Alternator Help!

[Cdog]

i have a new to me 21 ft Howard RC 1990 with a 1990 Mercruiser 454 365 hp bbc. I replaced the alternator because I was getting voltage readings all over the place at the battery when the engine was running. It has the 4 wire mondo style alt with red, purple, orange for a batt hot and a black ground.

Turns out the new 100amp alternator is doing the same thing. The dash gauge shows 11v. The meter at battery jumps from 8v to 14v when running. I know it’s charging because the battery started at 12.6v and ended up at 13v after running in garage.

Any ideas?

 

[Powermizer]

Check main harness plug. 8-14 volts at battery doesn’t make sence, unless you have a short, or the battery is almost dead to begin with. 11.5-14 would be more likely. Are you jumping battery to start it? Main plug is most likely your problem.

 

[Cdog]

Check main harness plug. 8-14 volts at battery doesn’t make sence, unless you have a short, or the battery is almost dead to begin with. 11.5-14 would be more likely. Are you jumping battery to start it? Main plug is most likely your problem.

Thanks for the reply. I’ll check the main plug and verify the connections. I bought two new optima blue top batts. Batts are wired in through a perko switch. I left the #1 batt on over night to see if there was a draw on the batt. It decreased marginally. Started at 13V ended up at 12.7 V after sitting for 24+ hours.

With the engine off, the batt reads a solid #. Like 12.8 or 13.2 V for example. But when I have the engine running the gauge read out jumps all over the place like a tach on a race engine. I get this erratic readout at the batt terminal & the back of the alternator, Orange & case ground on the harness (on the old one). Surprisingly Ive never seen this kind of issue before so I assumed the voltage regulator in the alternator was the issue and replaced it with a new 105 Amp model. The new one is doing the same thing as the old one. The issue that stumbles me is the fact that the battery is getting charge from the alt. After running I’m seeing an increase in voltage at the battery.

One other thing I just tried. I turned on the ignition to see if the gauge would show voltage. It appears it doesn’t do anything unless the engine is on, even though the key switch is in the run position. Is this odd? I could be wrong but doesn't the gauge get power when the ignition is on?

I doubt the ignition switch is the issue but I plan on replacing it with a switch that has a accessory position to run just the stereo.

 

[Powermizer]

Volt gauges on dash are sometimes not very accurate. I don’t understand why the gauge doesn’t read while key on engine off, but almost every gauge I have had reads about 11 volts key on engine off and about 12.5 engine running. I found some cleaner that works great on cleaning connections. When I get to work tomorrow, I can look to see what it is and let you know. I can’t remember the name of the stuff.

 

[Cdog]

Volt gauges on dash are sometimes not very accurate. I don’t understand why the gauge doesn’t read while key on engine off, but almost every gauge I have had reads about 11 volts key on engine off and about 12.5 engine running. I found some cleaner that works great on cleaning connections. When I get to work tomorrow, I can look to see what it is and let you know. I can’t remember the name of the stuff.

Thx! I’m gonna go throw a meter on it and see what it does. Who knows, it maybe the gauge shorting out causing the issue. It’s a VDO gauge from 1990 so anythings possible.

 

[Powermizer]

You might want to check what the voltage is on the purple wire while running. Also check voltage between positive terminal on battery and alternator case with volt meter while running. That will tell you real quick if you have a connection problem. Both should read close to 13.8 volts.

 

[Cdog]

I’m in the boat now metering the gauge. The gauge has purple for a + & black for ground. There is a 1.7 voltage drop from battery to the gauge and it takes approximately 10-20 seconds for the gauge to react to the ignition switch turning on. The voltage is there at the voltage gauge but it takes up to 20 seconds for it to react to it.

 

[Powermizer]

Do you have a testing volt meter? You can do the checks I mentioned. I think you have a bad connection.

 

[Cdog]

Do you have a testing volt meter? You can do the checks I mentioned. I think you have a bad connection.

I do. That’s what I’ve been testing with. After messing with it and accidentally arching the test leads to the gauge, it’s working better. I guess sitting for 25 years has made it lazy. LOL!

 

[Powermizer]

Cant solder electrical components with this stuff, but it cleans connectors really good. h
https://www.napaonline.com/es/p/WLD51022
I use a small paint brush to apply. let it sit for about five minutes and then wash off with brake cleaner. use on both plug ends.
Then if you want to go a step further use this on connectors too. https://www.napaonline.com/en/p/BK_09180

 

[Cdog]

So I ran the boat yesterday afternoon at lake Bartlett. I haven’t metered the charging system since Sunday morning but I had 12v + showing up on the dash gauge while driving the boat. I sprayed the ignition switch with electrical cleaner, arched the test leads a couple of times on accident and now the gauge is a little more responsive. I plan to play with it some more Friday. I’ll meter it at the gauge & see what it does. The gauge was metering 1-1.5 v less than what was coming from the ignition switch. Really weird..,,