Voltage drop

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71_dart_swinger

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Car idles fine. Runs fine. As soon as I turn on nitrous fuel pump. It’s drops to just about 12v on the dot. All my grounds are good and tight. The relay is working fine. All my connections are good. Can’t find the problem
Please throw those ideas this way. It drops basically a full bolt. From just over or around 13 down to 12. When I turn on the fuel pump for carb not nearly as dramatic of a drop. Maybe .3-.5 volt drop with the other. ????
 
Have you checked how much voltage and amps your alternator is putting out ?? And have you tested the battery
 
Without knowing details of the system, impossible to say.

First, WHERE is it dropping? At the battery? at the load device?

If it is at the battery, determine if the alternator is large enough and working properly. Look at wire size, and drop in switches and relays
 
Measure the voltage drops directly.
In other words place one test lead on the power source, and the other at a connection downstream..

A 1971 factory voltage regulator will try to maintain system voltage around 14 Volts.
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Yes measure drop to the relay, but also measure drop to regulator.

The problem could be the wiring and/or it could be the alternator's capacity has been maxed out at whatever rpm you are testing at.
When the alternator can not produce sufficient power for the demand (Power = volts x amps), system voltage drops. Then the battery becomes the highest voltage power source. But its output voltage will also drop as power is sucked out of it.
 
What size wire are you using , and how long ?
Longer run = Larger gauge wire.
How many amps is the pump ?
How many amps is the alt ?
Need more info
 
It’s a 65 amp After roughly adding up a bunch of my amp draws. Just guessing it’s going to need an alternator. Ignition is msd 6al2. So there .7 to .9 amps per 1k rpm. Dual Bosch 044 pumps. 13-15 amps each. Just broke my 4 post battery kill switch. One if the smaller 8-10 size wire was falling apart and barely on the post. Thinking it may be that. Waiting for me battery kill to get here. But for the mean time. Just going to connect it straight to the battery with the alt wire and see if that was it.
 
Measure the voltage drops directly.
In other words place one test lead on the power source, and the other at a connection downstream..

A 1971 factory voltage regulator will try to maintain system voltage around 14 Volts.
View attachment 1715572730

Yes measure drop to the relay, but also measure drop to regulator.

The problem could be the wiring and/or it could be the alternator's capacity has been maxed out at whatever rpm you are testing at.
When the alternator can not produce sufficient power for the demand (Power = volts x amps), system voltage drops. Then the battery becomes the highest voltage power source. But its output voltage will also drop as power is sucked out of it.
It just started doing it. I did a test hit the other day with 150 shot and it was great. 10 minutes later. Went for another and then it started acting up. Will try all of the above in the am. Thank you
 
Do you have a mechanical voltage regulator or solid state ? Mechanicals have a nasty habit of turning off at certain rpms. I remember my FOX radar detector getting tripped at 35 miles per hour.
 
A 65 amp alternator rating only means whatever the manufacturer wants it to mean.
In other words its a label, or viewed cynically, its a marketing number.
Usually the rating is something close to maximum output, which only occurs when the alternator is turning fast enough.
Whether this maximum is measured with the output regulated to 12 Volts. 13.5 Volts or 15 Volts is another thing only known by the manufacturer.
An alternator that can produce 65 amps at 15 Volts is producing 975 Watts vs one that can maxes 65 Watts at 12 Volts is only producing 780 Watts.
What we, the users, really care about is about is current available in the 14 to 15 Volt range.

If your fuel pumps are both running at idle, they are sucking a large amount of the alternator's capacity. In fact they are probably maxing it out, especially when the battery wants recharging. Alternator capacity at idle rpms is relatively low.
Depending on how the pumps have been wired in, pulling 26 to 30 amps may also be too much.
 
A 65 amp alternator rating only means whatever the manufacturer wants it to mean.
In other words its a label, or viewed cynically, its a marketing number.
Usually the rating is something close to maximum output, which only occurs when the alternator is turning fast enough.
Whether this maximum is measured with the output regulated to 12 Volts. 13.5 Volts or 15 Volts is another thing only known by the manufacturer.
An alternator that can produce 65 amps at 15 Volts is producing 975 Watts vs one that can maxes 65 Watts at 12 Volts is only producing 780 Watts.
What we, the users, really care about is about is current available in the 14 to 15 Volt range.

If your fuel pumps are both running at idle, they are sucking a large amount of the alternator's capacity. In fact they are probably maxing it out, especially when the battery wants recharging. Alternator capacity at idle rpms is relatively low.
Depending on how the pumps have been wired in, pulling 26 to 30 amps may also be too much.
Both pumps are wired through a relay. Thank you
 
A 65 amp alternator rating only means whatever the manufacturer wants it to mean.
.

