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Amp Reading and my alternator

 
Old 05-15-2019, 09:11 AM
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Joe79Vette
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Default Amp Reading and my alternator

I have a 72 that I added a Dewitt fan package radiator to. I kept the factory fan too, so the electric fan only comes on when sitting in traffic etc. It is great and I wired it though the starter. My question is when the fan kicks on the Amp Gauge goes to +20 and stays there until the fan turns off. I am still using my original 42 amp alternator that I rebuilt. I replaced all the wiring with new Lectric harnesses. Do I need to go to a bigger alternator? The car is a low option one, No AC, or power windows etc.

Thanks,

Joey
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Old 05-15-2019, 09:41 AM
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lakerider57
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My guess is that yes, you need to go with a bigger alternator since at idle that alternator is not putting out anywhere near 42A. Ideally, you should choose around a 100A alternator or better. If you do that, then you also need to upgrade your wiring.

I have a DeWitts radiator with dual SPAL fans and I went to a 100A alternator and upgraded from 10g to 4g wiring for hot & ground. I originally upgraded to 6g but DUB convinced to upgrade to 4g.

Here is the alternator I went to ($124), sorta plug-n-play, there are cheaper ones out there that are not chrome:

https://www.jegs.com/i/Proform/778/141-657/10002/-1

Richard

Last edited by lakerider57; 05-15-2019 at 09:43 AM.
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Old 05-15-2019, 10:12 AM
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7T1vette
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Your system is fine, IMO...except that your ammeter is reading backwards. It should read -20amps when the fan kicks on. It show 'discharge' when that happens because the 42 amp alternator can't put out an adequate amount of juice with the extra load at idle. So what? The battery makes it up for the short time you are at idle. I'm sure that when you are driving at 30 mph or higher, the ammeter goes down to almost '0'. Actually, it should read a little to the + [charging] side when you are at speed, which would indicate that your alternator is making up the lost charge from the battery.

Your system sounds like it is working exactly as it should. Now, if you intend to be idling for long periods of time, THEN you might need to upgrade the alternator. Otherwise, you're good to go.
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Old 05-15-2019, 03:10 PM
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lionelhutz
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The meter is working exactly like it should. It's showing the current from the alternator to the solenoid and since you wired the fans after the solenoid it's showing the fan current.

Is this one electric fan or two? The alternator isn't keeping up at idle if you have 2 fans because 20A is less than what 2 fans would draw. Both the alternator and the wiring should be upgraded to properly handle the electric fans. A CS144 alternator and #6 gauge wire would work for the upgrades.
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Old 05-15-2019, 03:15 PM
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Joe79Vette
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It is the single Dewitt fan. Which I think draws 21.2 amp or so per their website.

Thanks
Joey
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Old 05-15-2019, 08:19 PM
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carriljc
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You need a higher capacity alternator. Your indication are correct as mentioned by lionelhutz, however you are drawing down your battery every time that fan energizes.

You can check this by taking a voltage reading at the battery terminals. Take a reading before the fan starts, and another after the fan starts, you should notice a definite voltage drop. Especially with that little 42 amp alternator.
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Old 05-15-2019, 08:23 PM
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Joe79Vette
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Thank you guys. I am going to check the voltage before and after the fans come on. I have a 61 amp for a 72 AC car that I will try too to see if there is a difference.

Thanks again

Joey
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Old 05-16-2019, 01:45 PM
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Joe79Vette
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Ok. Now I am really confused. Drove the car today after installing the 63 amp alternator. Fans kick on the amp gauge goes to +20. Decided to see what the headlights would do, turn them on and the gauge goes to zero. Is the right?

