While driving the coach, the chassis batteries are quickly discharging

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While driving the coach, the chassis batteries are quickly discharging

March 17, 2019, 07:40:09 pm

On my 2004 U320, while driving the coach, the chassis batteries are quickly discharging to the point check engine light and alarm come on. The digital volt meter I have connected to chassis batteries and the dash voltmeter both drop below 12 volts. I turned on boost switch, that did not solve problem. I started generator, that provided temporary solution as at least it allowed me to continue and the voltage meter stays steady at about 13.2- 13.4 volts.. With generator running, boost switch can be on or off and chassis battery voltage remains strong and steady.
Here is what I have checked:
Serpentine belt driving alternator is tight.
All battery connections are tight
With the engine running, no generator, no pedestal power chassis batteries are reading 12.8-13 volts.
With the engine and generator shutdown and coach not connected to any power pedestal, and inverter shut off, the chassis batteries are reading 13 volts each.
I can continue to my final destination with generator running, but need to get to the bottom of this issue. I have a spare isolator with me. Not sure how to tell if alternator is causing the issue.
Any advice or guidance would be greatly appreciated. Thank you

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #1 – March 17, 2019, 08:03:44 pm

If you can get to a Autozone,have the individual batteries tested,sounds like one or more may be bad,make sure all connections are
clean and tight and go from there.Alternator should be putting out more voltage when running but bad battery may be hindering
it.

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #2 – March 17, 2019, 08:16:49 pm

With engine running and around 1000 RPM, your battery charging voltage should be about 14 volts. Looks like (from your data) that your alternator is not putting out what is should. Try eliminating all loads (turn every thing off) and see what your charging voltage is. You will need the boost switch on to get chassis battery voltage with engine running. With the boost sw. off, check your starting battery charging voltage and see if it compares to charging the chassis battery. If they are both low, you may need to replace the regulator on your alternator. Have a great day ---- Fritz

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #3 – March 17, 2019, 08:24:07 pm

There are 3 chassis batteries and the positive terminals are connected to each other and readily accessible. Can I do any harm if I carefully disconnect one battery at a time, let the coach run on the other 2 batteries and see if the output readings change? Reason would be to try to determine which battery, if any, might be bad

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #4 – March 17, 2019, 08:54:00 pm

If there is a battery disconnect switch, it could be bad. It could cause a chassis voltage drop.

Or, if there is corroded connection it too could cause a voltage drop. My U270 draws the following currents with various systems running, approximately:

16Amps - Engine running
24Amps - Defroster on high
34Amps - Defroster and lights on low beam

I once drove across the USA with a bad alternator during the day. The voltage slowly lowered, but I guess the batteries were good enough to supply the required 16Amps for 2500 miles.

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #5 – March 17, 2019, 08:55:58 pm

Quote from: cdm – March 17, 2019, 08:24:07 pm

Can I do any harm if I carefully disconnect one battery at a time, let the coach run on the other 2 batteries

No, the engine starter will have less amps to turn the engine but the engine shouldn't have any trouble turning over for testing. Now if you disconnect a good battery and you have one good battery and one bad battery the starter will struggle to start as 1 battery ain't going to have the oomp.

Mike

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #6 – March 17, 2019, 09:09:09 pm

You need to measure the voltage at the isolator. The center pin will be the alternator output. One end will be engine battery and will have the voltage sense wire attached with the engine battery wire. The other end will be the house batteries. Check your voltages with the engine running at high idle.

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #7 – March 17, 2019, 09:18:21 pm

You can test for a bad battery with a load tester which any shop should have, no need to do different things to battery connections. In any event, the voltage out from your alternator will be the same whether or not you have a bad battery. If your voltage checks of the alternator are good, then check for a bad battery. It wouldn't hurt to load test the batteries any way just so you'll know their status. I assume that you have 3 twelve volt batteries in parallel. and one bad one will not affect your charging voltage, but will reduce the amount of current that you will have available. Have a great day ---- Fritz

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #8 – March 17, 2019, 09:34:22 pm

Unless you have a dead short somewhere it has to be a bad alternator, isolator or battery. Divide and conquer. Should be fairly easy to figure out.

