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millerpb · 03-14-2010, 08:09 PM

My 06 Holiday Rambler Neptune with a Cummins 300 just started having an erratic tachometer. At cruising speed on a flat highway it periodically would jump from 2,000 to 3,000 and back several times. At lower speeds it would also jump back and forth erratically but with less amplitude. The engine was not racing and the transmission was not downshifting. It just seemed like an erratic tach. Has anyone else had similar problems? Was it most a faulty tach? Faulty sending unit?

Thanks.

KIX · 03-14-2010, 08:29 PM

I've had that problem on my coach. The tach was faulty and a new one fixed the problem. My coach is a Spartan MM chassis and a Cummins ISC 350hp engine. The tach on mine was very easy to change out.

Nonno · 03-14-2010, 08:34 PM

Haven't had the tach do that but a while back I have had it happen to the volts and also had the odometer go blank. Just recently I had all guages stop working (warning lights were ok). Didn't do anything for volts jumping because it corrected itself. To correct blank odometer and recent guage outage I did a reset by turning the chassis battery disconnect to the off position for about 5 min. When turned back on they were working. The reset may temporarily help with your problem but not sure what causes it or what the permanemt fix is.

millerpb · 03-14-2010, 08:59 PM

KIX,

Thanks for sharing your experience. I suspect that's the problem, just wanted to see if there were other possible solutions out there.

Bob,

I"ll try turning off the batteries for a while and see if that fixes it. If not I'll take it into Cummins.

Thanks.

walt2137 · 03-20-2010, 05:14 PM

Your alternator is going out, time for a rebuild.

millerpb · 03-20-2010, 06:57 PM

Walt2137,

Have you had this experience? If the alternator were going out I would expect the alternator light to indicate a problem, but that's not the case. Also, wouldn't low voltage affect other gauges too? Or can I expect the other gauges to start acting crazy soon?

walt2137 · 03-21-2010, 12:58 AM

Miller the alternator is what provides the signal to operate the tachometer. If the alternator is not charging and the tachometer quit or is/was erratic then the alternator is failing. Take the alternator off (pay attention to where the wires attach). Take the alternator to an alternator,generator and starter shop and have them rebuild it, lots cheaper than new.

Art Ward · 03-21-2010, 07:36 PM

The tach on my 2007 Monaco Cayman just started bouncing around. Thanks for the tip. Turning off the battery disconnect switche, will that diconnect the battery power source to the alternator? I'm new at this, probably be a good time to change the belt while I have it off.
I haven't had to do any thing on the engine, it looks like every thing is very hard to get at, any sugestions. Art Ward

millerpb · 03-22-2010, 06:11 AM

Thanks Walt, I'll check it out.

nplenzick · 03-22-2010, 06:27 AM

Quote:

Originally Posted by walt2137

Miller the alternator is what provides the signal to operate the tachometer. If the alternator is not charging and the tachometer quit or is/was erratic then the alternator is failing. Take the alternator off (pay attention to where the wires attach). Take the alternator to an alternator,generator and starter shop and have them rebuild it, lots cheaper than new.

My tach has done that since I've owned this coach for 2 1/2 years and 45K miles. When the alternator was replaced it did nothing to to cure the performance of the erratic tach. All lights started to flicker when idling and a short time latter the "alt light" came on. That's why we replaced the alternator. I strongly suspect a poor connection somewhere. My tach bounces more when turbo boost goes over 10 psi and bounces less when you have all the lights on. I hope one of these days soon I'll pull it out and clean ALL connections with DeOxit and see if that makes a difference. I wonder if the fellow who replaced his tach did that first if he would have had the same results before buying a new one.

RustyJC · 03-22-2010, 06:59 AM

I'm going to go out on a limb here because I'm not sure about the ISC, but on the ISB the common link between the intermittent tachometer and the alternator working/not working is the crank position sensor. This sensor generates a speed signal that the ECU sends over the CANbus to the tachometer. The ECU also must see an engine speed signal from the crank position sensor before it will send a "charge" signal to the voltage regulator in the alternator.

Crank position sensor failures aren't exactly commonplace, but they're not unheard of, either.

Just something to consider.....

Rusty

just_rving · 03-23-2010, 04:34 PM

On a 2006 the alternator doesn’t drive the tach the engine computer sends the signal thru J1939 communication

millerpb · 03-23-2010, 08:40 PM

I took Bob Russo's suggestion and disconnected the chassis battery for 5 minutes and the tach is acting normal now. We'll see if this is just temporary.

jimpattigeib · 03-06-2012, 12:49 PM

Millerpb, Are you still having problems with the tach, or did you find a fix. I have the erratic tach without much luck in a good repair, even checking all connections and disconnecting the battery.