Slow initial crank speed on Merc 7.4

[botebum]

Hello fellow boaters! I am a noob so please don't hurt me(much). Long time pleasure boater, moderately handy with wrenches, elec and nuclear engineer by trade. I purchased a boat last fall and it had this issue when I bought it, and now I"m ready for boating season but would like to get some insight into my failure mode, and where best to start t-shooting.

When I turn the key to the start position, the starter engages the starter ring. The engine will turn over very, very slowly at first, and then after about 3-4 seconds holding the key in the start position the cranking speed of the engine will eventually pick up and the engine will start. It is worse when the engine is warm, but pretty consistent.

Things I have already checked(easy stuff)

1. Battery specific gravity is good.
2. Charge level is good.
3. resting voltage is 12.9V.
4. Install batt in my Dodge 5.9L truck and it starts easily.
5. Remove negative cable to block, clean cable end, and block with wire brush, resistance from negative batt terminal to block bolt is 0.3 ohms.
6. Clean both terminals on top of battery.
7. Spin Perko switch around many times to wipe the batt switch surfaces inside.

The primary wires setup for daul batts and go to the Perko switch, and then the red primary goes to the starter lug of course. Once the starter is going around at good speed, the engine starts fine. It is just the initial 'kick' when I turn the key to the start position that is extremely slow.

Items I'm thinking in order are: A Perko switch contacts are high resistance(I can't get into the box because there is another box around it with lots of support wires). B Starter solenoid contactor dirty. C Starter motor brushes.

Let me know if you've had this before, and what you would check that maybe I haven't thought of. The starter is very hard to reach under the 7.4 ex manifold. I'll take the manifold off if needed, but hoping this might still be wiring.

Doc

 

[UncleWillie]

Re: Slow initial crank speed on Merc 7.4

What is the Battery voltage on the Battery Terminals WHILE you are cranking?
What is the voltage on the starter terminals WHILE you are cranking.
What size battery is it? What is the CCA Rating?
That is a big engine and needs a big battery!

Unless you have a 4 lead ohmmeter there is no way you can measure 0.3 ohms with any sense of accuracy.

A starter battery cable should measure in the neighborhood of 0.001 ohm or less.
0.3 ohms would be considered a totally failed connection!
Even a dead short across 0.3 ohms at 12 volts would only produce 40 amps.
The starter would not even engage, much less crank!

Crank it for a few seconds and then CAREFULLY! touch all the connections.
The Bad one is going to be HOT!!!

 

[dubs283]

Re: Slow initial crank speed on Merc 7.4

sounds like your starter is gettin tired

time for a rebuild

 

[tpenfield]

Re: Slow initial crank speed on Merc 7.4

My top list of things to check would be . . .

Is there a difference if the Perko is set to 'Both' batteries versus just one of the batteries? If so, then it is pointing to the batteries (1 or 2 of them) being the culprit.

Sounds like you checked your connections, etc.

The starter might be getting tired. So, that would be next if the batteries prove out OK. Also, check the connections at the battery, if you have not done so already, as I have been there, done that.

If you do have to take the starter out, you are looking at 2 bolts in an impossible to reach location. I had to move the battery out of the way in order to get at mine. Make sure you have an ample supply of band-aids, etc for the fingers and knuckles.

 

[botebum]

Re: Slow initial crank speed on Merc 7.4

OK, to close the loop, I checked another batt and same thing. I didn't get a chance to check voltage while cranking. The resistance measurement was my meter leads at .3 ohms, so the ground cable was secure and clean. I unbolted and joined the two terminals together on the Perko switch so the switch contacts would not be in the circuit and same thing.

The starter looked like the original large housing direct drive so I pulled it and put in a planetary geared starter. cranks fast and starts good. I'm guessing the motor windings are failing slowly, maybe due to being submerged.

So, for me the very slow crank starting was caused by an aged starter. Replacing those things is a mother bear at my age. I gotta work out more.

 

[Mischief Managed]

Re: Slow initial crank speed on Merc 7.4

OK, to close the loop, I checked another batt and same thing. I didn't get a chance to check voltage while cranking. The resistance measurement was my meter leads at .3 ohms, so the ground cable was secure and clean. I unbolted and joined the two terminals together on the Perko switch so the switch contacts would not be in the circuit and same thing.

The starter looked like the original large housing direct drive so I pulled it and put in a planetary geared starter. cranks fast and starts good. I'm guessing the motor windings are failing slowly, maybe due to being submerged.

So, for me the very slow crank starting was caused by an aged starter. Replacing those things is a mother bear at my age. I gotta work out more.

Glad you got it fixed. Probably not the windings though, worn brushes would be my first guess. Take it apart and see if it's just brushes. If it is, replace them and you'll have a working spare starter that can collect dust for the next 20 years, but will, through karma, prevent the installed started from ever failing.

Those planetary gear starters really spin the motor over nicely don't they? Don't seem to draw as much current as the old style either.

 

[tpenfield]

Re: Slow initial crank speed on Merc 7.4

Glad you got it fixed. Probably not the windings though, worn brushes would be my first guess. Take it apart and see if it's just brushes. If it is, replace them and you'll have a working spare starter that can collect dust for the next 20 years, but will, through karma, prevent the installed started from ever failing.

Those planetary gear starters really spin the motor over nicely don't they? Don't seem to draw as much current as the old style either.

Yup, karma works . . . I did that with a set of blowers in my last boat. The original ones were starting to fail, so I bought a new set and kept them on the boat in a storage compartment. The old ones lasted another 6 years before giving it up.

 

[Hoosmatroos]

Re: Slow initial crank speed on Merc 7.4

Where did you bought the starter? Perhaps a link?

 

[achris]

Re: Slow initial crank speed on Merc 7.4

... maybe due to being submerged.

:facepalm: Yep, that's a 'gotcha' every time...

So, for me the very slow crank starting was caused by an aged starter. Replacing those things is a mother bear at my age. I gotta work out more.

You and me too...

...Those planetary gear starters really spin the motor over nicely don't they? Don't seem to draw as much current as the old style either.

The main difference is permanent magnet vs field windings....

 

[Mischief Managed]

Re: Slow initial crank speed on Merc 7.4

Yup, karma works . . . I did that with a set of blowers in my last boat. The original ones were starting to fail, so I bought a new set and kept them on the boat in a storage compartment. The old ones lasted another 6 years before giving it up.

I replaced my starter a few years ago after the ring gear stripped due to hydro-lock. A few years later, the ring gears became available on ebay and I fixed the old starter for cheap money. I bet I'll never need it now...

 

[achris]

Re: Slow initial crank speed on Merc 7.4

Ring-gear? That's the big toothy thing on the FLYWHEEL... Do you mean the 'starter drive'?

 

[botebum]

Re: Slow initial crank speed on Merc 7.4

Where did you bought the starter? Perhaps a link?

MARINE COATED STARTER Mercruiser 450 7.4L 454ci 8cyl 1995-1996 10099 9000821 : Amazon.com : Automotive

Arrived quick, installed fine. Didn't get new bolts so I bought a pair cause the old ones were crusty.