Voltage out of computer is .5v instead of 5 volts.to sensors. Engine shut down. Plenty of battery, cranks over fine.No Spark My skilled certified mechanic is tracing downhill for shorts, thinks computer is fine.
Any ideas ? 2003 mag ho w/ under 300 hours. Does the 2 beep alarm.

Sounds like the ECU, Have had this same thing several times in cars I've repaired. The stepdown dc regulators where fubared.

Is there just one 5volt wire coming out of ECU or several ?

Edit in: Looked up paperwork of one that my memory was having issues with. Expedition. TPS had ,005 volts going in. Was issue somewherein that section of 5volt reference wire, so ran a wire from a closer to ECu Section.

The 5V go to all sensors, and one of them is probably shorting. Start unplugging them one by one until the 5V comes back up.
MAP, IAC, TPS Water pressure, etc
Suggest start with IAC

Yep,,as long as you have a good ground and power supply to the ecm, that would be the first step in diagnosing the issue.

your mechanic should know the proper testing procedures to find the 5 V reference signal issues hopefully they are trained well for the 555.. I have posted the testing procedures for the 5 V reference in the past up here if someone wants to search for it as those procedures helped the original posters find his problems.

at .5 V ( a half of VOLT ) for a 5 V signal -- I would have to ask does the engine even run ?? -- if it does start and run, surely it will not stay running for long ? Is this correct ?

If proper testing does not find the issue(s) --- do not rule out the ECM. I am in Dallas area if help is needed and surely could find the issues at hand to fix in a timely manner with your app.

Also watch out what you do and possible follow any wrong info for testing so that you do NOT cause ECM damage, if it has not already taken place. They do not like incorrect voltage nor voltage spikes. One voltage spike can do damage. just saying as a word of warning

Engine does not run. Stopped suddenly. Laptop indicated TPS, that replaced it still doesn't run. Plenty of battery, turns over fine. A mystery for my mechanic who is Mercury certified. Luckily it died pulling into Bremerton marina. We were able to sit in the restaurant waiting for Vessel Assist for the tow back to Seattle. Hopefully he will figure it out soon. Thanks for all the responses. I will post the results

Thanks I kind of knew it did nt start / run but if it did - the engine would not nor could not stay running

My guess would be the water pressure sensor and or that whole circuit . Next possible thing to look at is any bad grounds, Next possible could be the ECM itself ( lost internal ground ) or one of the chips internally. If you need the ECM / PCM checked out and or repaired give me a shout.

There are alot of 5volt ref. power and ground wires, but the IAC circuit isn't one of them.

Hey MUC I know you will know the good old smart transom harness and connection(s) haha

But have to add do not rule out any of the 5 V connections / wires / harnesses

Anyways - I forgot to post this inn the mix as well - double check to see if you even have a TACH signal if NOT --- zero tach signal - The crankshaft sensor is BAD. That is on and is a 5 V sensor / 5 V reference signal / circuit .

There is a reason why the TPS code faulted most likely at engine shut down or just before it stalled..

Hi BUP Yes that transom harness can be tough to get to. Iv'e always been against using the same 5v ref for both essential and non-essential items. When Volvo started using 5v for the trim sender we had a few trim sender problems kill the engine. They came out with a inline resistor to keep that from happening and the next year added a second 5v ref for non-essential sensors. I think MerCruiser started doing that with the PCM 112?

@rickyc4 hope your tech gets to the problem quickly. I would think they will pull pin 23 out of connector A at the PCM to isolate the 5v ref. That should tell them if this is a PCM problem or an harness/sensor issue. If it is a PCM problem. Make sure they Ohm out the harness while doing a "wiggle" test so it doesn't happen again. Maybe @BUP can confirm, but I think the main enemy of the 5v ref circuit in the PCM is getting battery voltage on the 5v ref ground. My guess is that because you have 0.5 volts the problem will be in the harness/sensors, And you will have to give your tech the time to find it. This can be a very time consuming job because sometimes depending on the boat you have to cut 20 to 30 tie wraps (some of them by feel alone) to inspect the harness. These time consuming jobs are most often best taken (surprisingly) to the high price shop. Because you have a better chance of getting a top notch tech and they will get to the cause quicker. Very rare to find a really good tech working for a cheap shop. Why would they?

The IAC gets battery voltage from the MPR and it's ground from the PCM


Good Luck MUC