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Please vote on fault P1793

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  #1  
Old 06-29-2018, 01:10 AM
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Default Please vote on fault P1793

This is my first post in 13 years because I've always found my answers with just a search... Just fixed a persistent limp home mode with a used throttle body(TB). For posterity, the TB problem was, I think, a P1122 fault. Took me a while to figure out there's a peddle sensor on the left side of the TB and a throttle sensor on the right. Cleaning the plug for the peddle sensor has kept me running for months, but it finally totally went out. I have an 8/99 XJ8, 82K miles BTW... I think the TB's changed on MY 1999.5 and are the same until around 2003. I've never posted because I've had little to add to the wealth of knowledge contained herein...

With the TB issue sorted, I can now tackle P1793. That's for a voltage >16V or <7V. I replaced the alternator last year with a rebuilt Jag Alt, not an aftermarket. I think I'm still under warranty. My battery is pretty fresh too.

Been through the wiring check mentioned in post #14 of the following thread and everything seems to check out:

https://www.jaguarforums.com/forum/x...-again-149174/

Here's what's happening and seems to have began shortly after the Alt replacement: Occasionally, I get a "battery not charging" and battery light for a second or two, but not long enough to check if the battery is not charging. Get about 14V when I check the battery with car running. At about 2500 rpm, I can see a blip of the dash lights and that triggers P1793 and "Gearbox Fault" display. The Gearbox Fault is clearly caused by the momentary loss of electrical power and resets on restart. This is repeatable either by brisk acceleration or just revving the engine in the driveway. I surmise that means it's not a ground interruption-- you'd expect that on a pot hole or something--, but I'm confounded as to how power is interrupted to the dash, trans, etc. if the battery is supplying electrical power. My best guess is some computer, somewhere, is disrupting electrical power because of an over-voltage from the Alt????

Like "Who Wants to be a Millionaire", I'm asking for an audience vote.... The majority is usually right.... What say you?

I ask because, those who have been there know, replacing the Alt is a real pain....

Regards to all... Mark
 
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Old 06-29-2018, 05:26 AM
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Hey Mark,
Two options to check, first the body grounding lanyard coming off the ground of the battery. In my case I had zero power (happened randomly twice) after just driving it. I was lost to the cause and found that lanyard strap warm, replaced, no issue in 2 yrs. Another thing to check would be the Mega fuses (just above the spare)...
Amazon Amazon
...they are difficult if not impossible to check with a standard meter, but it was recommended by my inde' before I found the lanyard issue. They are cheap and its a quick change at minimum.
 
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Old 06-29-2018, 05:41 AM
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It may be the engine flexing and thus causing a momentary bad connection or short (probably ground/power but could even be the alt wiring).

It would flex more if one of its mounts is tired, too.
 
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Old 06-29-2018, 06:14 PM
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Thanks guys:

I went through all the electrical troubleshooting in the attachment below and didn't find anything wrong. Also, the power loss is not random. I would expect engine flexing or grounding not to be repeatable. I can get the blip (milliseconds) every time at about 2500 rpm whether moving, slow revving or fast revving-- if that makes any sense... I keep thinking sudden loss of electric would come from vibration, not a specific engine speed... No?

Now, the mega fuses would almost fit, but for the regularity of the fault. But, that's a cheap enough fix to give it a try. I did check them as per the troubleshooting below, but maybe they're the culprit if as Highhorse says they're hard to test.

One more thing and I'm a little vague on this because I was getting a bunch of faults from the bad TB, but I recall getting P1632. iCarsoft OBD said something about a fault with the "Smart Generator (or Smart Alternator) Faults Sensor/Circuit Malfunction???" That fault is not repeatable like the P1793, but it was there.

Which leads to the next question.... Is there anything, other than a loss of battery voltage, that can momentarily shut off electrical at the dash, lights and tranny? IOW's, can those be shut down by the ECU or some other computer? That's what I feel like is happening... but find it hard to wrap my mind around...

Thanks again...Mark
 
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Old 06-29-2018, 06:18 PM
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Thanks Guys;

I tried to post another question, but said a moderator had to approve???
 