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #9 – March 17, 2019, 11:00:45 pm

Quote from: sedelange – March 17, 2019, 09:09:09 pm

You need to measure the voltage at the isolator. The center pin will be the alternator output. One end will be engine battery and will have the voltage sense wire attached with the engine battery wire. The other end will be the house batteries. Check your voltages with the engine running at high idle.

Please post the readings between these points and a good clean ground. That will start you on the road to isolating the problem. MUCH better than jumping to conclusions.

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #10 – March 18, 2019, 03:09:34 pm

Here are readings taken this morning:
See attached pictures. I have Digital BEP Voltage sensitive relay connected to the Isolator which was installed by previous owner

ALL READINGS BELOW TAKEN AT 1200 RPM
ISOLATOR READINGS AT HIGH IDLE- DISCONNECTED FROM PEDESTAL- NO GENERATOR, NO BOOST ON
HOUSE PIN TO GROUND- 12.8 VOLTS
CENTER PIN TO GROUND- (ALTERNATOR) - 48 VOLTS
CHASSIS PIN TO GROUND -12.4 VOLTS

ISOLATOR READINGS AT HIGH IDLE- DISCONNECTED FROM PEDESTAL- NO GENERATOR, WITH BOOST ON
HOUSE PIN TO GROUND - 12.8 VOLTS
CENTER PIN TO GROUND- (ALTERNATOR) -47.7 VOLTS
CHASSIS PIN TO GROUND - 12.46 VOLTS

ISOLATOR READINGS AT HIGH IDLE- GENERATOR RUNNING DISCONNECTED FROM PEDESTAL- NO BOOST ON

HOUSE PIN TO GROUND -13.47 VOLTS
CENTER PIN TO GROUND- (ALTERNATOR) - 47.8 VOLTS
CHASSIS PIN TO GROUND 13.4-VOLTS

ISOLATOR READINGS AT HIGH IDLE- GENERATOR RUNNING DISCONNECTED FROM PEDESTAL- BOOST ON
HOUSE PIN TO GROUND- 13.25 VOLTS
CENTER PIN TO GROUND- (ALTERNATOR) -47.7 VOLTS
CHASSIS PIN TO GROUND- 13.35 VOLTS

ALTERNATOR READING from positive terminal on back of alternator: At 1300 engine rpm, no generator, not connected to power pedestal alternator putting out 48.4 volts. Battery read 13.3

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #11 – March 18, 2019, 03:18:24 pm

Wow, the alternator is sure trying to do its best (or is bad)! 48 VDC is crazy high.

SO, either the isolator is bad or the alternator/regulator is bad.

Easy diagnose: With everything OFF, AND ALL HOUSE 12 VDC LOADS OFF (in case voltage really is 47+, you don't want to harm anything that runs off that bank) remove the cable from the center of the isolator (the one from the alternator B+) and add it to the cable/stud for the HOUSE battery bank.

You will need to move the sense wire to that same lug. The sense wire is normally on the chassis battery-- can be on any connection between isolator and chassis battery. BUT (yes, large BUT, you don't want to take a chance on exposing your engine ECU to 47+ VDC so you are going to use the house battery bank for the test!). Does not need to be large-gauge wire but needs to go from that lug with B+ and house battery cable to the sense terminal of the alternator. If tracing the battery end of the sense wire proves difficult, just run a new temporary 14-16 gauge wire.

Start engine and run as the same RPM as before. If readings are "normal" (around 14), the isolator is bad. If still 47+ VDC, the alternator/regulator is bad.

I am betting on the isolator being the culprit.

