Carver 404-Engine Alarm Merc 7.4 - Part 2

[RReardon]

Hi Everyone

This is a new post regarding what I thought were engine alarms (and may still be). I recently received the boat back from the mechanic with a new "engine knock" sensor, as that is what the diagnostic computer indicated was the issue. Unfortunately, after about 15 minutes of cruising this weekend, the alarm sounded (was above 3000 rpm).

Call is in to the mechanic, but I did a little investigating behind the helm. This is what I see:
1) The alarm seems to be coming from a small black box (maybe 3"x3"x2")
2 There are five leads (black and white mostly) coming out of this box
- one to starboard tach
- one to port tach
- two to the engine sync gauge
- one to a general ground (I think)
There don't seem to be any other leads going anywhere else.

As I mentioned, I am pretty sure the alarm is coming from this source. So, my basic questions / thoughts:
1) Is this even an engine alarm, since there does not seem to be a lead to the engine (although it does go to the gauges, which are getting data from the engine)
2) If I disconnect the gauges from this alarm, It seems like it should kill the alarm.
3) I am assuming the gauges would still work since they would be receiving their data from the engines?

Would be interested on any thoughts on the above.

When the mechanic replaced the knock sensor, due to the codes he was seeing, he felt the engines were preforming without issue and the issue was the knock sensor. That does not sound like the case, but until he figures out what is going on, it would be really nice to run the boat without the alarm on....ugh.


Thanks
Rich

[tomschauer]

is this the same alarm that's sounds for a second when you turn on the key?
if so I found a small label on my boat (1998 355 with 7.4 mercury bluewaters) that says the alarm will sound with high engine temp, high transmission temp or low oil pressure. I do not believe it reads the ecm at all.

[bud37]

Just a guess here, but sounds like the sending unit for the engine sync gage....tach signal in, sync gage out for comparison.Have someone turn the key on while you are in there and listen for the location of the beeps, that will be the engine alarm, they are quite small.

[Viper]

Was wondering if the knock sensor was going to be the end of it.
The black box is the module for the sync gauge. Never seen one of those with an alarm in it. Don't know why it would incorporate one. The engine alarms/horns are little round cylinders attached to the engine harnesses for your vintage I believe.

[RReardon]

Thanks Tom/Bud/Vip

I really appreciate your thoughts. A couple of bits of information:

* It definitely sounds like the alarm (when I start it up) is coming from the Sync box (which makes sense given the lead placement into the tachs and the sync gauge). Maybe my hearing is off, but I couldn't see anything in there that looked like an engine alarms (cylinders) anywhere in terms of noise generation. I will look closer depending on what the mechanic comes up with tomorrow.
* The alarm (beep) sounds like the tone I hear when I turn the engines on. Its the only tone I hear, so I am not sure there are any other startup test alarms or tones.
* The engine psi and coolant temp all stay within what I would call normal ranges (plus or minus a quarter of the guage) while cruising.
* historically, my engines have never been in perfect sync. In other words, the starboard engine always needs to run about 200 rpm higher than the port to keep centerline with the sync gauge. I have asked the mechanic about this and it never seems to get him excited. The gap between rpm narrows as I increase to cruising rpm and speed ( so it might start our at a 300 rpm gap when I am around 1500 to 2000 rpm, then as I climb in speed and approach kind of a typical cruise rpm around 3300 rpm, the gap is only 100 to 200 rpm).
* The alarm definitely does not trigger until I surpass 3000 rpm.

When I dropped the boat back off tonight, the mechanic was going to hook it back up to the diaganotsitc, run a sea trial and mentioned something about the ecu (in his defense, this was a really quick dockside convo).

Thanks for the additional thoughts, as it really helps me converse with the mechanic.

Regard
Rich

[bud37]

Okay, another one for you...make a bit of a stethoscope from some tubing/hose....put it against whatever you want to listen to, other end to ear of course, you will hear exactly where the sound is from that way, useful for other hard to diagnose sounds.

As for the sync....if it were me I would go by your ears and the tachs....just my opinion though... good luck man....

[Viper]

The alarm is not coming from your tachs or sync. The discrepancy between one tach and the other could be a number of things; one or both tachs not working properly, throttle cables not set up the same, one engine not running up to snuff, etc. The only way to tell for sure is to get a reading off the computers and compare.

They should consider taking her out on a sea trial with the diagnostic computer hooked up, take it over 3K rpm and see what pops up when the alarm sounds off.

When this happens, does your rpm drop? Have you tried increasing rpm when the alarm sounds? If so, does it let you increase rpm when the alarm sounds?

[Viper]

The alarm for your vintage will likely look like the one in the link below. It produces an electronic beep rather than a buzzer. They're usually tied to or dangling from the harness under the dash. It's not very big, the face is about the size of a quarter:
http://www.marineengine.com/newparts/part_details.php?pnum=MER816492A+6

If diagnostics indicates a knock fault again, it's likely a wiring issue or ECM issue. The knock sensor reacting to bad fuel or an engine runability issue shouldn't set a code for that particular part.

[RReardon]

Hi Vip

Thanks so much for that link. That is definitely not what I "thought" (wrongly) was the source of the beep (which is exactly as you described). I received an update from the mechanic this afternoon and I think you are right on the money with respect to wiring. He took it out for a sea trial and is convinced now it is the wiring to the sensor and is going to reinforce / rework the wiring connections, etc.

We will see how things go, as we are headed out on a 4 day tomorrow morning when we pick it up.

Thanks again for all your great help.

Kindest Regards,
Rich

[Viper]

Rich, keep us posted.