Transmission, Turbo, Tachometer, oh my...

[jleonard]

think what you are describing is a mechanical tachometer that actualy spins a cable off of the flywheel? My boat has an electronic tachometer. It's not a Cummins brand. I want to say it's a VDO?

No I am not describing a mechanical tach. I have also not seen a Cummins 6B series with an alternator signal driven tach but perhaps in 1989 they did.
Normally they use a VDO tach that gets it's signal from a proximity switch that is threaded into the belhousing and reads the pulses from the flywheel teeth. I have seen several circa mid to late 1980s 6Bs and thet all had this kind of setup.

And Yes I repowered a boat in 1999 with a new Cummins 6B and I chose to power my tach with the alternator pulse..but only because I already two tachometers in place that were like that. I calibrated the tachs, each individually with a hand held photo tachometer.
And yes, the tach CAN be ok at idle and way off at higher rpm. I was told by the Cummins tech that did my warantee inspection that the best way to calibrate a tach is at NO LOAD WOT. The engine will always repeat this rpm. Meaning you could claibrate any time, just run it up in neutral and adjust.
This will be closer to your cruise rpm than idle is...plus who cares about accuracy at idle rpm anyway.

So first you need to check no load WOT rpm and make sure that is within spec. Then do a test run at WOT and record the rpm you can achieve.
If you can't get to rated rpm at that point the troubleshooting can begin. Make note of any smoke. Do you have a boost gage? That will tell a lot and help with troubleshooting. Boost gages are cheap and very easy to install. You can use an air pressure gage and plumb it temporarily if you want.
A Pyrometer to measure EGT (Exhaust Gas Temp) is a nice addition but will run approx $150 per side.

[jruesr]

I was on a friends boat yesterday and used my photo tach to verify and calibrate his tachs. He has twin Cummins 5.9 diesels/3000rpm/370hp (2002 models). His tachs are not connected to the alternator. On the back of the tachs there is a switch and an calibration adjustment. The switch has six positions. The positions correspond to different numbers of teeth on the flywheel. Its not a Cummins tach. These tachs are connected to a little sensor that detects the flywheel teeth as the flywheel turns. I call this a proximity sensor. Its not a mechanical unit running off a cable as you envision.

My boat has the original Cummins tach with hour meter. It connects to the alternator with a gray wire.

Both tacks need calibration. As I stated in my earlier reply (please read it again) my photo tack came from harbor freight and it cost $60.00. Its extremely accurate and easy to use. You are wasting a lot more money than that. It is a simple task to determine your gear ratio if the boat is running and in the water. This could prove or disprove your fear regarding the transmission.

I have a 30 page book on the transmission and could scan it for you if necessary but the photo tach check will tell you the ratio with just a little checking. Check the speed of your main pulley on the engine front (ballancer). Then check the speed of the intermediate shaft connecting the transmission to the vdrive. Do both without changing the throttle position. I did mine while tied up in my slip. Divide the engine speed by the transmission shaft speed and you will get the ratio. Mine came out exactly 1.12.

I believe you are saying that the tach reads correctly in neutral. I am not sure how you know that but if it runs up to 3000 rpm (about) with no load but will not go past 2100 in gear then I agree that you should be concerned.

My boat is the same year as yours ( 32 sportsfisher 1990). I have had three different pitch props and the boat always goes 21 to 22 kts WOT. The max rpm on the engine changes but the max speed does not seem to care what prop I use????!!!!

If you cannot find a photo tach in your area then send me $60.00 plus tax and freight and I will send you one. You need to calibrate your tach and you need to check the ratio as I described in my previous email.

Good luck

John
Passages III
Still shaking at 1 to 3 oscillations per second depending on speed
Punta Gorda, Florida

PS: Thanks for the replys on the kiel covers and the possible drive shaft wobble. Still thinking about both of these and do not know what to do next.

[Halcyon]

To verify your tachometer's accuracy use a hand held laser tachometer. They work on a piece of reflective tape you put on the motor, like the main pully. You can get laser tachometers on e-bay or harbor freight for under $20 shipped.

[Seaslug44]

Thank you for all the great replies and information. I should have time today to go down to the boat and inspect everything to figure out what's going on. The tach on my boat is original but isn't a Cummins. I will take a photo today and try to figure out what make and model but I have seen photos of the helms of other 89 to 92 SF's with Cummins and original instruments and they all seem to have the same gauges.

Now I'm wondering... is my tach supposed to be wired to that "proximity sensor" and when they wired the new alternator they accidentally wired the tach to the alternator instead? That would explain some things.

Question- where on my engine can I find this proximity sensor? I want to see if there is anything wired to it.

[Seaslug44]

Normally they use a VDO tach that gets it's signal from a proximity switch that is threaded into the belhousing and reads the pulses from the flywheel teeth. I have seen several circa mid to late 1980s 6Bs and thet all had this kind of setup.

Is there one wire than runs from the "proximity switch" to the tachometer? where on the engine block do I find this wire terminal? I'm thinking they wired my tach to the alternator by accident.

