Genset interferes with tach - any theories?

bobofthenorth · June 28, 2009, 03:35:27 PM

My VDO tach takes its input from the AC signal on my alternator. Now that I have my genset working it appears that the genset is somehow interfering with the tach. If I run the genset while we are travelling my tach is extremely erratic - it works briefly, drops to zero, jumps up to roughly the right speed, sometimes seems to stick at one speed and just generally isn't all that useful. It obviously not a deal breaker but I'm thinking there's likely some little electronic bit that I could put in somewhere to cure this behaviour. Like a diode or a capacitor or (gasp) even maybe a resistor.

All I know for sure is that it is way beyond my theoretical skills to figure this one out.

Jerry Liebler · June 28, 2009, 05:09:17 PM

Bob,
The genset is producing 120 VAC & should have no effect on the tach which is in the bus DC circuits. The only thing common should be chassis ground & at least in the case of AC any current flowing in ground is an indication of a problem. So I'd suggest trouble shooting by turning off AC breakers to find which circuit has the problem. Correcting the excess leakage in an AC load should cure the problem but just for good measure you could add a ground wire from the tach back to the alternator which would short out any coupling from the genset.
Regards
Jerry 4107 1120

olebusman · June 28, 2009, 05:15:47 PM

Maybe your inverter is charging your batts and confusing your bus alt. regulator?

olebusman

gmbusguy1 · June 28, 2009, 05:46:28 PM

Bob, what does your voltage read when the tach screws up???

do these Prevost's have a no generator light on the dash???

You may need to reset your voltage regulator on the Coach Generator to be just above the charge setting from the inverter/charger

Chris

Jerry Liebler · June 28, 2009, 06:44:59 PM

To avoid the issue of the inverter charging the coach battery disconnect any battery intertie while running the genset.

Kwajdiver · June 28, 2009, 06:52:02 PM

Is it possible to have someone flip off breakers while your driving? See if the problem goes away with just one breaker. As Jerry said, you may have some coupling going on. This may prove it so. or it could tell you nothing at all.

Bill

RTS/Daytona · June 28, 2009, 08:01:12 PM

The "R" terminal is the 12 volt (inverted sine wave) output of the 50DN generator

many coaches use that output to feed a very poorly designed 12 volt charging system to eqaulize the batteries when using 12 volt devices - such as 12v headlights

Because it is connected (indirectly with the 12volt side of the 24 volt battery battery bank) - this can reflect back onto the "R" terminal line

First - see if your tack goes crazy with your inverter/charger off - If the problem disappears - your tacks goes crazy because your inverter is charging the 24 volt side of the bank - and some noise is getting thru to the 12 volt side - especially if you have a VANNER

BUT MUCH MORE LIKELY

the tack goes crazy cause the generator is connected to the 12volt side of the bus start batteries - I believe your generator has an unfiltered built-in 12 volt charging system that being reflected back to the 12Volt "R" terminal - If this is the case - give the genny it's own small 12 volt battery

I have noticed on my tack - the 24 regulator makes my needle bouce after I first start the bus - but quites down after a few minutes - I've always attributed that to a noisey regulator

Hope this helps - let us know if thats the problem

P.S. Some Buses use a fregency detection device connected to the "R" terminal to turn off the "generator" light - It looks for a frequency from the "R" terminal that above the 400 rpms (about 100 pulse/sec) frequency from the 50DM to turn off the GEN LIGHT - SOMETIMES WHEN runing the engine while your still plugged in - your inverter/charger will actually cancel out the pulse wave - makinging it flat and you GEN LIGHT will come on - Happens a lot with RTS's

Melbo · June 28, 2009, 08:19:30 PM

hey bob would the trimetric you installed have anything to do with this???

The trimetric monitors the charge and you just added it

Trying to think out of the box here

Melbo

BUR · June 28, 2009, 09:05:24 PM

Bob I have a 2500 watt Prosine inverter, before I start a trip I turn the charger off on the Prosine or tack will sometimes drop to O rpms. Hope this helps. BUR

Sean · June 28, 2009, 09:19:45 PM

Bob,

Jerry L. has the right answer.

When your genny is running, your inverter/charger is trying to charge the batteries. If the house and chassis batteries are tied together while this is happening, when the batteries get to be mostly charged, the alternator regulator sees the charge voltage coming from the 12-volt charger as an "overcharged" condition on the batteries, and basically turns the alternator field windings off altogether. This causes alternator output on both the main output and "R" terminals to drop to zero.

In addition to illuminating the "not generating" or "charge" light on the dash, this will cause your alternator-driven tach to drop to zero.

As the 120-volt charger cycles in and out (in "float" mode), this condition will come and go, leading to erratic tach and "charge" indications.

This happens exactly this way on my bus if I happen to run the genny when I drive. I have a manual switch to disconnect the house-to-chassis tie when this happens.

HTH,

-Sean
http://ourodyssey.blogspot.com

bobofthenorth · June 28, 2009, 09:43:11 PM

Jerry & Sean that makes good sense. I'll try disabling the house to chassis connection the next time we are on the move and see what happens.

TomC · June 28, 2009, 10:01:36 PM

I have my gen hooked to my starting batteries. The little alternator on the generator was making the big 50DN not charge, so my no charge light came on my dash. I simply disconnected the little alternator on the generator and problem solved. Good Luck, TomC

JackConrad · June 29, 2009, 04:43:16 AM

Just a thought. Could it be EMI/RFI from unshielded wire? Jack

bobofthenorth · June 29, 2009, 07:58:08 AM

That's what I was thinking too Jack but Sean & Jerry have me convinced that the bus alternator is just shutting down when it sees the output from my Freedom 25. We're sitting for a couple of weeks now so I can't test the theory but I will check it when we move and post the results. I don't have a "not gen" light on the dash so I can't look for that clue but most of the behaviour fits with the alternator simply shutting down.