Voltage issues

First-on-race-day

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So ive been having an issue where my voltage shown on my sct device goes as low as around 11.3 volts. But if i measure for voltage at the battery or fuse box i get a good 13.2v. I also get 13.2v at pin 16 on the obd2 port.
So my question is. Where does the sct x4 get its voltage value from?

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First-on-race-day

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It goes down to 11.3v at idle with blower motor on high, and high beam lights on. With no loads on, it hovers in the mid 12s. And also during a wot pull the sct will show it goes down to the 11v range. But i left my multi meter on the battery during a wot pull and used my min,max function and the lowest the voltage got was in the low 13s during a wot pull.
So my issue seems to be maybe a wiring issue to the pcm. Because my battery and alternator seem to be doing just fine and if i test at the battery with a multimeter the voltages fall into an acceptable range

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SVT Hess

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I had similar issues with WOT voltage being low according to my SCT handheld but I have a volt gauge on my triple pillar that would show a different voltage, normally always higher. What fixed the issues for me was getting a high quality built alternator such as a Mechman, Nations, ect and upgrading the power wire and ground wires. I even added extra grounds from the block to the chassis just for extra insurance. The stock style units unfortunately just don’t keep up with the demand of our cars and tend to already be overworked being on the blower belt. After going to a better alternator I haven’t had the first issue with WOT voltage between my X4 readings and what my volt gauge reads.
 

me32

BEASTLY SHELBY GT500 TVS
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I had similar issues with WOT voltage being low according to my SCT handheld but I have a volt gauge on my triple pillar that would show a different voltage, normally always higher. What fixed the issues for me was getting a high quality built alternator such as a Mechman, Nations, ect and upgrading the power wire and ground wires. I even added extra grounds from the block to the chassis just for extra insurance. The stock style units unfortunately just don’t keep up with the demand of our cars and tend to already be overworked being on the blower belt. After going to a better alternator I haven’t had the first issue with WOT voltage between my X4 readings and what my volt gauge reads.
This right hear.
 

01yellercobra

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It goes down to 11.3v at idle with blower motor on high, and high beam lights on. With no loads on, it hovers in the mid 12s. And also during a wot pull the sct will show it goes down to the 11v range. But i left my multi meter on the battery during a wot pull and used my min,max function and the lowest the voltage got was in the low 13s during a wot pull.
So my issue seems to be maybe a wiring issue to the pcm. Because my battery and alternator seem to be doing just fine and if i test at the battery with a multimeter the voltages fall into an acceptable range

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It would be better to record a video or run really long leads to the cabin. All it has to do is spike for a half second and it'll show up as a max reading. You need to make sure it stays there and doesn't fall off as you accelerate.
 

SVT Hess

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Be aware that voltage at the battery could be in the 13s during WOT but that isn’t the voltage the pcm is seeing. I found my issue while doing remote tuning on my TVS, my tuner had stated I had a voltage issue getting to the pcm and I needed to be cautious when doing pulls as that meant the fuel pumps may not be getting the volts needed. Luckily for me I have a wide and and was logging it in the data log to make sure I wasn’t going lean but it was still a issue cause everything the pcm is feeding is also getting lower volts. I got a 240 mechman unit shortly after and no more issues. Occasionally I’ll have low voltage at idle but that’s only due to the 240s needing slightly higher rpms than idle to start pumping the volts. Yeah it was a expensive upgrade between the alternator and doing the Big 3 wiring but it’s a lot cheaper than leaning out and popping a engine. Also remember when doing the upgraded wiring, run a ground wire that’s the same size from one of the alternator mounting bolts up to the negative terminal on the battery. @Bdubbs did a excellent write up on installing his Nations alternator which is what I used for reference when doing my mechman. Believe this is the thread
https://www.svtperformance.com/threads/my-first-review-of-nations-200-amp-alternator.1118055/
 
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04torchred

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I have noticed my voltage gauge reads about 0.5 volts or so higher than what I log consistently at any rpm range.

I now run a rebuilt 200amp unit from a local alternator shop.
 

First-on-race-day

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Be aware that voltage at the battery could be in the 13s during WOT but that isn’t the voltage the pcm is seeing. I found my issue while doing remote tuning on my TVS, my tuner had stated I had a voltage issue getting to the pcm and I needed to be cautious when doing pulls as that meant the fuel pumps may not be getting the volts needed. Luckily for me I have a wide and and was logging it in the data log to make sure I wasn’t going lean but it was still a issue cause everything the pcm is feeding is also getting lower volts. I got a 240 mechman unit shortly after and no more issues. Occasionally I’ll have low voltage at idle but that’s only due to the 240s needing slightly higher rpms than idle to start pumping the volts. Yeah it was a expensive upgrade between the alternator and doing the Big 3 wiring but it’s a lot cheaper than leaning out and popping a engine. Also remember when doing the upgraded wiring, run a ground wire that’s the same size from one of the alternator mounting bolts up to the negative terminal on the battery. @Bdubbs did a excellent write up on installing his Nations alternator which is what I used for reference when doing my mechman. Believe this is the thread
https://www.svtperformance.com/threads/my-first-review-of-nations-200-amp-alternator.1118055/
Thanks for the info. I just put in a new ebay 180amp alternator about a month ago hoping it would fix my issues and its putting out about 60+amps at idle with my blower motor on high and high beams on. And my sct shows around mid to high 11v but if i test at my battery my battery shows 13.2-.4 volts. So based of that im thinking that i might have a wiring issue on the wire that supplies power to the pcm.

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First-on-race-day

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When you upgraded your alternator did you do the big wire upgrade? If not i would recommend doing that and then re check it.
Well the car came with a battery relocation using 4gauge wire so i upgraded all the wiring to 0 gauge. It seems like the issue is the power going to the pcm

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First-on-race-day

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Does anyone have any idea what these 3 connectors are for? Located right beside the pcm. So far found no issues with the inputs to the pcm but some of the wires on this connector that im holding match what the sct reads so i might be getting close to the issue
20190326_212514.jpeg


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First-on-race-day

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I finally had time to diag my baby and so far i found 2 voltage inputs to the pcm that are not within spec. My guess to why im getting low voltage to these inputs is probably going to be high resistance. On page 2/3 i have narrowed down the issue to be between the fuse box under the dash and constant control relay module. Splice 169 could also be my issue.
20190403_204017.jpeg
20190403_203955.jpeg
20190403_203920.jpeg


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First-on-race-day

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So after all my testing i have finally found the issue. So i was not getting good voltage to 2 out of the 3 B+ inputs to the pcm. After some testing i found the issue to be a bad ground to the body. The one good B+ input to the pcm must have been getting its ground from the engine and the other 2 from the body. Problem solved


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