Why only 12.9 volts at wot?

[2QWIK4U]

Installed the 140 amp alternator. At idle voltage on scanmaster is 13.5-14.2. At wot it still drops to 12.4-13. Why??

 

[Boost231]

Good question, I'm looking at a data log voltage was at 15 wot 12.5, once off the throttle back to 15.

 

[nolanator]

Ive had this issue as well. My alternator will read between 13.8-14.3v in lower RPMs, but drop to 12.5-12.9v under hard acceleration.

 

[John Larkin]

Belt slippage? Not likely but maybe.

 

[Gmoney]

Have you recently changed battery cables,alternator wiring etc...
As the price of copper has gone up some sellers have gone with Copper Clad aluminum in there cables vs copper which is not as conductive!! Especially the chinese Crap

 

[mikestertwo]

Installed the 140 amp alternator. At idle voltage on scanmaster is 13.5-14.2. At wot it still drops to 12.4-13. Why??

It's caused by voltage drop in the wiring. That voltage you are reading is ignition voltage not battery voltage. As your rpms go up the current draw from the ignition system is higher causing a higher voltage drop in the wiring. If you read battery voltage you will find it does not drop.

 

[2QWIK4U]

I thought maybe it was my problem, not enough voltage to fuel pump.
I noticed today if I eased into the throttle and kept it around 3/4 of the way, around 14-15 lbs of boost,that fp stayed around 60 lbs.
As soon as I push pedal to floor the fp drops to 50-52 lbs. Boost around 18 psi, didn't matter if idle fp was set @ 42 or 46.
Only thing I can think of is that fuel pump is weak.

 

[mikestertwo]

I thought maybe it was my problem, not enough voltage to fuel pump.
I noticed today if I eased into the throttle and kept it around 3/4 of the way, around 14-15 lbs of boost,that fp stayed around 60 lbs.
As soon as I push pedal to floor the fp drops to 50-52 lbs. Boost around 18 psi, didn't matter if idle fp was set @ 42 or 46.
Only thing I can think of is that fuel pump is weak.

Is your fuel pump hot wired? As with the ignition system as when rpm and boost increase the current draw by the pump increases do to the increase in pressure which gives more of a voltage drop in the wiring.

 

[Nick Micale]

Good question, I'm looking at a data log voltage was at 15 wot 12.5, once off the throttle back to 15.

The data log he is referring to is my race car today at the track, and this has been an ongoing issue plus I have seen this issue in other turbo Regals.

Common sense is when volts like that drops at high RPM, and returns at lower RPM it would more likely be a mechanical issue than an electrical issue, so that should be investigated first?

Many aftermarket belts for a 1986-7 GN are much longer than stock and will cause this problem.

 

[Chuck Leeper]

30 y/o key sw causing issues?
Corroded connections on/inside batt cables?
Bad grounds at other locations?
Powder coated brkts causing poor grounds?

In the case of dropping fuel psi, are you outrunning the injs?

 

[2QWIK4U]

I'm not sure if I'm out running the injectors. They're 50 lbs, boost not that high on a TE-44.

 

[GNRick]

It's caused by voltage drop in the wiring. That voltage you are reading is ignition voltage not battery voltage. As your rpms go up the current draw from the ignition system is higher causing a higher voltage drop in the wiring. If you read battery voltage you will find it does not drop.

Hook up a 3 wire volt meter and read voltage direct from battery http://www.amazon.com/gp/aw/d/B00Q7...er+3+wire&dpPl=1&dpID=5105SRg81AL&ref=plSrch#

 

[mikestertwo]

The issue with this perceived voltage drop is caused by our alternators not being wired for remote sensing. Not sure why GM decided to do that on our cars but not on others. The alternator cannot sense the voltage drop in the electrical system as the loads increase. It can only sense the voltage at the output of the alternator. I had this same issue until I wired in the sense line. My voltage never goes below 13.2 volts showing on Powerlogger.

 

[Chuck Leeper]

I had this same issue until I wired in the sense line.

Haven't seen a schematic for this mod.. Might you have such?

Tia,

 

[Ktbregal]

Subscribed do tell

 

[mikestertwo]

Haven't seen a schematic for this mod.. Might you have such?

Tia,

Here is a picture of the connector I took off of a front wheel drive car. The brown wire is the normal lead that goes to the idiot light in the dash. The red lead I ran down to the starter terminal where the fusible links are.

 

[Ktbregal]

What year and make front we drive car? Did you just hook it up to the hot side at the starter?

 

[SS/GN]

I also thought the T/A belt was shorter and recommended for all cars to limit belt slippage and tensioner rattle.

 

[dynoman]

I had this same issue until I wired in the sense line. My voltage never goes below 13.2 volts showing on Powerlogger.

The problem I see with this , is the alternator is regulating its voltage output based on what it's "seeing" somewhere down stream in the harness after many questionable 30yr old connections . The voltage you need to "see" is at the output of the hot wire relay to the pumps & or injector harness . Those are the places where low voltage under load will be a problem , 12.5 v at the powerlogger and ecm is enough for it to function properly . If the alternator is putting out 15v but it "sees" only 12.5v due to a remote sense lead , it will try to raise the voltage even higher ( more than 15v ) to a point where it can damage other components . If the overvoltage is prolonged it also can overcharge the battery . I believe sensing the voltage at the back of the alternator is best and if there is big voltage drops down stream you have wiring issues . Sam

 

[mikestertwo]

The problem I see with this , is the alternator is regulating its voltage output based on what it's "seeing" somewhere down stream in the harness after many questionable 30yr old connections . The voltage you need to "see" is at the output of the hot wire relay to the pumps & or injector harness . Those are the places where low voltage under load will be a problem , 12.5 v at the powerlogger and ecm is enough for it to function properly . If the alternator is putting out 15v but it "sees" only 12.5v due to a remote sense lead , it will try to raise the voltage even higher ( more than 15v ) to a point where it can damage other components . If the overvoltage is prolonged it also can overcharge the battery . I believe sensing the voltage at the back of the alternator is best and if there is big voltage drops down stream you have wiring issues . Sam

I guess GM doesn't know what they are doing again Just about every GM vehicle is wired this way. It's common practice in power supply wiring in electronic equipment. If you have a wiring problem...fix it! The sense line is tied to the positive battery cable down at the starter solenoid where the fusible links are that power the electrical system. Sensing the voltage at the back of the alternator is actually the worse place to sense the voltage since it can't sense the true demand on the electrical system because of line drop in the feed line to the battery. It won't "over charge" the battery since the sense line is tied in just after the battery on a very heavy cable which has very little line drop.