it acted like a dead battery. This is what I checked:

  1. Voltmeter says battery putting out 12.5 volts
  2. Load tester agrees with voltage but under load shows a bad battery. With cable removed, load tester shows good battery under load. I conclude the battery is good, but problem is with cable or downstream
  3. Ohm meter at positive cable at battery shows resistance suggesting positive circuit is grounded somewhere.
  4. Check the passenger side positive lug on the engine compartment firewall and it shows grounding somewhere.
  5. Remove all cables from that lug.
  6. None of the cables show grounding, suggesting it is nothing related to the motor.
  7. With cable to positive lug on drivers side removed from the passenger side, no grounding on the drivers side, again suggesting problem is between the positive circuit in the trunk and the passenger side firewall lug.
    Today I will run an ohm test between the positive cable in the trunk, and the passenger side positive lug.
    I could find nothing in the manual that covers the positive at cable, its routing, a replacement procedure [god help me] or the strange tap in the positive cable just before the terminal sporting two brown wires [where to they go?]
    I’m guessing that the cable somehow feeds the dash before it reaches the firewall lug, so the problem could be there I suppose.

Apologies for not tracing previous topic.
What about starter motor itsef? Any dimming on dahboard while engaging it with key turn? Is it working when overpassed with additional wire? Have you checked the ground of the engine and grounding band itself?

I disconnected the cables from the positive lug that go to starter and alternator, and tested continuity to ground but got nothing. I assume neither are the cause of the grounding of that lug.
The car started and ran before I disconnected the battery to replace the IAC and TPS. Again, there is no continuity to ground for any of the cables that go to the engine when removed from that passenger side lug.
Thanks for weighing in Janusz. I’m guessing if i took another battery and ran a cable to the starter cable, it would start. But wouldn’t that just tell me what I already know: That the problem is somewhere between the positive cable in the trunk, and the lug in the engine compartment?
I was did determine that the engine is grounded. There are at least 4 grounds to the engine. None were added between the time the car ran and now. I’m afraid I don’t understand why an engine ground failure would cause the positive cable lug to ground out?

Hi Richard-
Trying to be helpful, so please don’t take this the wrong way… I think your approach is a bit flawed.
Your premise is that there is a short to ground somewhere which is drawing off enough current to prevent a good battery from starting the car. This is because the battery tests “good” with nothing connected to it, yet it still won’t start the car, and it tests “bad” when connected to the car. Do I have that right?

If so, I assume you are getting a major arc when you hook up the positive cable. This short would have to be 20A or more to impede starting, methinks, and if the battery was left connected for any length of time it would soon be flat. My old truck starts just fine with the AC and headlights on. Everything slows down and the lights dim but it cranks right over, and between those two items it’s at least 10 amps.

What you are looking for is a current drain (a big one if it’s impeding starting), and you are testing resistance because low resistance indicates a path for current. But measuring resistance introduces unnecessary complexity. It varies by wire length, quality of probe connection, moisture or crud on the surface, etc. Plus there are all kinds of electronics in the car that may show a “path to ground” on the ohm meter that is not really there, e.g. when you measure a capacitor with an ohm meter it shows a momentary short, then infinite resistance.

I think you’d be better off measuring current between the positive post of the battery and the positive cable. If it’s more than about 100mA (someone will chime in with what is really “normal” in an XJS) then you have a parasitic drain. Leave the meter where it is and start disconnecting stuff (like fuses) downstream. Lots of posts here in the forum about parasitic drain.

HOWEVER, Unless you are measuring a whole lot of current, your starting problem is not a positive cable shorted to ground. It’s a bad connection somewhere between the battery and the starter, or the starter itself, or the battery ground to chassis, or an engine ground to chassis, which is what let’s the starter current get back to the battery ground. I feel like one of these is more likely. I’ve had many experiences where a poor connection (that appeared visibly “fine”) made me think “dead battery.” And the battery will load test just fine. Because it’s not dead.

My .02, I could be wrong, wouldn’t be the first time today, but it might give you some other things to consider.

1 Like

Bob is correct. Measuring resistance is useless. Measure current as he outlined. On an XJS, parasitic draw should be less than 20ma. One you have your meter set up, start pulling fuses one by one until you find the draw.

BUT, unless you have a huge power draw, that won’t stop you starting the car. You have other issues, assuming the battery is truly good.

To my mind if the pos cable is shorting to ground then surely the car would have gone up in flames by now?

1 Like

Or go to the starter if it fails to crank, then the engine can not run

Common culprit. The starter relay. Jump 12v from the fire wall post to the starter solenoid. Make sure the ignition switch is not in run. And that the transmission is in park. I suspect the engine will crank.