And the other thing is that, buried in some engineering books somewhere, are the RPM/ vs output for various alternators. If you drag race, or have a swapped system and therefore have mismatched pulleys, you may have slowed down the alternator to the point that it is below decent output RPM. "In general" most alternators do not put out full output until somewhere around 5-6K alternator shaft RPM. So figure your pulley ratios (just measure crank and alternator pulley diameter, it's a ratio) and estimate at what RPM the thing starts to work OK. If that estimate puts the alternator at 5K then you for sure have a problem.......too small, partially not working (1 or 2 bad diodes, bad stator, etc) and of course don't discount BELT SLIPPAGE as a possibility
 
Update if sorts
I have 14.2v at the battery. Identical pumps. One pump has 12.2 v going to it. The other for nitrous has 10.2 Both are through identical relays. Both are basically set up the same. Both switches have 12.1-12.2v going to each of them. At this point I’m going to think bad pump. Replaced everything electrical with good connections throughout. 4 days of laying on my back. In and out of car. I can only assume it’s the pump. It has a different sound to it as well. Like it’s dragging. Ordered new pump. Shall see how it goes on Tuesday
 
Why would you think it is a pump when it could be a bad wire connection or a bad relay?

And why replace the expensive pump when a length of wire can confirm?

Go to anyplace that sells any wire. Look up the pump specs Determine the size wire it needs for the power. Buy say, 20' of wire that is at least one size larger than needed. Wire it temporarily direct from the battery to the pump. If it seems problematic, obtain a multimeter that can handle the pump current and measure it's current draw.
 
Why would you think it is a pump when it could be a bad wire connection or a bad relay?

And why replace the expensive pump when a length of wire can confirm?

Go to anyplace that sells any wire. Look up the pump specs Determine the size wire it needs for the power. Buy say, 20' of wire that is at least one size larger than needed. Wire it temporarily direct from the battery to the pump. If it seems problematic, obtain a multimeter that can handle the pump current and measure it's current draw.
sorry. Like I said. Just literally re did all connections. The new wire is now 12g From dash to just under trunk. So say 8’or so. Still exact outcome as the 14g wire that was on it before. The exact same pump that is on the motor side if things has the exact same pump and relay. Both relays are working fine and are brand new. Both switches are brand new and working just fine.
Everything about the electrical is as it should from my understand. But one pump only has 10 volts and is making a different noise from. The exact same pump with the exact same setup that is working just fine
 
Well look. It does not seem to me you are actually checking anything. When checking voltage drop in a system you must check each possible point along the way and measure every single terminal, switch, and connection.

You can do this two ways......clip your meter to battery ground and measure each voltage, and note them, and compare them...........

Or clip your meter to battery PLUS and the drop along the way will be shown immediately

Follow this example path in your head. Let's say we have power to the pump from the starter relay, through a breaker, through a crash switch, and to a relay, and that the relay is activated by the key "ign" circuit. We can ignore the IGN circuit and troubleshoot just the power circuit

So we have starter relay...........breaker..........crash switch..........relay.............to the pump..........to ground

So get a long piece of wire, long enough to reach from the battery to every place your meter needs to access. hook solidly to a battery PLUS point, and best if you fuse that with a low value fuse. Hook "safely" to your meter, and activate the pump

Measure at the pump side of the breaker. Should be zero or nearly so. Measure both terminals of the crash switch. Should be zero on the breaker side and near zero, maybe 1/10 of one volt (.1) on the pump side.

Now check both relay terminals, the contacts. if you had .1 on the relay side of the crash switch, it should be .1 at the relay, and maybe .1--.15--.2 on the pump contact. Now go to the pump. The pump terminal might be say, .2-.3, because by now the wire length (to the pump) is adding up. NOW STAB the meter onto the pump itself or the pump ground wire. It should read "same as battery." Exactly.

Or, move your extension wire from battery PLUS to battery GROUND, and stab meter into the pump ground/ pump body. Should be ZERO. if something is dirty, there, the pump might not be grounding

Next, would be current. you will never know if the pump is drawing too much current, if you don't check two things..........the pump specs for maximum current, and then measure it. If the pump is drawing much more than spec, then obviously something dragging in pump

Also how about pump plumbing and filters? If the return is plugged, or a filter plugged, the pump might be drawing over current.

How about pump pressure? Does it develop rated pressure?

