Please help auxiliary start!

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Aarondj

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Aug 29, 2018
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I've had this 1987 Komfort motorhome for a little over a year now we started having a problem with the chassis battery draining down to nothing and not being charged off the shore line, so I went to use the auxiliary start today and it failed to work. I checked the fuse under the hood it was blown so I replaced it then I pulled out a multimeter and I checked the solenoid both sides, the side with the house batteries read 13.7, the side with the chassis battery obviously read zero because I've been having a problem with it so I hooked a booster charger to my chassis battery and then tested the other side of the solenoid and it read about 7.5 at the time which isn't bad considering the battery was completely dead and I just had the booster charge on it. At the front of the cab I have a little 12 volt light that I had on while I was looking at my Dash and when I push my auxiliary start just that light dims really bad the rest of them stay fine. Any suggestions as to what's wrong with my auxiliary start for starters?
 
Hi Aarondj, and welcome to the RVForum. The symptom you described of the light going dim when you push the auxiliary start button tells me that your house battery may be dead too, or very weak. How old is it? That 13.7 volt reading is voltage from the charger, not the battery. A fully charged 12 volt battery will indicate 12.6 volts, but you won't get an accurate reading with a volt meter unless you disconnect the charger and let the battery sit for an hour (or so) with no loads on it, then test it.

Unfortunately, the chassis battery sounds like it might be ready for the big battery house in the sky. Running a lead/acid battery dead is really bad for the battery, especially if it's done repeatedly and not fully recharged soon thereafter. Depending on its age and condition, it may never be able to fully charge.

It would be a good idea to check all the battery cable connections, but without knowing more, my guess is both your house and chassis batteries may need to be replaced.

Kev
 
The aux start function isn't magic - all it does is connect the house battery(s) to the chassis battery, in parallel so that the house battery can help.  If the chassis battery is severely discharged or has a shorted cell, it still drags down the overall system voltage. And if the house battery is not performing well, it can't help much either.  As Kevin mentioned, the 13.7v sounds like the output from the house converter/charger, so probably does not represent house battery status.

As long as the Aux Start solenoid closes when engaged, it is doing all it can do. The rest of the function depends on battery condition and (of course) the engine starter circuit itself.
 
I do know that my chassis battery may need to be replaced but my house batteries are perfectly fine I checked them without any sort of charge going to them and there reading a solid 12.7 both of them. The side of my auxiliary start solenoid that is attached to my auxiliary batteries is reading charge but not the side that's hooked to the chassis battery. Could a weak chassis battery be causing all of this?
 
Are you running your generator while trying to use the emergency start?  I have a Fleetwood and mine is intended to use in conjunction with the generator.  In fact, it will not work without the generator being on.  It does a great job of starting my motorhome when the vehicle battery is dead, however. 
 
No I'm not using it while the generator is running, I've had to use the auxiliary start before in the past and the generator was never running before it wasn't even hooked up to shore power at the time when I used it.
 
If you get zero volts on the chassis side (measured to chassis ground), then the solenoid hasn't closed to complete the circuit.  Disconnect the cable on the chassis side, have somebody push & hold the Aux Start switch, and take a voltage measurement to ground.  The chassis side should read the same as the house side if the solenoid is closing.
 
The aux starting switch is just built in jumper cables from the house batteries to the chassis battery. It is just like using jumpers from another vehicle. All of the techniques still apply.

If the chassis battery is shorted you will not get enough power to start the engine. It will pull the whole circuit down.

If the chassis battery is very low you will have to hold the aux switch for a long time to let the house batteries bring the chassis battery up sufficiently to get enough power to the starter. Just like having to let another car idle for a bit when using jumper cables. At first the starter will barely turn the engine, then, with patience, it will finally have the power to crank over enough to start the engine.
 
Thank you that was possibly the most direct answer to my auxiliary start question possible and knowing this helps me move forward a bit thank you very much!
 
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