Alternator and tack eratic

[bigcontender]

My tack seems to guit working , goes to 0 then bounces back up. The voltage drops at the same time then returns to normal when the tack starts working. I've cleaned the battery terminals and cable groung connections. Should I just repalce the alternator. Truck had 2 alternators put on it when I bought it used 3/02 , now has 72000 miles.
Thanks

[tom.mcinerney]

The fact that you see voltage dropping when tach screwy could point to instrument panel ground, or alternator , or alternator wiring {did check alternator output wire(it has fusible link)?; reseat connector that plugs into alternator?}.
If you monitor alternator output voltage while the panel voltmeter dips that could indicate whether problem in alternator or panel wiring.
A good load test of alternator will detect poor output; if it passes it should mantain the voltmeter. But it's possible for the alternator to pass a service station's load test, yet still be bad enough to screw up the tachometer .
The engine wiring harness grounds could be problem too.

[ChevyDooley]

Mine did that just before the alt'nater crapped out. ;)

[bigcontender]

This morning I took along my booster battery in case I had problems starting the truck today. I plugged it in to the cigaret lighter to charge the booster bat up. Guess what the bat voltage and tack poped up to 14v shartly after it started and was rock solid with the aux bat plugged in, so I was thinking an insturment pannel ground or insturment voltage regulator. I then unplugged the aux bat tonight for the trip homa and the voltage was still a solid 14v on 2 gauges. Go figure.

[tom.mcinerney]

Alternator output becomes intermittent when brushes wear down. The recent extreme cold could have opened a connection somewhere.

[bigcontender]

Yup I replaced the alternator last night and the voltage looks fine now. I'm just wondering why an alternator (2 replaced by dealer in 3 weeks) lasted for only 2 years / 20000 miles. I'm wondering if my glow plug controller is working right. The glow plugs cycle 3 or 4 times everytime it's started. Parked in garage about 30 deg. Comments?
Thanks

[bobt]

That's normal for my glow plug controller.
Correction: That's normal for me when the vehicle is started the first time or until it gets warmed up.
Bob

[ 02-07-2004, 07:59 PM: Message edited by: Bob T ]

[Beedee]

The tach runs off the alt, it is only an approximation of the real engine speed, but good enough for most people. If your alt packs in the tach will quit working.
My alt just quit with out warning today, daughter came home from downtown with " a red light on". Sure enough no tach and only reading about 10 volts, took the alt off and down to the local auto electric store, cost me $40.00 (can) to have the rectifier bridge replaced. The intresting part was, the tech just took one look and said rectifier. Could the stock GM alt be a problem child??? :D