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  #21  
Old 09-29-2017, 10:40 PM
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Go I think your best bet is to get fuel rail pressure under load. Use a DC/AC inverter to operate your computer from 12v if no battery. At least it starts and runs. Much easier to track down a gremlin on a working car


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  #22  
Old 11-07-2018, 06:59 AM
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hmm gonna have a digs in that fuse box see if anything is a miss, do i need to disconnect battery before removing fuses to check?
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Old 11-07-2018, 03:34 PM
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Quote:
Originally Posted by omodos View Post
hmm gonna have a digs in that fuse box see if anything is a miss, do i need to disconnect battery before removing fuses to check?
What makes you want to check these fuses? If any of these blow, the car won't run... It seems unlikely there's any issue there if your car is running.
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  #24  
Old 11-07-2018, 03:53 PM
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Curiosity...but i guess if it aint broke dont fix comes to mind now
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Old 11-07-2018, 04:11 PM
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X5 3.0d won't start



Just performed the simple test of monitoring the voltage drop on the starter cable when cranking the starter.

I have a 2001 m54 gas 3.0i. The engine was slightly not cold (a few minutes of operation after sitting overnight then a minute or two rest when I realized I've been meaning to take this measurement for a while).

I started the car twice in a row the first was actually about 75/100v second start 63/100 and after waiting just 3-4 minutes took 75/100 again.

I'm going on gut to say my starter hasn't started the escalation of current into failure and is pulling between 200-225A.

That means 0.75/212.5 means about 3.5 mΩ cable resistance.

If you take the voltage drop measured from the cig. lighter socket to the B+ jump point under the hood and multiply by 285 you should arrive at starter current.

Somebody with access to a clamp ammeter could perform the voltage drop measure to get a more accurate value but this will get you in the ballpark.

It also will give you trend and comparison info. Eg. Take the measurement at 30C and 0C it will be a lot different.

So back testing: 0.75v x 285 = 285*0.75 = 213. The immediate restart (oil still on all the journal bearings etc) was .63*285 or 180A both numbers seem right on target.

I was able to have the shrink wrap on the B+ hold my positive lead to make it very easy to do the measurement.

At some point it will be nice to have a control taken to measure the current draw and voltage drop at the same time but the 285 figure will probably work very well for most people to get a sense of if their starter is wearing out.

I'll take the measurement on wife's car when I'm in the same zip code as the car to DOUBLE my sample size.

Best if your meter has a peak scale (in fact probably a requirement. I just tried to watch live and only saw .30v or something. Maybe with analog meter you can watch the needle.

After I get a bigger sample size and especially if I can get a simultaneous clamp ammeter reading I'll make a thread about the topic and post more details.

1.38v would be 392A for comparison.
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Last edited by andrewwynn; 11-07-2018 at 04:23 PM.
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  #26  
Old 11-07-2018, 04:34 PM
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@andrewwynn clearly you are an electrical whizz..alas dont have a fluke like you but can try capture on phone camera what i see and watch frame by frame...excuse my ignorance but where in lighter socket do i touch multimeter probe to do your below step:

If you take the voltage drop measured from the cig. lighter socket to the B+ jump point under the hood and multiply by 285 you should arrive at starter current.
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Old 11-07-2018, 04:37 PM
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X5 3.0d won't start

Take slow motion video of multimeter when starting that might get you a useful reading; smart.

Center of the lighter socket is positive. Handy there is a little dent to hold the probe dead center.
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Old 11-07-2018, 04:48 PM
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I have the same (well, very similar) meter, Andrew - I'll do the test on my 3.0d and post up the numbers here.

Great work!
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Old 11-07-2018, 04:53 PM
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Ok sir thanks again will stick the probe into cig lighter socket..the other probe connect with a plastic peg to B+ set meter to 12v start car whist having someone record meter...will get back to you later on some of the maths but your explanation is much appreciated even though my brain is literally in sleep mode...
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Old 11-07-2018, 09:43 PM
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Quote:
Originally Posted by wpoll View Post
I have the same (well, very similar) meter, Andrew - I'll do the test on my 3.0d and post up the numbers here.

Great work!


Thanks and that would be awesome to have a diesel example. I've been wanting to get this concept of there ever since I read about the exponential growth of current draw as a starter wears out. Since many of not most DIY car fixers have a DMM and with the X5 having two separate paths for electric flow from the battery it makes this trick possible.

The cig socket gets its power from the feed that comes up the right side of the car, the starter gets the power from the big cable that goes up the left side past the B+ jumper and the alternator.

I'm not sure what power is cut off during the start but it's possible the value can be thrown off if something like the fan or wipers was drawing power up the right side. That said this technique should work very well to get a read on the starter current. I don't think the diesel or V8 has a thicker starter cable (which would change the 285 factor)
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