Tach Mystery

[karista]

Yesterday I started the engine with the back-up starter battery as a test. The starter battery has its own switch and is connected directly to the starter. The house Bank Batteries were turned off. Engine started and ran fine using the starter battery only, but the Tach needle would jump back and forth from 8RPM (Idle) to zero.
I then turned on the house bank switch and the needle stopped jumping. I then turned the house bank switch back off and the Tach started jumping again.
Turned the starter battery off and started using the house bank and Tach was fine (No needle Jumping)
So, it is only jumping when running with the starter. Any ideas what could cause this??

[Craig Illman]

I believe the alternator needs someplace to charge to, to get output from the tap for the tachometer.

Craig

[mainesail]

Careful you don't blow your alternator doing that!!!!!! Your house bank likely feeds direct to house and turning the house switch on and off with the alt running can blow the diodes...

[KWKloeber]

Yesterday I started the engine with the back-up starter battery as a test. The starter battery has its own switch and is connected directly to the starter. The house Bank Batteries were turned off. Engine started and ran fine using the starter battery only, but the Tach needle would jump back and forth from 8RPM (Idle) to zero.
I then turned on the house bank switch and the needle stopped jumping. I then turned the house bank switch back off and the Tach started jumping again.
Turned the starter battery off and started using the house bank and Tach was fine (No needle Jumping)
So, it is only jumping when running with the starter. Any ideas what could cause this??

Need more info to diagnose:

Where's your SB, length of cable run? SB age/condition? vs. HB?
Were your SB and/or HB on a shore charger beforehand?
i.e., was the start SB charged and but the HB called for a charge? What was the voltmeter reading in each case?
Or due to the length of run, the alt "thinks" the house calls for a charge, but the start is a short run and fully charged?

Ken

[Stu Jackson]

You have to possibility of blowing out the diodes in your alternator if it is not connected to a battery.

If your AO runs to the switch first and it was off, you could damage the diodes.

The tach was jumping because the AO had nowhere to go.

[karista]

Stu!
The starting battery was connected so the alternator was always connected to a battery. Also my main HB battery switch allows switching w/o affecting the diodes.
I believe Ken's response most likely clarified this issue. The wire length from the Starting Battery to the starter is only about 30" whereby the HB wire run is very long.

Ken
Both the starter and HB was on a charger prior to starting the engine, so they were both fully charged.

[KWKloeber]

Stu!
The starting battery was connected so the alternator was always connected to a battery. Also my main HB battery switch allows switching w/o affecting the diodes.
I believe Ken's response most likely clarified this issue. The wire length from the Starting Battery to the starter is only about 30" whereby the HB wire run is very long.

Ken
Both the starter and HB was on a charger prior to starting the engine, so they were both fully charged.

Ok, so test it out - you know where I'm going w/ this....

Run the SB down a little, then start and see what the tach does.

Get nominative --- bulk charge and well top 'em both off -- wait for surface charges to dissipate. 
Check voltage at the connection from both batts -- not AT each battery - ie, each one at the starter.  See what you have and if makes sense with what happened.

Was the SD on charge right up to before cranking -- i.e., did it still hold a surface charge?  i.e. maybe higher then the HB if both were on charge? 

Are they both the same age. condition, TYPE?? etc.  is one accepting a charge to 100% or is the HB at 95%? 


Lotsa possibilities here.

ken