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Old 06-29-2018, 07:37 PM
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Go to... https://www.jaguarforums.com/forum/n...-intro-must-5/ ...and read the forum rules on such. While there please introduce yourself to the community.
 
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Old 06-29-2018, 07:52 PM
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Thanks Highhorse.... I'm not new to the forum just posting...
 
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Old 06-29-2018, 07:57 PM
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Okay, let’s try this again with no attachments.

As said, I had been through the troubleshooting from the link above with everything checking out. Today, I put jumper cables 1)battery to ground and 2)battery to both the upper posts on the mega fuses behind the tire. That essentially eliminates the mega fuses and the ground strap… No? Still get the loss of electrical power blip at about 2500 rpm.

One more thing, and I’m a little vague on this because I was getting a lot of faults before replacing the TB, but one code that showed up on iCarsoft was P1632. That reads, “Smart Generator (or Smart Alternator) Faults Sensor/Circuit Malfunction." This isn’t necessarily triggered with P1793 but it was there at one point.

So with these tests, and the fact that the P1793 fault seems only dependant on engine speed, (moving, standing with slow rev up or fast rev up) and not vibration, makes me think it's not related to cables or grounds--- wouldn't be the first time I was wrong--.

So the further question is: Is there some way-- other than loss of battery power-- that the electric to dash, lights, tranny, etc. can be momentarily shut off by the ECU or some other computer? I feel that’s what's happening, but am having a hard time wrapping my head around it... For instance, if the Alt is going over-voltage, will the ECU-- or whatever—shut down power to prevent damage?

Thanks again guys. Believe me, I don’t want it to be the Alt and wish it was cables or fuses--- probably not so much engine flex—but think I’ve eliminated the usual suspects, and any suggestion is welcome. Also, never had this problem before replacing the Alt, although, I can’t say it started just then--- might not even notice, since I drive mostly in a 35 mph zone and not too much even then. Have a V-12 MB for that…

Regards to all… Mark
 
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Old 06-29-2018, 09:08 PM
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Originally Posted by mcypert View Post
Thanks Highhorse.... I'm not new to the forum just posting...
Understood, but there are rules to posting for less than a set amount of posts.
 
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Old 06-29-2018, 09:18 PM
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It could be a voltage regulator, but we are getting deep into the guessing game. Bottom line, the coincidence of a fairly new alternator and your electrical issue, plus the type of code you are getting, comes right back to the alternator and when you have a flux of power, its usually a voltage regulator. I say get it tested.
 
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Old 06-29-2018, 10:03 PM
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Thanks again Highhorse. I read the rules and understand why they apply. I've just been using the forum for so long (since about 2005, IIRC) I felt like a member...

I think what bumped me was trying to attach the pdf attached in post #14 of the link in my #1 post. It's a troubleshooting of electrical problems specifically put together for the X308. Very useful and thought I'd share. Anyway, it's referenced there and I'd suggest anyone having electrical gremlins to start there...

IIRC-- and often I don't-- there's some computer circuitry built into the Alt. I was wondering if anyone knew if and how the Alt could tell the car to cut off electrical power. Way beyond my pay-grade...

Again, trying to avoid pulling the Alt, if not necessary. Been there, done that, don't like that. BTW, pulled mine through the wheel well-- there are various methods posted herein-- the wheel well is just one and possibly both the easiest and bloodiest...

Next need to contact the Seller of the rebuilt Alt and see if they'll sell me another with the understanding I'll be returning the old one???

As said, at this point, any suggestions are appreciated...

Regards...Mark
 
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Old 06-29-2018, 10:05 PM
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Once again... Blocked... Thought I was following the rules.

But sentiment was thanks and any suggestions appreciated...
 
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Old 06-30-2018, 08:28 AM
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Make a couple posts in the New Member forum to get the post count up to allow you to post attachments. The rule is made so sales people can't come and post their garbage and run off.
 
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Old 06-30-2018, 04:23 PM
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Originally Posted by Highhorse View Post
Make a couple posts in the New Member forum to get the post count up to allow you to post attachments. The rule is made so sales people can't come and post their garbage and run off.
Thanks again, doesn't seem to mind short relies.

