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Old 06-01-2015, 02:28 AM   #1
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1991 25' Airstream 250
Oxford , Oxfordshire
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12V gremlins; what's the best process to fault find?

Hope you guys can help, because this problem is killing me.

I've had some 12v gremlins on the coach since last Summer. We had the first big camping trip this weekend and had further 12V issues. There appears to be 12v leakage across the auto and coach systems. Specifically, there is a fault which can be replicated which I assume relates to a 12v thermal breaker.

Position is as follows:

Starter and Leisure batteries both fully charged, with good voltage readings and terminals cleaned/tight connections (I have triple checked this);
I replaced the Isolator last year with a like for like replacement, so that should be fine;
Starter solenoid does not appear to be functioning correctly, but is yet to be replaced;
We were boon docking this weekend and working mostly gas with the only real electrical drain coming from the Dometic fridge controller (fridge running on gas);
Internal 12v systems work fine and voltage on 12 V distribution board matches the leisure battery (12.68v);
If you then run additional circuits concurrently e.g. cigarette lighter to charge an iPhone, the interior lights or fan - the fridge 12v supply cuts out - no lights on the fridge.

I then went to check the voltages again - results as follows;

These tests were all done with the Auto battery disconnected (so not to confuse the position).
Voltage on 12V distribution board is now only 4v (was 12.68v);
Voltage across leisure batteries is still a healthy 12.68v;
Voltage on rear of kill switch (which feeds 12 v board is only 4v)
Voltage on battery side of kill switch is only 4v;
Voltage on either side of thermal breaker on rear of battery box is only 4v;
Voltage on lesiure battery side of solenoid is only 4v;
HERE IS THE REALLY WEIRD BIT, the voltage on the end of the heavy red cable coming directly from the leisure batteries is only 4V - SEEMS IMPOSSIBLE BECAUSE THE VOLTAGE ON THE BATTERY TERMINAL READS 12.68v;
After a while (and I am assuming this is where a thermal breaker is somehow involved) all comes back to normal where fridge works, voltage on 12v distribution panel all matches the batteries (12.68v).

I then did some further tests. The 12v kill switch does kill voltage to the 12V distribution board, so that is all working fine.
I then reconnected the starter battery with the kill switch off, but the fridge still worked as did other interior 12v services, which suggests that the auto and coach batteries are connected in some way (I am assuming via faulty solenoid or someone splicing together a circuit).

Can any one advise on where i might start with fault finding on this one?

Thanks in advance for any thoughts on how to move this one forward.


PS I have got a Sure Power: 12V 200A Battery Separator to replace the stock solenoid on the back of the battery tray, so will try and fix that over the weekend. Sure Power 12V Bi-Directional Battery Separator (200 Amp) - Part#: 1315-200

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Old 06-01-2015, 04:00 AM   #2
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12V gremlins; what's the best process to fault find?

Assuming the problems are new I would expect bad connections. Cleanup and re torque everything you can get at. Especially grounds. Check all the crimp on connectors. They are notorious for voltage drops as they age and corrode. The plastic sleeves are great at hiding problems.

As far what some PO did... One can only guess.

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Old 06-01-2015, 04:19 AM   #3
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After 40 years working with electronics and acoustic equipment on and under the ocean, I've arrived at Gringo's Rule #1: It's Always Connectors.

Something is either not getting to where it needs to go, or something is getting into somewhere you don't want it. Either way, it's through a bad connector of some kind.
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Old 06-01-2015, 06:50 AM   #4
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Ditto on the GROUNDING!. The second year I had my AS, was camping in Yellowstone in early May and the lights slowly started to dim and in about 3 hours everything that required 12v stop working. I was connected to 30 Amp shore power and even went so far as to, in the middle of the night, connected my truck to the AS to get 12v power. No joy! After cutting the trip short and arriving home, went to Parallax (located about 20 minutes from my home) and they swarmed all over the AS. They diagnosed that there was a major grounding issue and that this had damaged the AGM batteries. Parallax contacted Airstream with their results. AS called and had me bring the AS to Jackson Center (about 2 hour drive from the house). There they found that the main ground wire from the converter to the ground bus bar had come loose and spark welded just a couple of strands of the wire to the bar and that made things work, but as the load increased it cause multiple problems. The techs at AS replaced the wire and also replaced the AGM's all at no cost.
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Old 06-01-2015, 10:09 AM   #5
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1991 25' Airstream 250
Oxford , Oxfordshire
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Thanks all, I will go round the loop again with the connections and I totally get that because I have been caught out by them before. My sense is that this is something different. If I find that someone has spliced together the auto and coach batteries, what is the fault finding technique? I am assuming that it is pulling the fuses on the chevy fuse block to see if that causes any change to the voltage issue.

Does anyone know whether those small 12V thermal breakers in the 12 V distribution board are prone to failure?