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #12 – March 18, 2019, 04:03:53 pm

Brett,
Thank you for that input. I switched the cables as you recommended, disconnected from pedestal, shut off all loads on the house batteries, started coach , ran at 1200 rpm. I am now getting 14 volt readings off back of alternator, at dash gauges, and at the house battery post on isolator.
if this means isolator is the problem, I have following questions:
1- can I run the coach 800 miles with wires as the are now on isolator?
2 - or should I install the new isolator now?
3- Any reason to discontinue use of Digital BEP Voltage sensitive relay? I also have on board a new Trik-L- Start I could install when new Isolator being installed.
Thank you for your help

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #13 – March 18, 2019, 04:39:21 pm

No urgent need to install a new isolator.

You could put ALL wires on one lug. Only negative is that if dry camping, all batteries will discharge as one. Basically you will have one LARGE battery bank.

So: B+, cable to chassis battery, cable to house battery and alternator sense wire are all together on one post.

Wire it like this and re-test. All should be around 14 VDC with engine running.

No first hand experience with the Digital BEP Voltage sensitive relay. If just a method of charging chassis battery from house bank (smart combiner), leaving it is just fine, but unnecessary, as the inverter/charger will charge all batteries since they are combined.

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #14 – March 18, 2019, 05:50:04 pm

CDM your picture of alternator shows it as a 14 volt unit which is what I'd expect. If you're getting 48 volts then wouldn't it be the voltage regulator gone wacky? Or alternator? I don't see how the isolator would be bad, don't see how it could raise the voltage from 14 to 48 ? But then I've never seen the inside of one.

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #15 – March 18, 2019, 05:53:34 pm

If the sense wire tells the alternator that voltage is too low, the alternator will work hard to "bring it up". Certainly, if a diode in the isolator is not allowing charging to the connection that has the sense wire on it (generally on the chassis battery side of the isolator), the sense wire will give a false-low signal to the alternator.

I have no idea how high it will go-- have never tested that. But, if connecting it directly results in the correct voltage I don't see a problem.

If the results when all hooked to the same lug are not around 14 VDC, then back to trouble-shooting.

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #16 – March 18, 2019, 06:18:15 pm

I installed the new isolator, disconnected coach from pedestal power, started coach and ran idle up to 1200 rpm. I checked all 3 terminals on new isolator and was reading 14 volts at each. Gauge on dash also reading 13.8-14 volts. Thanks to the input I received from all of you, I believe I have solved the problem.

Many thanks to everyone

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #17 – March 18, 2019, 06:22:22 pm

Actually, with a diode-based isolator, the center lug/alternator B+ will read about .7 VDC higher than the two other lugs, as that .7 VDC is lost to HEAT in the isolator. That is why there are cooling fins on the isolator-- to dissipate the heat.

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #18 – March 18, 2019, 06:32:37 pm

Thanks Brett, didn't know the isolator could cause that kind of problem. 48 volts? Wow!

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #19 – March 18, 2019, 06:49:17 pm

Quote from: folivier – March 18, 2019, 06:32:37 pm

Thanks Brett, didn't know the isolator could cause that kind of problem. 48 volts? Wow!

Ya, that is higher than I have seen as well. MUCH more common issue is poor connection/corrosion on the small-gauge sense wire connections.

As an example, even if chassis battery voltage is 13.5 to start, if there is sufficient resistance, the sense terminal of the alternator may only see 12.3 so it keeps raising the voltage-- commonly into the 15-16 VDC range. So, if those kinds of readings appear, start by checking the sense wire connections. If difficult to trace, just run a new one from the chassis battery lug of the isolator. EASY.

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #20 – March 18, 2019, 07:24:52 pm

cdm,was that your original isolator?

Re: While driving the coach, the chassis batteries are quickly discharging

Reply #21 – March 18, 2019, 09:22:58 pm

I have only owned the coach for a couple of years, and this is the only time I have changed the isolator. The one I removed does not look like its been there 15 years, neither does the BEP voltage sensitive relay. On my 2005 U270, I changed the isolator after it failed in a much different manner. It failed suddenly and voltage spiked very high as I was driving, burning out the dash radio system in a matter of seconds and causing the house batteries to boil and buckle.That isolator looked as though it was original to the coach. The mileage on both of these coaches was under 30,000 miles at the time of failure

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