Joey
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Old 05-16-2019, 11:17 PM
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lionelhutz
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That probably loaded the alternator output voltage down enough that the alternator wasn't running the fan at all and it was only running off the battery.
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Old 05-17-2019, 02:49 PM
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I'm fairly sure that the power for the headlamps comes off the alternator side of the ammeter wiring. If so, then what you're doing is rebalancing the ammeter reading and it just means that the alternator output is supplying the headlamps and it isn't capable of supplying amps to keep your battery supplied... and that means that your battery is being drawn down. (too lazy to go look at drawing so take that as a guess)

I suggest you take the BEFORE and AFTER fan start voltage readings at the battery terminals.... I am quite certain you will find that the alternator cannot maintain voltage at the battery terminals.... but you really ought to check before you find yourself stranded with a dead battery.
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Old 05-17-2019, 08:18 PM
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Rescue Rogers
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Did your lights come on when the fan was running. Youre running a 20 amp fan and 20 amp lights. You have a 60 amp alternator so your still on the plus side. The amp gauge isnt really your friend. I had one and I wanted to keep the gauges stock. I had the same situation as you with the ups and downs of fans and lights and weird readings. When my alternator mounting bolts broke/ sheared and I lost the belt and didnt know it until I started overheating since I didnt have a water pump anymore. With the new voltmeter that looks stock, I'll know if I loose the alternator again
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Old 05-17-2019, 08:25 PM
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Joe79Vette
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Yes. Lights came on when the fan was on. I am going to check the before and after tomorrow to see what the amp drop is when the fans kick on. See if the alternator is keeping up.

Joey
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Old 05-18-2019, 02:36 AM
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lionelhutz
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Originally Posted by Rescue Rogers View Post
Did your lights come on when the fan was running. Youre running a 20 amp fan and 20 amp lights. You have a 60 amp alternator so your still on the plus side.
If only it was that simple. A 63A 10SI alternator is capable of about 25A at idle.
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Old 05-18-2019, 10:49 PM
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Joe79Vette
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I measured the voltage on my car.

At idle with fans not running. Voltage was 13.7

After driving the car, I came back and at idle with fans, radio and headlight on. Voltage was 12.6

strange as well was at idle the amp gauge worked its way back to near zero. But when driving it was near +20 and stayed there.

Joey
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Old 05-18-2019, 11:10 PM
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carriljc
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Joey,
I encourage you to upgrade. 12.6 VDC at the battery is terrible while the car is running.
Upgrade preferably to a CS-144 since they're quite common and relatively cheap. You're living on the edge and it will eventually end up drawing down your battery. If you don't want to trouble yourself with the CS-144 you can plug-&-play at 94-amp 12si, but for the cost I'd really suggest just going to a CS-144.

BUT, if you want to go with the 94 amp 12 SI I'll go look up the part number I bought now..... be back in a sec.
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Old 05-18-2019, 11:18 PM
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Here you go. https://www.autozone.com/batteries-s...rnator/12352_0

and here is one with a lifetime warranty:
https://www.autozone.com/batteries-s...rnator/91298_0

I have this alternator sitting in my garage as an emergency backup. Works as designed, but cannot maintain the battery terminal voltage at 13.7 to 14.0 at idle with all my loads running. Works just great at a little higher rpm, but that does us no good at night with lights on and loads running.

At least it's a cheap experiment if you want to try it.

Those webpages also give you vehicle fitment in case you want to ask for some vehicle instead of part number.

Last edited by carriljc; 05-18-2019 at 11:22 PM. Reason: add 2nd link and other info
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Old 05-18-2019, 11:24 PM
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Joe79Vette
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Thank you. Did you upgrade the wiring too? That is the part that worries me the most. I replaced all the wiring with Lectric do everything is new and not cut up.

Thanks
Joey
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Old 05-18-2019, 11:34 PM
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7T1vette
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You guys are just way too picky. So the alternator doesn't keep up when it's at idle. So what? He isn't going to be idling very long and as soon as the engine revs a bit, that current draw goes down to nothing. I think a battery in decent condition can handle a 20 amp discharge for a few seconds without any problem.

To the OP: Go for a stronger alternator if you want. You don't NEED it...but if it makes you feel more 'warm and fuzzy', go for it.
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Old 05-19-2019, 01:21 AM
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carriljc
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I have lived with a barely adequate alternator and it's not pleasant. I am not making up theoretical stories. The upgraded wiring is simple too. I think you would be a lot happier.

Originally Posted by Joe79Vette View Post
Thank you. Did you upgrade the wiring too? That is the part that worries me the most. I replaced all the wiring with Lectric do everything is new and not cut up.

Thanks
Joey
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Old 05-19-2019, 07:37 AM
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if wanting stock look, put a 100 amp stator in your existing alternator. get a voltmeter that plugs into cig lighter.
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