Why only 12.9 volts at wot?
- Thread starter 2QWIK4U
- Start date
- Joined
- May 26, 2001
I find it hard to believe the issue here is a "wiring defect"?
I am not an "electrical expert" and I just had my race car completely re-wired with a new FAST stand-alone harness and the cost of parts and labor for this job is $6000.
There are NO electrical gremlins in the car for it to drop voltage according to the FAST computer.
It is very consistent when RPM reaches 3000+ voltage drops from 15 volts to 12.5 volts and returns to 15 volts when RPM comes back down to 3000 RPM.
I checked the alternator on a test machine and and the voltage never drops or varies from the max output voltage, so I am convinced that my issue is belt slippage.
As far as this being the issue of the OP, I cannot verify it, but I do know all the belts listed from the many brands I have seen or used, even the GM replacement, are ALL oversize from the original 1986-7 GM belt, some as much as 25mm longer compared to my original unused belts from GM.
- Joined
- May 26, 2001
Duplicate post deleted, sorry?
- Joined
- Feb 18, 2006
Factory wiring is too small for any added load. Not a wiring defect. The wires are just plain too small.
Solution: Remove all aftermarket load from the factory fuse panel. Bring a feed wire into the car from the alternator and run a relay or 2 to switch power on and off for the accessories.
Doing that to my car made a huge improvement in wot ecm voltage.
My $.02
Solution: Remove all aftermarket load from the factory fuse panel. Bring a feed wire into the car from the alternator and run a relay or 2 to switch power on and off for the accessories.
Doing that to my car made a huge improvement in wot ecm voltage.
My $.02
- Joined
- Nov 30, 2009
I guess GM doesn't know what they are doing againJust 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.
You just proved my point , but maybe I wasn't clear . With the sense leads tied in on the main battery cable , it is almost the same as the back of the alternator ( all very heavy wiring ) to that point . I was talking about inside the car under the dash from the fuse box where the voltage has gone through old connectors and smaller wiring .
Do you see who I work for , for the past 32yrs .
Every panel I have been in , if the power supplies had sense terminals , most were jumped to the power out on the power supply . The only time it would be different was if it was a high precision circuit , but then it would have a dedicated power supply for that circuit alone .Has anyone monitored voltage at the heavy draw areas ( after hot wire relays ) ? I don't believe that a constant 12.5v at fuse panel during a WOT run will hurt anything or effect the performance of the car as long as fuel pump , injectors , & coils are not being " starved " for voltage .
If no one has done this , I think I just might do this to help solve this issue . To bad I'm putting the car up for winter this week , but it will give me a winter project.
This stuff is so much easier to explain face to face .
Sam
- Joined
- May 26, 2001
.....I don't believe that a constant 12.5v at fuse panel during a WOT run will hurt anything or effect the performance of the car as long as fuel pump , injectors , & coils are not being " starved " for voltage .....................
Generally dropping to 12.5 volts will not hurt or affect performance with street use, but at the track, especially in competition use, voltage must be consistent and much more than 12.5 volts at sustained WOT for proper performance as well as reliability in my world!
Another concern I have is most in-tank fuel pumps I have used specify 13.5 volts as their baseline of output performance.
Since I have a Boost-a-Pump inline directly from the battery to my external fuel pump on my race car, I am confident that I always have much more voltage there than needed.
On my performance street car, 12.5 volts would not be acceptable for me at WOT.
- Joined
- Nov 30, 2009
Where Mike is sensing voltage is very close to the back of the alternator because of it being on heavy trunk line wiring . Nothing wrong with that , but maybe his ECM voltages improved because of marginal connections at the fusible links that were corrected when the sense lead was installed . What I'm saying is that the voltages dropping to 12.5 at the ECM during a WOT run should be ok as long as the heavy current draw components don't . You would need to monitor the voltage at the alternator to see if it is losing voltage as current draw goes up , or is it marginal wiring at the fuse block . If the alternator holds 13.5v-15v throughout the run ( monitored at the alternator ) and the powerlogger shows a voltage drop I think you found your problem , marginal wiring and or connection . Couldn't you run a dedicated wire from the alternator to a powerlogger input and monitor both voltages at the same time ? Sam
- Joined
- Nov 30, 2009
The misconception of if something is designed to operate at 12v then 13.5 is better , is all wrong as far as the ECM is concerned . They all have an operating range in which they will work exactly the same , and giving them more voltage will not improve there operation ( to much will hurt ) . Fuel pumps , injectors & ignition coils are all hot wired for a reason , to maintain full output from the alternator . I am not saying that dropping to 12.5v for the whole car is ok . I'm saying that dropping to 12.5v at the ECM will not hurt which is where most are monitoring the voltage from . We have PLC's (programmable logic controllers ) at work that run on 24vdc . Sometimes they see 22v and sometimes see 28v with no change in function and these are WAY more sophisticated than anything in our cars . To tell if the alternator is dropping voltage you HAVE to monitor it at the alternator .
- Joined
- Nov 30, 2009
My injectors are hot wired & there is a hot wire kit for CCCI also .
http://www.casperselectronics.com/store2/product_info.php?cPath=5_6_10&products_id=619
http://www.casperselectronics.com/store2/product_info.php?cPath=5_6_10&products_id=660
http://www.casperselectronics.com/store2/product_info.php?cPath=5_6_10&products_id=619
http://www.casperselectronics.com/store2/product_info.php?cPath=5_6_10&products_id=660
- Joined
- Nov 30, 2009
To the OP , I just looked at my own PL files and my voltage drops from 13.4v @ 2000rpm to 13v @ 3000 to 12.7 @ 4000rpm then mantains until the end of the run . There was one dip to 12.5 ( for a 1/10 sec. ) when the second fuel pump kicked on from the hobbs switch . That is from the PL , I'm going to monitor at the alternator next spring to see the difference between the two .
- Joined
- Nov 30, 2009
Your welcome.
Similar threads
