Mercruiser troubleshooting

I'm reviving an old Galaxie with a Mercruiser 470 I/O. The motor seems to be in pretty good shape-it hasn't been started since 1990, but it's only got 140 hours on it and it looks pretty good...The trim wiring seems to have rotted out, but that's my next worry. First I want to try to start the thing. I have a new battery, and besides the main battery cables, there were red and black lines in the battery compartment-both look like they go to different sides of what I thought was the starter solenoid (although it's on the opposite side of the motor as the starter, so maybe it's the trim solenoid). Anyway, when I hook those wires up, the battery terminals get super hot. The lead cable connector on the red battery lead to the starter began to melt. I disconnected everything, charged the battery, pulled the rotten trim wires (which I think were shorting things out) and tried it again. Still hot, although I left it on for a much shorter time. So I'm thinking now that the starter might be shorting out? Anyway, I'm looking for a wiring diagram for a 470, maybe one that can explain what looks like it used to be a fusible link to the ignition wiring, and a fuse across the red and black terminals...
I can't tell what color this thing that looks like a fusible link used to be. I think my next move is to disconnect the starter and put a multimeter across it. Should be open, or pretty high resistance, right?
Can anybody point me to such a wiring diagram, or offer suggestions on troubleshooting this system?

Thanks,
-Dave

Re: Mercruiser troubleshooting

Oh, yes, and there was a 15 amp fuse connected to the fusible link, but one side of that's missing. I'm clueless about how that's supposed to be wired...

Re: Mercruiser troubleshooting

Re: Mercruiser troubleshooting

Before you even think about starting that engine (on muffs I hope) you need to change the impeller in the outdrive. It is unlikely that the current 23 year old impeller will pump any water and you will instantly overheat and melt the exhaust shutters and probably the engine exhaust bellows, and within a couple of minutes overheat the engine and probably blow a head gasket. Good cooling is essential to a 470. Also need the year and serial number of the engine.

Re: Mercruiser troubleshooting

I think it was build in 1980; but the motor S/N is 5039258; the numbers on the outdrive are 5046479 and 1_84R. And thank you very much-that's a good call- I hadn't even thought about the impeller. I took a hammer to the starter, and have gotten it to crank, but I'm probably lucky there didn't seem to be any spark coming from the coil (probably because of that fusible link and missing fuse wire?). Where should I go for info on the outdrive removal/impeller replacement?

Re: Mercruiser troubleshooting

By the way, if I have the muffs on and the hose hooked up, wouldn't I be able to watch it and see if the impeller is working before I go pulling the outdrive apart for the replacement? Not sure exactly where, but I should see water squirting out the outdrive when I hook it up and the motor is working, right? I should be able to see that in an instant or two, before the thing got too hot, right? Or is that just too risky?

Re: Mercruiser troubleshooting

Here is a manual for your engine: Boatinfo - Mercruiser Service Manual nr. 3 - 1978-1984 - All 4 cylinders, L6 & V8 Engines and a manual for your out drive (I think it is the correct one): Boatinfo - Mercruiser Service Manual nr. 4 - 1978-1982 - MC 120 to 260 Sterndrives To start troubleshooting turn the key on, and check the voltage on the coil +, should read about 8 volts ( voltage will be reduced by the resistance wire). If no voltage trace the wires back, look for voltage on the purple wires. The problem with starting it with the old impeller in is that the fins may be broken off or set so badly they could actually block the water flow. So if you are serious about getting this running get the outdrive work done first, change the impeller, check the gimbal bearing, grease the universal joints and refill the outdrive with oil. With the outdrive off check the bellows for cracks and check the function of the shift cable. I know you would like to get the engine running, but if you don't do the outdrive work first you will be asking for disaster. With the outdrive maintenance done you can just hook up the muffs and get it running without worrying about damaging your engine. Just IMHO. Welcome to the "470 Club" :facepalm:

Re: Mercruiser troubleshooting

Thanks for the tips, Terry. Even though this '78 only has 140 hours on it, and has never seen salt water, I'm taking your advice. I've bought an impeller kit, am draining the oil from the outdrive right now, and hope to install it tonight. I've traced what's left of the rotten trim wires back-they're the ones that go through the transom, and I've been told by different mechanics either 1) I'll have to take the bell housing off, or 2) if I'm skilled, I can run the wires through with fishtape... Any experience with that one? I'm paying $101 for the harness, which should be here in a couple of days. Quotes to install it run around $350 (I have a feeling I'm going to wish I had spent that money). Then it's back to figuring out why I've got no spark...

Re: Mercruiser troubleshooting

Well actually I wouldn't worry too much about those wires right now, because you are going to have to change the bellows anyway, and when you do the bellows it is relatively easy to do the wires, because you will have the bellhousing off. The spark should be easy. Put in a new set of points, and if you can get voltage to the coil + you should get spark. One thing to do, check the voltage on the coil + while the engine is cranking, should be the same as battery voltage. Also check and see if you have a purple/yellow wire coming from the slave solenoid (on the side of the engine) to the coil +.

Re: Mercruiser troubleshooting

I have a boat from '78 also, and with a 470. I also had rotten trim limit and sender wires, all I did was take the switches off, cut lengths of red and black wires from a spool, crimped connectors on them and screwed them down inside the units. All you need is a long extension and a wobble (1/4") and you can unbolt the piece that goes through the transom without taking anything apart. Run the wires through, connect them and you're good to go. This was 3 years ago, and after 3 years of almost every weekend in salt water - still no issues. Just a thought, if you don't want to blow $350...

However, I do agree that you'll need to change the bellows anyway. I thought that once I got my '78 running I could just drop it in the water and go. I was totally wrong: bellows, gimbal bearing, shift cables, hoses...the list goes on...

Capnkirkpatrick said:

Thanks for the tips, Terry. Even though this '78 only has 140 hours on it, and has never seen salt water, I'm taking your advice. I've bought an impeller kit, am draining the oil from the outdrive right now, and hope to install it tonight. I've traced what's left of the rotten trim wires back-they're the ones that go through the transom, and I've been told by different mechanics either 1) I'll have to take the bell housing off, or 2) if I'm skilled, I can run the wires through with fishtape... Any experience with that one? I'm paying $101 for the harness, which should be here in a couple of days. Quotes to install it run around $350 (I have a feeling I'm going to wish I had spent that money). Then it's back to figuring out why I've got no spark...

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Re: Mercruiser troubleshooting

I like the idea of not taking the thing apart...too much. I haven't gotten up in there to see how bad the rot is, and whether it continues on through the transom...