And last you might need to uninstall the pump and test it for the same conditions above, to control it's operating conditions.

Now I might be full of crap. It might be a bad pump. If you like throwing money at it , go ahead. How much is those pumps worth? One of my EFI pumps is over 200 bucks
 
Well look. It does not seem to me you are actually checking anything. When checking voltage drop in a system you must check each possible point along the way and measure every single terminal, switch, and connection.

You can do this two ways......clip your meter to battery ground and measure each voltage, and note them, and compare them...........

Or clip your meter to battery PLUS and the drop along the way will be shown immediately

Follow this example path in your head. Let's say we have power to the pump from the starter relay, through a breaker, through a crash switch, and to a relay, and that the relay is activated by the key "ign" circuit. We can ignore the IGN circuit and troubleshoot just the power circuit

So we have starter relay...........breaker..........crash switch..........relay.............to the pump..........to ground

So get a long piece of wire, long enough to reach from the battery to every place your meter needs to access. hook solidly to a battery PLUS point, and best if you fuse that with a low value fuse. Hook "safely" to your meter, and activate the pump

Measure at the pump side of the breaker. Should be zero or nearly so. Measure both terminals of the crash switch. Should be zero on the breaker side and near zero, maybe 1/10 of one volt (.1) on the pump side.

Now check both relay terminals, the contacts. if you had .1 on the relay side of the crash switch, it should be .1 at the relay, and maybe .1--.15--.2 on the pump contact. Now go to the pump. The pump terminal might be say, .2-.3, because by now the wire length (to the pump) is adding up. NOW STAB the meter onto the pump itself or the pump ground wire. It should read "same as battery." Exactly.

Or, move your extension wire from battery PLUS to battery GROUND, and stab meter into the pump ground/ pump body. Should be ZERO. if something is dirty, there, the pump might not be grounding

Next, would be current. you will never know if the pump is drawing too much current, if you don't check two things..........the pump specs for maximum current, and then measure it. If the pump is drawing much more than spec, then obviously something dragging in pump

Also how about pump plumbing and filters? If the return is plugged, or a filter plugged, the pump might be drawing over current.

How about pump pressure? Does it develop rated pressure?

And last you might need to uninstall the pump and test it for the same conditions above, to control it's operating conditions.

Now I might be full of crap. It might be a bad pump. If you like throwing money at it , go ahead. How much is those pumps worth? One of my EFI pumps is over 200 bucks
Ok. Will use a long piece if 12g in the morning Plumbing is a trunk mounted 15gal fuel cell. 6an braided line to Bosch pump. 6an all way to aeromotive return style regulator back and dumped into cell. Summit racing screen filter. Engine side is braided 8an from cell to another aeromotive return style regulator then to carb
 
Will use a long piece if 12g in the morning
It's just probe extension. Any reasonable size will do.

The objective is to measure voltage drop in the positive wiring.
Voltage drops when current flows through resistance.
Current is electrons moving. The current flowing out of the alternator or battery's positive is the same as the current flowing though it's ground connection.
If there is a drop in voltage between the power source and the equipment, then there is resistance to flow in the positive wiring.
If voltage drop to the pump is high, then there is significant resistance. Probe additional locations until its found.

What is high resistance?
That depends, but lets use an easy example.
Say a fuel pump draws 11 amps at 14 Volts, and 10 amps at 12 Volts.

Battery supplying power around 12.5 Volts, we measure a 0.5 Volt drop between the battery and the pump.
V=IR
0.5 Volts = 10 amps x Resistance
0.05 ohms = Resistance

Think about that.
Just 0.1 ohm of resistance will cause a 1 volt drop to our example pump.
 
Another way to think of this is to compare to 120V systems. People sometimes dismiss "only 1/2 volt" in 12/ 14V systems but remember........

If you are comparing to 120V AC that is 5 volts. For some systems this is not much, for others it's a lot. When I grew up the "farm" voltage was often lingering on 105V. "Normal" was maybe 115. Back in the "tube" TV days, the whole famn damily would be watching something and the well pump would start. The TV would "pull in" the screen visibly and the lights would all dim!!! I've actually no idea how far the volltage drooped during pump start.
 
Turns out it was a bad fuel pump. Battery is in trunk so easy to make short jumper leads. Just out of curiosity I put it straight to battery. It still only had 11v at pump. Did same with my other pump. It had 12.2v. Got a uro fuel pump from Autozone. Basically a Bosch 044 replacement. No more crazy voltage drop. Thank you for all the input gentlemen.
 
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