Jump to content

Generator Starting Issues


Recommended Posts

For reference.

My 09 42’ Scepter with a 10K gen is slow cranking except when driving or the batteries are being charged. And much more so in cooler weather. I’ve known for some time that the voltage is low at the gen. The gen is connected to the house batteries. 

Summary: ALL connections are clean and good including lug to wire connections. I used a battery load tester connected to the gen end of the positive  cable to measure voltage with a 85A load. The 1/O 44’ positive cable has a 0.4V drop (same as a wire wire calculator) and the ground which uses the frame has a 0.4V drop and the Monaco Blue Seas switch has a 0.4V drop. Other drops are small.  Replacing the switch and using 2 4/O cables will help but would still be  marginal. Not to mention what’s involved adding those cables.

Clearly the best solution is to install a small battery and battery isolator next to the gen.

Link to comment
Share on other sites

Odd that the gen would start from the house batts.

Mine is wired to start from the chassis batt, so if the house batts have been pulled down it will still start and recharge them.

A garden tractor battery and isolator could be a good solution though, and a lot less trouble to install than pulling larger wire throughout the coach.

Link to comment
Share on other sites

Monaco wired some to the chassis battery and some to the house battery.  The thinking for the house battery wiring may have been that the chassis battery is smaller and could become discharged for various reasons and the priority remains with starting the engine. 

Plus using the battery boost switch with the gen connected to the house batteries is a user option for gen starting. 

There are pros and cons to either one. The biggest con for either one is the long distance from the gen to the DP rear batteries. Makes you wonder if the house batteries should be forward and better located for the inverter and gen. My inverter cables are 4/O and 21' one way and can easily pull 150A. 

At any rate this is not the only marginal wiring that Monaco used in my MH.

Link to comment
Share on other sites

  • 2 months later...

Its possible that if the batteries are not the issue, then the issue could be in the starter.  I have an old Jeep that I used to blame the battery on a slow start, but after the starter was replaced, all I can say is Wow, fires right up.  Same battery is still in the Jeep. 

Link to comment
Share on other sites

  • 1 year later...

Just did this. My coach is wired such that the generator starts from house batteries. I tried wiring up the gen start switch to trigger the Big Boy and still had starting issues. Onan manual for my 12.5 recommends 2/0 cable for the run length where Monaco has 2ga coming from the FRB. I riveted a battery tray to the front of the thing and rigged up some cables and an isolator. Highly recommend.


Edit: measure the genset pos/neg terminals when cranking.. the ECM cuts out starting when the voltage sags to 6V, which mine definitely did. 40 something feet of 4/0 plus 20 something feet of 2ga plus the ground cable and frame plus crappy crimp joints.. there's a LOT of loss from the rear of the coach to the front. Cranking power I measured at 300A.


Edited by trailmug
  • Like 1
Link to comment
Share on other sites

  • 2 months later...

Sure thing, I had some cables made from custombatterycables.com (details below).

Battery isolator: https://www.amazon.com/gp/product/B00WTAFR84
Battery cage: https://www.summitracing.com/parts/TGI-301347-KIT
Battery: Optima Red Top 78
NSI ITOG-1 wire splice (for splicing the Monaco 2ga hot wire to the battery isolator cable).

Painted battery cage and riveted to generator front cover.
Ran 1/0 ground cable to generator negative terminal, leaving the existing ground wire in place.
Ran new #2 from battery isolator to rear of generator, cut off and discarded the factory red lug, spliced to new red 2ga battery isolator cable.
Ran new 1/0 hot cable to generator positive terminal.
Attached isolator battery side terminal to aux 4ga from red battery cable.
Butt spliced isolator ground wire to aux 10ga from black battery cable.

The side terminals they used were too thick for regular battery bolts, so I had to get extended ones. Maybe next time I'd get a 34/78 battery and use regular clamp lugs. I like to sit tools on top of the generator, though, and didn't want one to fall down across the battery. Using some air compressor hose worked well to measure the necessary cable lengths (roughly the same size and flexibility). I recommend following the existing wire paths as much as possible, as if you have a generator slide, the cables need to lay in such a way that they're not pinched when you retract it.


#70 - Custom: 112 in, 1/0 ga SGT Black, Side Mount (short bolt) , std 5/16 lug AUX: 10 in, 10 ga L no end $88.89 USD 1 $88.89 USD
#70 - Custom: 74 in, 2 ga SGT Red, std 1/4 lug , no end $34.25 USD 1 $34.25 USD
#889 - Add Loom - Per cable Options: $5.00 USD 1 $5.00 USD
#70 - Custom: 98 in, 1/0 ga SGT Red, Side Mount (short bolt) w/cov, HD 5/16 lug AUX: 24 in, 4 ga L std 1/4 lug $100.36 USD 1 $100.36 USD
#889 - Add Loom - Per cable Options: $5.00 USD 1

$5.00 USD

Edited by trailmug
Link to comment
Share on other sites

I spent a fair amount of time decreasing the losses between my batteries and the inverter, had close to 0.8 V under high load…150 amps. This helped the generator starting and on occasion I still have to use the boost switch. 

Had thought about adding a separate battery for the generator with a large $10 diode as the isolator… 50 amps for charging.  At this point I’ve put this project on the shelf.


Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...