Voltage Drop

[KANE]

Here is the question-

What would cause a voltage drop of 3 volts ONLY under WOT (Wide Open Throttle)?

This is the set-up on my '78-
91 TPI with painless harness (MAP equipped speed/density system, reads as a 91 F-body on a TECH 1)
91 Corvette 105 amp alternator (swapped pulleys and soldered in new connector)
4 gauge charge wire from alternator to starter (yes-its HUGE!!)
LT4 injectors, super coil, 8.8mm wires
This is a MAP car... no ESC like MAF cars
New cap, rotor and coil.

I checked for an open or gounding out curcuit. None. No shorts. I checked the battery at an idle- 13.7 volts and the Alternator at idle is the same. The voltage does increase as it should under light throttle over 1k rpm.

The voltage drop seems to be directly related to how long I keep it under WOT. For a quick downshift- its okay. The drop lasts for a second. But if I hold it in WOT... well, it drops steadily below 11 volts and then the TPI hesitates due to insufficient current to run it. There is a direct relationship with voltage drop and engine speed ONLY under WOT.

My thought is the CONTROL MODULE for the HEI. It seems to me that when its under load that the module can't keep up.


Any ideas?

 

[geekinavette]

Tell me if I understand correctly...

There is a direct relationship with voltage drop and engine speed ONLY under WOT.

So...if you're going up a hill at WOT at say 3000RPM (JUST as an example) then the voltage will drop as RPM increases, but if you're just sitting in the driveway and hold it at 3000RPM then the voltage is ok?

Bill

 

[KANE]

Yes- thats it.

No sigificant drop while in the driveway just out on the road.

The voltage will "twitch" under no load while in the driveway.

 

[geekinavette]

Well if it ONLY happens under WOT conditions then I don't think I would suspect an ignition problem, 3000RPM is 3000RPM whether under heavy load or not...unless there is something else involved that I'm not considering (entirely possible!)

It DOES sound like something is putting too much of a load on the alternator under WOT (drawing too much current). Since it only seems to happen during WOT...AND gets worse with increasing RPM...the only thing I can think of off the top of my head that would be affected by both WOT AND RPM is the electrical signal to the injectors. Maybe something "funky" there?

A couple of things I would check out (you probably have already):
What was the current rating of the 91 F-body alternator with the injection system you are using? Did that setup use LT1 injectors...if not do the LT1 injectors draw significantly more current and does your alternator take that into account?


Just trying to throw some ideas out there. Wish I could be more help, I just don't know much about TPI systems. This is interesting though...I'll be following your progress.

Bill

 

[vigman]

Another thought

When you are romping on it...

The engine is twisting ( slightly ) on it's motor mounts... I could be either OPENING a circut on the ALT... or messing with a ground. I would really look at the problem from that angle first...

Hook a volt meter DIRECTLY to the ALT stud ( and a local ground ) via wires running under the hood..into the drivers compartment... if the voltage doesn't change... you have narrowed that down as a possibility!


Vig!

 

[geekinavette]

Vig's right, check the simple stuff first! I have SUCH a bad habit of not doing that...D'OH! :L

Especially make sure the main battery-to-engine ground is in good shape!

Bill

 

[KANE]

Good news-

After driving around with my voltmeter hooked up to the starter, alternator, red hot wire to the distributor and even the battery itself I have come up with this: the voltage drop is less than .2 (or +.2) per under WOT on any one of the above. My average voltage is between 13.6 and 14.8 volts depending upon weather the lights, heater and AC are on. BUT no less than 13.5 volts EVER. Long live the 105 amp alternator and 4 ga charge wire!!

But the factory gauge is off a litle at times. Dash wiring or the unit itself. Oh well. I CAN LIVE WITH THAT!!! I kinda thought that I should get a trouble code from low voltage at some point if that were it. The '91 TPI is a 256k system, so it does have some serious power to it and can recognize insufficient and will trip a check engine light.

I also found the source of my WOT problems- KINKED FUEL LINE. What I need to do now is to cut the line at the fuel rail and plumb in an AN fitting to match the rest of my fuel system.

New runners, new front bumper, fix my paint chips, 3.73 rear end, throttle body, get my AC re-charged and new power window regulators. THATS ALL THAT IS LEFT!!! AHHHHH I SEE THE LIGHT AT THE END OF THE TUNNEL!!!!

Ironic what a few "coincidences" can do to throw you off in terms of trouble shooting a problem.

 

[geekinavette]

Coolness! Glad to hear it was nothing serious.

Bill

 

[vigman]

So you need to

Pull the center console and I'll bet the PC flex board for the instrument center panel behind the dash is funky, or the nuts worked loose.. or the connector is corroded... something like that....

Good news!

Vig!