One more bit of info... I'm getting a 0.4A parasitic draw on the battery with everything off. Diode?
 

Last edited by mcypert; 06-30-2018 at 06:17 PM.
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Old 06-30-2018, 06:12 PM
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Apparently, I’m wrong again... about an Alt diode that is. Checked the stud connection under the hood, right-hand side that passes through the bulkhead, see x308 troubleshooting mentioned in #1 post.

Got a 0.0A parasitic draw from the battery when measuring from there. My understanding is that that cable only provides power to the starter and Alt, thus neither of those is draining the battery.

So, I have a new problem that’s unrelated or not??? Any comments welcome.

Regards to all… Mark
 
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Old 06-30-2018, 08:10 PM
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Oops… I don’t think I did the parasitic draw test correctly. Maybe didn't wait long enough. Will redo as Jag says to and report back...
 
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Old 07-01-2018, 11:20 PM
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Okay, I rechecked for parasitic battery draw using Jag’s procedure as outlined in something called ”Quiescent (quiet or sleeping) current testing" (attached). Basically, you put the meter’s (DMM) amperage tester in line between the battery and cable, close the trunk (I put the DMM on the bumper) and wait an hour. Amperage, as I checked it, dropped from about 0.4A to 0.05A, which I figure is close enough to the spec of <0.03A.

So, I’ve now ruled out Alt diode drain or some other parasitic draw. I also put a full charge on the battery and let it sit for about 24 hours. Voltage was 12.89 the next day, so guess my battery is fine.

Even after a full charge, still getting the occasional red battery light. So, guess I’m back to defective Alt, unless someone has any different ideas... Highhorse's DX of voltage regulator seems the most likely, and that's built into the Alt, correct? The blip/loss of electrical power still has me baffled ??? and Alt seems to be keeping the battery charged... For now, it's not something that's really creating any problems. But, obviously something I want to fix, especially if the Alt is still under warranty...

Regards…. Mark
 
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Old 07-02-2018, 07:42 AM
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Think of your alternator and battery this way and it may help your aspect. The battery is only there initially to provide power to start the car. Its secondary purpose is to maintain secondary energy draws like the clock, memory on the radio and lights when you open the door. After that, once you put the key in the ignition, its the alternators job to recharge the battery and run the car. The battery is now the secondary item. But if the battery isn't good (yours is from the tests), it will cause the alternator to keep attempting to charge it and create issues and run down because its running the vehicle. These draws are thus controlled through the voltage regulator as needed, hence its name. Each manufacturer has its version of such for the needs of its vehicles. The regulator may be having an issue switching from a higher power draw to going to standard operation or vise versa.

You stated earlier you believe your still under warranty. Even if not, have you contacted your rebuild source to see if there is anything you can meter/check without having to haul it back out of the car?
 

Last edited by Highhorse; 07-02-2018 at 07:44 AM.
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Old 07-02-2018, 08:08 AM
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Going to do a demo of true ignorance here. Is there any chance of one or more of the diodes "stuttering" a bit at the engine's 2500?
With the alternator still under warranty, could you substitute in a 'CHEAP' after-market and look at the results? Especially valuable if the sub is a different design, and more especially with different diodes. If it were to work well you'd really be ready to converse with the rebuilder!
 
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Old 07-02-2018, 01:42 PM
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To me, it is a bad alternator. Either a bad connection to/from or a bad ground or a bad board (vibrating a loose connection around 2500 rpm).. Find your neighborhood alternator/starter repair shop and let them have a crack at it.

As I noted many times before, I dropped mine off at the rebuilder's facility and before my wife could get me home, five miles away, he had found the fault, rebuilt and tested, and reinstalled the alternator. It's all he does . . . . and he's pretty damn good at it.

Had the same issue with a Cadillac 20 years ago - up in Fairfax County VA -- visited this shop - an Albanian who rebuilds for everybody, had hundreds of alternators ready to go. "$150 if I have to take it off the car; $50 if you bring it in!" Ten minutes later, a rebuilt alternator. These things aren't rocket science, but it is nice to have a fully instrumented test bench . . .
 
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