Next. is the start interrupt switch on the shift tower. It completes the ground leg of the start relay. Just wiggling the shifter might get a connection and the starter should crank.

1 Like

Thank you all so much. What got me started down this road was that when trying to start the car everything looked OK but when the key was turned all the way to “start”, nothing. Not even cranking. So I tested the batt voltage and got 12.35. So I put my batt load tester on it and it measured 12.35v but when the load switch was thrown, the meter dropped all the way to “bad”. So I took the batt cables off the battery and tried the load test and everything was fine. So it does not appear to be the battery. Where I apparently went wrong was then going to the lug in the engine compartment and finding some continuity between that and ground.
If I understand the comments, instead of checking continuity at the lug in the engine compartment, I should be looking at current.
Robin, at this level of frustration, I’m unsure what emotion would have resulted from your scenario.
Carl, the trans interrupt switch was my second thought. Being a conversion that shift linkage is always suspect so I did play with the shift lever to see if it changed the outcome. It did not.

Hi Richard-
Battery voltage without load is pretty meaningless. Good job going to the load tester. But I am not sure load testing the battery in situ is valid. I’ve never tried that, but with a lot of other stuff hooked up via the chassis of the car I would find the results suspect.
Regarding the No Start, my money is on:

  1. A poor connection at the battery or at a ground. Make sure everything is clean and tight. Then make sure again.
  2. A fault at the starter or starter relay. Prove this by jumping with a known good battery. If same “no click” result, definitely not the battery.
  3. Is there a security system or some sort of other starter interrupt?
  4. A parasitic drain has flattened your battery to the point it won’t click or crank. Test for current draw and find the culprit. This would mean your load tester is lying to you.

I do not think that you have a current draw or short significant enough to prevent starting. If you do, your battery would be way dead and there would be much smoke and running about happening. At least there would be if it were in my garage. Back in the days when we had ammeters in cars I neglected to secure the ammeter wire when I had the ammeter out for repair and much excitement ensued…


1 Like

Yes, it’s called the alternator :slight_smile: Unplugging it will fix your problem, but the problem is not a problem at all with working rectifier unit (inside of the alternator)

You cannot have any drain over the positive cable or grounded cable. As mentioned above - your car would change into fireball shortly.

Plugging the amp meter between negative clamp and negative battery terminal would give you more indicated drain info.

What engine version have you got?

Have you checked starter motor’s connectors/ground for oxy? Oxy prevents amps from going through even with indicated continuity.
If you have AJ6 engine on board, there is a black tubular plug just next to the heater matrix valve. Inspect and clean it, check cables for stretch/crack damage - that your started motor’s engage power…

The car is a 92 XJSC, originally a V12. Now it is LT1 powered out of a 94 Buick Roadmaster.

Actually unplugging the alternator had no effect. I disconnected all the cables from the positive lug, one of which goes directly to the alternator. There the positive lug on the passenger side still showed connectivity to ground. However there was no connectivity between ground and the now disconnected alternator cable or any of the others. Likewise the starter. With the cable between the positive lug on the passenger side and the positive lug on the driver’s side connected, my multimeter showed connectivity to ground which of course included anything running off that lug. Disconnecting the cable between the two positive lugs on the firewall eliminated the grounding of the drivers side, including the cable to the starter.

However others have pointed out that measuring resistance on a small multimeter is the wrong methodology measuring the wrong thing. They suggest that I measure current, which I plan to do tomorrow. I will also do as you suggest.

Well, with all cables removed from the positive lug on the passenger side of the engine bay, a good battery measuring 12.66v, and properly grounded in the trunk, there is no voltage at at the lug. I am still unsure what the tap in the positive battery cable feeding 2 pairs of brown wires powers. All 4 wires are getting 12.5v at the tap. The tap is an English part so I assume it is OEM.
I’m thinking that the only thing that cuts voltage off to the positive lug is the transmission neutral/park switch. I’m wondering if in the contortions involved in replacing the spark plug wires may have disconnected to neutral/park switch. On the other hand, the LT1 did fire right up after the wires were changed, but the high idle problem persisted. So I removed and cleaned the intake and replaced the IAC and TPS. When all was back together and the battery cables reattached is when it acted like a dead battery. The load tester would show 12.6v until the load switch was thrown in which case it showed the battery was bad. Remove the positive cable from the battery, and both tests showed a good battery. Both the battery neg and the engine are grounded. But it’s not getting any current to the positive lug.

Have you tried jumping the car or using a known good battery?

IAC and TPS on an LT1 installation??? My LT1 has no such items.

The neutral start switch merely completes the ground leg of the starter relay. Sure you have the right switch. Green multi purpose on the shift tower.

Although the installer made changes, the S57 jaguar wire schematic is extremely helpful.