[jleonard]

The switch if you have one would thread into the bellhousing which is the large dioameter aluminum piece that makes the transition from the engine to the transmission. Let's see how good my memory is...I believe it would be on the port side of the engine.
The switch is maybe 3/4 inch diameter and would be sticking up and have a wire epoxied into the end similar to a fishfinder's transducer.
I don't think the tach would work at all if it were wored to the incorrect device.

But based on the answer above it appears that many of the older 6Bs had alternator driven tachometers. And alternator driven tachs are notorious for going out of calibration. If you do get a handheld tachometer to check your rpm, there is likely a small hole in the back of the tach that is access to a set screw that will adjust the reading.

[Seaslug44]

Here are some photos. It is a VDO tachometer. I thought I was going to find just three or four wires but instead I found a maze. I was thinking one purple from the ignition and then one signal wire from either the alternator or proximity switch and one black wire to power the lamp. Instead I have a circuit board attached to the tach and under the board a few wires in series. I'm assuming that maybe there are other gauges that need to feed off of the tach? What am I looking at here?

[Seaslug44]

Can anyone tell me if they have this same original tachometer and if it's wired to their alternator or their proximity switch. Here are more photos. This is the circuit board that is on top of the rear of the tachometer. Not sure if this has anything to do with the tach at all. Second photo is the maze of wires for all of the gauges. Third photo you can see some of the wires attached to the tach under the circuit board as weel as some small holes that look like there could be an adjustment screw inside. The photo also shows some numbers if they mean anything to anyone.

[jruesr]

I have the same boat and the same controls that you have. Find attached a wiring diagram of our boats Cummins panel.

Click on the diagram and it will appear larger. Then put the cursor on it and a little + sign appears. Click again and it will be larger still.

The circuit board is an Electronic control module that connects into the gage circuits and controls the four warning lights such as low voltage. over temp, low oil pressure etc. You do not need to be concerned with the board if you are adjusting the tach.

My tach has a calibration screw on the back that I used to set it with. I do not have a photo of the back of the tach but I do not recall any confusion in finding the screw.

The gray with from the terminal on the tach goes to the alternator. The alternator should have a tach output terminal.

Hope this helps.

John

My issue with this board is that the overtemp alarm is calibrated to 205 degrees +- 3. Mine starts alarming at 202. I have a newer engine and when I run WOT it sets off the alarm. I would like to know what temp the alarm should be set to for on a 6bta370/3000.

[jruesr]

One more comment regarding your transmission. According to the manual on the transmission and the page that I attached, all other gear ratios possible would result in a slower shaft output speed. Your suspicion is that your gear is turning the prop too fast thus slowing your engine. I do not think this is possible for our transmission.

John

[Seaslug44]

Thanks John. The diagram is extremely helpful. So the fact that I have that gray wire coming off of the tach going to the alternator is proof that the boat yard was correct in telling me that my tach is indeed wired off of the alternator. Guess that's how Cummins did things back then. The point you make about the transmission gears is a good one. I have to get a photo tach on this boat while underway to see what is going on. The boatyard has one but they are not available today. I am wondering if I can pick one up at a Pep Boys, Auto Barn or some other auto parts store. Maybe even Walmart, Sears or Home Depot? The one I have is an electronic timing gun for gasoline engines. won't help in this application.

[Seaslug44]

Just got back from sea trial. WOT only 2100 rpms 17 to 18 knts. No black smoke. No sign engine is stressed or overworked. Could hear new turbo (forgot how good that sounds). I've concluded that I must have some sort of fuel restriction or throttle problems.

Any thoughts?

[jruesr]

Did your prop shaft panels fall off again. You said that if they are missing the result would be reduced RPM and lower top end.???

Maybe there us a life jacket tied to your rudder. Or perhaps a crab trap.

John
Passages III

PS I installed a vacuum gauge in my fuel line to measure fuel restriction through the lines, filter and tank pickup. As a result I wound up replacing my fuel pickups and all the lines plus the filter assembly. I believe you said the boat was stored in its slip for a long time while the transmission was being done. Lots can grow in an idle fuel tank. My fuel tanks had to be replaced. Both of them. Thus I learned more than I wanted about this also. The tanks have a removable manifold plate where all the fuel lines connect to the tank. Also included is a dip stick. You can remove the plate and look down into the tank. I did and took some pictures. Guess what I found.

The pick up tube can be removed and replaced. I replaced mine with half inch pickups. They were less than 3/8 inch dia and were full of crud. Raycor sells the vacuum gage. Its nice. The gage records the maximum vacuum your system has seen. It has a movable pointer that moves up with the vacuum reading and it stays at the highest point recorded. You can look at the gage after your test run and see what the highest vacuum was since it was reset.

Good luck again.

[Seaslug44]

Very interesting. Where on the fuel tanks can I find the manifold plates? I didn't think I had much clearance at the top of my tanks to gain any type of access.

[jruesr]

My boat has a small door just under my electrical panel on port. The easiest access to the starboard tank manifold plate is behind the fishing storage locker in the cockpit, starboard front of the cockpit. It can also be reached through a small door in the aft stbd cabin.