Well let me start by saying I've been on here for years reading all the great threads with Ray, lowpsi, dave_n,.. So I'm really hoping one of you experts out there can help with what appears to be a rare issue I"m having with one of my 496 mags. It's a 2002 formula pc with 496 mags/ bravo 3s. Last week the motor ran fine, this week I go to take it out for a weekend trip and I notice the idle is a little rough. After getting some gas and pulling out, the port engine is really struggling to keep up RPM with the starboard engine. I decide to cancel and turn her around. I did the following: Changed the fuel filter/water seperator, checked all spark plugs which all looked lean and clean (none stood out as different), put my Rhinda on it and found the following code and volt readings: Sensor power = 3.30 volts , TPS at idle(closed) .44 and 2.99 at WOT. I noticed the TPS Position at WOT says 66%. It seems like I'm having a voltage issue with the 5 volt sensor system. I replaced the port TPS sensor with the other good motor and the voltage readings were identical (on the port side with the known good sensor). I"m not getting any other codes but I have googled to no end and can't find anyone with a code a "133 5 vdc pwr low" and position readings that are way low. I have tried to check grounds as best I can, but the way they buried these motors back ends under the hull on my model 34 PC, it's very hard to see anything. I replaced the battery on that side as well with no improvement.

Any help would be appreciated. I feel my season ending sooner then later. I was thinking about calling Whipple tomorrow to see about a reflash but I"d like to know what one of you experienced 496 guys thinks about the code 133 5 vdc pwr low, and the fact that I'm getting 3.3 volts as sensor power. It's like whatever is causing that is causing my TPS to be off by a huge amount that is making the engine idle low and rough and have no responsiveness.


THANKS A BUNCH for any input!

Hi there, for a starting point, are there any other faults showing? You'll want/need to double check as that could show you a starting point of where.

Sounds like you have a short to ground either in the 5v system, wire harness, splice101, one of the 3 wire sensors, weak power or ground connection someplace or hopefully not within the PCM.

Splice101, 5v transducer power for sensors (Gray wires)

But don't rule out splice 100, 5v transducer ground. (Black/Brown wires)

You'll want to carefully pull apart every connector you can for inspection. Then while watching the 5v system on the scan tool, carefully wiggle all wires around to see if you have any changes live.

Search for service manual #33 merc part number 90-863757-1

Hope this helps, Good luck.

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Thanks for the response Lakematdude!

I did notice a trim ckt low as well when I first ran the codes. Of course I have the usual pilot, and fuel 1 and 2 ckt high as well but I've always had those and they are normal since I don't have those sensors. I cleared the faults and the 5 vdc pwr low was the only one that came back. I'm afraid to drive the boat to generate codes since the engine is running so crappy... the plugs looked lean I didn't want to risk burning a valve or something.

Jiggling connections with the Rhinda attached sounds like a good start. So are you saying a sensor can cause the entire 5 volt system to have reduced voltage? Or are we talking a short in a wire going to a sensor, not the sensor itself? Wouldn't a sensor that caused that much of a change in volts on the system show up as out of range? Should I unplug sensors with the scanner attached? to see if it changes the volts or is that just going to give me a bunch of codes? Should I use a volt/ohms meter and tap in to the gray or brown side to see something, or do it at the actual sensors unplugged?

I've read people not being able to start the motor with a short on that circuit, but I haven't heard of anyone seeing less the 5 volts but enough to start things and not having any codes showing up other then the 133. Has anyone seen a code 133?

Also, does anyone have a recommendation for a good 496 tech in the Annapolis area? If I can't find something tomorrow I'm not going to have much of a choice but to call a tech... I'm just worried since its electrical a shop might bend me over a barrel with diagnostic hours...

i had lot of cases with 5 vdc pwr low
usualy two sensors make that , first is water pressure sensor and second one is IAC .
just disconect one by one and look do you have 5 volt sistem back

I have heard that you should replace the IAC on the 496 as a PM item. It is my understanding that the IAC often fails which causes idling issues and is inexpensive to repalce. The bigger problem is the IAC can short internally and cause damage to PCM. I believe that Whipple can repair the PCM for $500.00 plus shipping.

Thank you all for your responses! I didn't realize how the wiring/sensors were so tied together that one faulty one, even if not related to symptoms, can cause issues with the whole system. It turns out the trim ckt low was right and unplugging the trim sensor (3 wire plug buried in the back) brought my 5 volt system back to life.. engine runs great again. Thanks a lot for the advice lakematdude and babalu!! So it looks like I have to pull the boat to get this out. I think I'm going to try and stretch it a few more weeks and do it all during winter break. I need another couple crabs runs!! Oh, and GMC, I replaced the IAC on this motor already but I guess I'm due on the other motor so will probably do that this winter as well. Just curious, if I'm running a stock 375HP system, no mods, and I had extra money, which I dont, would Whipple be able to put a better reflash on my computers to maybe get a better power curve or a couple extra ponies?

Well, I really really appreciate ALL of you helping out on this and being so responsive, sincerely! It's awesome to know that I'm not totally on my own when troubleshooting .

THANKS!!