Basic engine synchronization gauge question

Odayssea

New Member
Apr 10, 2008
102
Annapolis, MD
Boat Info
340 Sundancer 2000
Engines
Mercruiser 7.4L
This question pertains to the engine synchronization gauge, not to an automaitc (maste/slave) synchronizer. Most of the time - 80% or so - the syncheonizer gauge centers when both engines are running at the same RPM. There are times however when the synchronizer gauge is way off even thought the RPM's are the same, or conversely, the gauge shows perfectly centered even though the tachometers are different. The difference may be small (within <100 RPM) but sometimes it shows the engines are synchronized even there is a difference of 200 or 300 RPM (e.g. 3,300 starboard and 3,000 port). When this happens I tend to ignore the synchronizer and keep the engines at the same RPM. and note the sound also settles into the same pitch when the RPMs match, but then the gauge insists they are imbalnaced.

Is it correct to maintain the same engine RPMs even when the synchronization gauge shows they are off or should the synchronization gauge be paid more attentions or preferred over the tachometer readings? What are possible causes of the difference?
 
Larry,
I had nothing but trouble with my gauges. While I cannot speak with certainty to the exact issue - let me relate what I experienced.

My gauges (Tachs) shifted a few hundred rpm a lot. And they failed entirely. However, during this process, the syncro looked fine. I suspect (others may know for certain) that the synchro gets the feed direct from the engines, and the tachs as well. So if a tach fails, the syncro works regardless (mine did).

With that, I paid more attention to the syncro and the throttle position. And tapped the tachs periodically which for some odd reason would jolt them back. Then they failed! The first under warrantee (2003), the second on it's trip back to PWM as my trade in (2008- years of trouble).
 
Glad you asked this question Odayssea as I'm now having the same problem. Last season the sync (our first season owning this boat) worked perfectly, I could rely on it and always tried to center the needle. When I did the engines sounded to be in harmony and the boat would hold course. Happiness.

This season the sync guage wasn't working at first, then it worked fine for the next couple of runs and on Sat. it wasn't even close. The rpms were the same and the needle stayed just where it wanted, off to the right mostly. The way the needle is moving it looks like it is stuck and needs lubrication, but the rear panel of the instrument seems to be sealed. I'm thinking that it's got to be replaced. But I asked a friend and he said he had this problem - applied an electrical sealant to the wires at the rear of the guage after cleaning the terminals and poof, problem gone. Does anyone know if there are potential other fixes before I try to source a replacement? Obviously repair will be my first approach, but I welcome ideas to solve this one.. to me the sync is essential!

Thanks for raising this question,
Warren
 
You are likely right Ed. I replaced both tachs last year - it seems to be a common problem with SR tachs, see earlier threads about replacing them with digital tachs - and the engine sync guage kept working without a hitch all through 07.

Not so this year.
Warren
 
You guys have newer boats but I experience variation with my "89 as well. I have checked both tachs with an Engine Analyzer and port is right on but the starboard one sometimes moves around a bit.

I use the port tach to tell me the accurate RPMs and the syncronizer to....sync.

I am hoping to build a new gauge panel next year with all new gauges so I will live with it for now.
 
When in doubt I go with the sync gauge. Tachs can be finicky.
Is it possible that you dinged one of your props? I did that last year to my starboard prop and as a result the starboard tach was always a few hundred rpm higher than the port one when the sync gauge was dead center. I changed both props this year before the boat went back in the water and both tachs are reading almost exactly the same again now when the sync gauge is centered.
Also, try to access the back of the gauges and make sure everything is tight. It might help to give all the connections a quick spray with CDC contact cleaner, that stuff disolves the green film that develops on electrical connections and cause eratic readings.
 
Ah, so I am not alone.

Ed, you pointed out on our maiden cruise that my starboard tach would momentarily go haywire and that you had seen this with your previous boat. Based on that I focused more on the sync gauge, always tapping the throttles to get a finer adjustment than I could by gripping. Then I started doubting the sync gauge and relying more on the tach gauges.

Now I feel a little more at ease ignoring the tach gaueg and relying solely on the sync gauge, and listening for the hum of both engines in pitch.
 
The tachs in my 420 are typically about 100 RPM off. On the few occasions when I do not use the Glendenning synchronizer, I advance the throttles and keep an eye on the synch gauge. When the needle is straight up and down the boat stays on course and fuel burn is identical between the engines. Same for the 320 I owned, less the Glendinning synchronizer, which my 02 320 did not have.
CJ fixed the 320's tach problem by installing new ones, along with a slick smartcraft install.

regards
Skip
 
I believe that 2002 was the first year for Smartcraft compatibility, could be wrong though...
 
The contact cleaner sprayed on the sync terminals seems to have done the trick! After checking the terminals and spraying them the problem is solved. The guage is now operating normally.

I was told that if it had become necessary to replace the gauge that the current SR part # is 1291525, which replaces old SR # 912378 but fortunately ordering a replacement won't be necessary.

Thanks for the input guys!
Warren
 
Glad to hear it was an easy fix.
I keep a can of CDC contact cleaner and a can of WD 40 aboard all the time.
 
Hello,

I have a 1997 330 DA, 7.4 V-drives, with an erratic tach on the starboard engine. I was curious at how you resolved your tach problem?

Thanks for any help, Dan
 
My sync problems returned. When it's working it's perfect. But it's intermittant so I think it's time to break some connections, cut the wires back and reconnect. I think it's that simple.

Dan the tach problems that you are experiencing are probably something else. I experienced the same issue with both tach needles dancing around when we purchased the boat. My fix - I bought 2 new SR tachs. Sorry, I know that you didn't want to hear that, but it seems to be a common problem with the tachs themselves. I have a friend who opened one of his tachs and fixed it with lubrication, but that fix is unlikely in my opinion. Check back 6-12 months ago for another thread on tachs - I know Dominic replaced both his with digital ones and he was very happy. You might want to explore that.
Good luck!
Warren
 
mine sync gauge has never been right. I just rely on the sound of the engines. I'll try taking it out and cleaning the terminals.
 
As I said earlier the tachs on my '89 340 bounced all over the place. This past weekend I removed the panel and on the back of the tachs is a cylinder # selector. I turned them back and forth in both directions several times and now both are rock steady and the right RPMs.
 
being new to twin engines, this is a great subject. When I'm running on plane, how "far" is it ok to have your engines off sync? Do I ned to make sure the needle is exactly at 90 degrees, or it a hair to the left or right going to do any damage to the boat?
 
I just keep mine within a needle width or so. It's too much to try to keep them EXACTLY sync'd by hand. Trim, and seas also seems to have an effect on the sync. So I just keep it "best I can".

I also have the tach problems where one will be 800 or so higher than the other and the sync is dead on. I'll try some contact cleaner and move the screw around in the back as others have done.
 

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