Jumping Alternator and Tach
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Senior User
Joined: Oct 2013
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From: Jordan Station, ON
Jumping Alternator and Tach
So, I just recently (this fall) installed a voltage gauge in my truck. I noticed that most of the time at idle, the voltage jumps around pretty rapidly (needle bouncing back and forth), and so does my tach (has ever since I installed it, even with a resistor in line). HOWEVER, sometimes my volt gauge reads rock solid, usually when I'm running the truck at some RPM above idle, but not moving (sitting in driveway at 2k RPM). At this time, the tach is also rock solid, no movement. So, my question is, is this normal? I don't think it should be, which leads to my 2nd question, what should I look into for fixing it?
Laughing Gas
Joined: Jun 2016
Posts: 855
Likes: 56
From: Des Moines, Washington
I know this sounds like I'm blowing smoke but the first thing I'd check is the fan/alternator belts. If they are loose or glazed, they'll slip at idle and could cause your symptoms.
Other things to check are the usual grounds and have the alternator checked for open diodes. An alternator isn't called a generator 'cause it actually creates 3 phase AC which is rectified by 6 diodes into a relatively smooth (some ripple) DC which is then "filtered" by the battery to a much smoother (very little ripple) DC. An open diode will increase the ripple, maybe to the point it's visible as a wiggling gauge needle. The voltage regulator could also be needing replacement.
Other things to check are the usual grounds and have the alternator checked for open diodes. An alternator isn't called a generator 'cause it actually creates 3 phase AC which is rectified by 6 diodes into a relatively smooth (some ripple) DC which is then "filtered" by the battery to a much smoother (very little ripple) DC. An open diode will increase the ripple, maybe to the point it's visible as a wiggling gauge needle. The voltage regulator could also be needing replacement.
Thread Starter
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Senior User
Joined: Oct 2013
Posts: 328
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From: Jordan Station, ON
Ok. So, I do have new belts, so that's probably not a glazing issue, but I'll check the tension for sure. Now, I did 'rebuild' the alternator with a little kit from partsource about 2 yrs ago... I think it was those diodes that you're talking about. My alternator wasn't charging, and that fixed it. I'll double check on the grounds. One thing I just realized since you mentioned grounds is I didn't re-install the bellhousing to frame ground strap after I had my engine out this winter... That *could* be an issue potentially. It also might be why my CB SWR is way out the roof, but that's a separate issue. So, I'll start with fixing those things. Thanks for the recommendation! Also, with our voltage regualtors, is there a surefire way of testing them?
Laughing Gas
Joined: Jun 2016
Posts: 855
Likes: 56
From: Des Moines, Washington
New belts do stretch so it could be low tension.
The alternator kit could have been just brushes or may have included diodes. The diodes are mounted to one or two semi-circular plates and would need soldering to replace. That'd be something you'd remember as it takes a pretty good sized soldering iron.
Most regulator tests are going to check for whether it's functioning and within the correct voltage range. I personally don't know of a test for the problem you're having.
The missing ground strap could be an issue. Without a good ground reference, the regulator might struggle at low speeds to keep things under control while higher speeds sort of smooths thing out.
High SWR could be a number of things. Loose or corroded connections, water in the coax, crushed coax, poor grounding of adjacent body panels and installation/removal of something near the antenna such as a roof rack. I never did figure out why the SWR on my 2M base station co-linear went through the roof. I pulled the antenna, looked at it then reinstalled it and it's been fine since.
The alternator kit could have been just brushes or may have included diodes. The diodes are mounted to one or two semi-circular plates and would need soldering to replace. That'd be something you'd remember as it takes a pretty good sized soldering iron.
Most regulator tests are going to check for whether it's functioning and within the correct voltage range. I personally don't know of a test for the problem you're having.
The missing ground strap could be an issue. Without a good ground reference, the regulator might struggle at low speeds to keep things under control while higher speeds sort of smooths thing out.
High SWR could be a number of things. Loose or corroded connections, water in the coax, crushed coax, poor grounding of adjacent body panels and installation/removal of something near the antenna such as a roof rack. I never did figure out why the SWR on my 2M base station co-linear went through the roof. I pulled the antenna, looked at it then reinstalled it and it's been fine since.
Last edited by lasermike; Oct 24, 2016 at 02:17 PM. Reason: Can't spell
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