PS Any other thoughts gratefully receieved.
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Old 06-01-2015, 10:26 AM   #6
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We still have one funky 12v circuit, but our major problems were solved when my son found that one of the big cable terminals going to the solenoid that you are about to replace on the back of the battery tray had broken and would shift every time the tray was opened and closed.
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Old 06-01-2015, 10:33 AM   #7
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All I keep seeing is voltage..voltage...voltage....voltage. How about you hook up your probes and see how much current you are drawing. Disconnect everything....then start plugging in one thing after the next and see whats drawing all the current. the measurement of current.......That's how I would trace it. I would hook the probes in series with the fuse panel and then connect one thing at a time. When you see the current start to go up then you know there is an issue if the item is suppose to be off. I kept blowing my fuse yesterday on the motherboard for my home A/C. I don't know a damn thing about A/C, but I know electronics. I followed the Schematic and found a bad coil that was drawing too much current. Kept blowing a 3 amp fuse. That with Youtube and I was able to fix the problem with an A/C tech coming to the house. It cost me $11.53 what I found out would normally cost $300 service call.
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Old 06-01-2015, 10:34 AM   #8
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1) Loose or dirty connections?? Check!!

2) Bad ground?? Check!!

3) Have you checked battery isolator??.....connects car battery to house battery when engine is running - allows alternator to charge both batteries. Isolates house battery from car battery when engine is off - allows fully charged, "isolated" car battery to start motor.

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Old 06-01-2015, 10:36 AM   #9
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A couple of years ago I was trying to trace a voltage drop in some cables on our Airstream by using a Digital multi-meter. A pal of mine walked by. He works as a linesman on very high voltage overhead power lines. He asked me what I was seeking. He then ran one hand over the cables, and said "That's your problem". He had detected the warmth of a resistance from a partly broken invisible internal wire causing heat. Sure enough, he was exactly right. He had no use for my meter. I learnt something that day.
(I have a digital "gun" thermometer which I use to balance the brakes on the trailer, so perhaps that could be used for fault detection.)
Nick Crowhurst, Excella 25 1988, Dodge Ram 2500 Cummins Diesel. England in summer, USA in winter.
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Old 06-01-2015, 01:46 PM   #10
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Portland , Oregon
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Boom Sounds, you wrote:

"HERE IS THE REALLY WEIRD BIT, the voltage on the end of the heavy red cable coming directly from the leisure batteries is only 4V - SEEMS IMPOSSIBLE BECAUSE THE VOLTAGE ON THE BATTERY TERMINAL READS 12.68v"

When you got that 4V reading at the end of the big cable, was there anything connected to the cable besides the battery at the other end? Were you using the battery for your meter's ground, or getting it from the chassis?

If there was anything connected besides your meter's plus lead, disconnect everything from that cable and check voltage again. If it's still low, voltage drop is being caused either by a faulty cable or, if you were using the chassis for your meter's ground, a bad ground at the battery. Check the end of the ground cable where it attaches to the chassis or ground buss.
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Old 06-01-2015, 03:27 PM   #11
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Feeling re-invigorated and ready for battle! Thanks all and will keep you posted as to how I go on.
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Old 06-01-2015, 05:27 PM   #12
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+1 on connectors. When I had an electrical gremlin problem on my then 3 month old Airstream Interstate discovered a loose connector on the battery isolator. It was obviously not fully tightened during assembly.

- - Mike
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Old 06-02-2015, 08:13 AM   #13
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champaign , Illinois
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Whether its the dause of the problem or not it would be a good time to clean up all ground connections to the frame. As everyone knows a bad or intermitten ground connection can do some wild and crazy things. Plus, with electrical gremlins, Ive found that I need to ALWAYS ALWAYS start with the easiest thing first. Cant tell you how many times Ive dug in with all kinds of theories and it ends up being something quite simple. Dont forget your ground from the converter to frame also, that connector corrodes pretty bad. Not part of your existing problem but as long as you are cleaning up your grounds might as well clean them all up then you know thats done. (genset ground strap also)
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Old 06-02-2015, 09:04 AM   #14
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Hot Springs , Arkansas
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As stated several times, check all the ground connections. Make sure the terminal connection is clean as well as the chassis connection. That part is often overlooked and is a big cause of problems.

Look for splices on the circuits that cause problems. Does the power plug on the 12v charger get excessively warm? If there is a short or partial short to the wrong circuit, the wiring can act as a big resistor and will eventually melt or burn through. Any funky fried wiring smells?

Next, check the resistance on the circuit with the battery(ies) disconnected. Check the ground side at the device and outlet to chassis. With the places that you are having difficulty working correctly, that will help you eliminate them one at a time.
Are your suspected wiring issues in the same area of the coach?
What are your voltages with Key On, Engine On?
I hope it is something simple and embarrassingly obvious. Your secret will be safe with us......

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