8.1 mercruiser keeps frying the computer

[05 sangerdlx]

I have an 05 Sanger dlx with the 8.1 mercruiser. Rod bearing went out in it so I pull the engine and rebuilt the bottom end. Went to start the boat back up and would not start, just turned over. Took it to the mechanic And found that the computer had been burnt up. Sent it to whipple to have it rebuilt and found that it burnt the sensor grounds inside the computer. Was told to put it back in and try to start it again when I got it back. Burnt it up again so I sent it back to get rebuilt. Boats back at the mechanic now but we are afraid to put the computer back in it. He did some checks and found it's getting 12v to the sensors when he plugs in only the big plug to the computer, the sensors I assume. Then if he wiggles the wiring harness where it attaches to the computer it will drop to 8v, then back to 12v. So we are thinking the wiring harness is bad and causing a voltage spike, and burning up the computer. Anyone felt with this before? Sound like a reasonable assumption? When I pulled the engine I left the wiring harness on it and never took off anything from the top end. Only took off or disconnected what I had to.

 

[Bt Doctur]

is the battery hooked up backwards?

 

[05 sangerdlx]

No it's hooked up right

 

[Bt Doctur]

only other way to get a surge is connecting the power side first then the ground or loose grounds causing a spike

 

[Fun Times]

Hi there, welcome to iboats!

Ohms checking the MPI wire harness may help tell you if you have a wiring harness problem before just replacing it.

One common failure to the PCM especially on 8.1's is the idle air control system. The IAC has been known to get real hot and either melt the plug connector at the IAC and/or damage the PCM inside.

You may want to first start with a full diagnostics check and cleaning of the IAC system and maybe try a new IAC or have the old IAC disconnected when you plug in the rebuilt PCM as a precautionary measure. The IAC is run through the 12 volt system while most of the other sensors are the 5 volt system. Just a thought.

Hopefully he has pulled off the PCM connector plugs and inspected the wire pins for damage or wire pins backing out of the connector while wiggling.

Also it may be a good idea to try separating the boat wiring harness and engine wiring harness in order to run the engine only to help narrow down where the problem may lie.

Keep us updated on what you find, good luck.

 

[05 sangerdlx]

Whipple said that the computer wasn't burnt up because of the IAC. He said there is a specific part that gets damaged when the IAC goes and it wasn't damaged. Only problem was that the sensor ground was burnt up. I'll see if he did the checks that you mentioned. Thanks

 

[wellcraft-classic210]

One thought-- Has anyone checked the voltage while the engine is running with a volt meter? Wondering if the voltage regulator is failing to keep the voltage at 13.6 Volts -- // Pretty sure Anything over 15 volts would be a concern.

 

[05 sangerdlx]

No because the boat keeps burning the computer up and it won't start at all. Everytime I've tried to start it it burnt the computer up so I don't want to put it back on until I know what's causing this.

 

[wellcraft-classic210]

Well that does seem to rule out the alternator anyway. -- Sounds like a short circuit in one of the components on the output or mis-wiring of some sort.
In a perfect design the fuses would protect the components but that's not always feasible or done properly.

Are the fuses the correct value?

I suppose if you could find out the computer ratings for max. current you could add some fuses for bebug purposes.

 

[Fun Times]

Rod bearing went out in it so I pull the engine and rebuilt the bottom end. Went to start the boat back up and would not start, just turned over. Took it to the mechanic And found that the computer had been burnt up. Sent it to whipple to have it rebuilt and found that it burnt the sensor grounds inside the computer. Was told to put it back in and try to start it again when I got it back. Burnt it up again so I sent it back to get rebuilt.

Since the engine was completely torn down, It sounds like maybe one or more of the wires on the engine harness got pinched together, smashed or connected wrong somehow.

Do you or Whipple know if it's the 12 volt "engine ground" system that fried or the 5 volt "sensor ground" system that fried? Knowing may help narrow something down to look for.

Splice 100 is sensor ground...Black/Pink wire.

http://boatinfo.no/lib/mercruiser/manuals/mercruiser36.html#/108

 

[05 sangerdlx]

We left the engine as complete as possible. Only disconnected the main wiring harness from the boat and the grounds basically. So there were only a few plugs to plug back in. I didn't think there was a way to hook the wrong plugs together. Thought they would only hook up to their matching plug. No wires or plugs hooked up wrong that I've found or that the mechanic has found. Whipple said it was an "open sensor ground". I remember him saying it was 5v. I just don't understand how this computer is getting burnt up but no fuses or relays are. Or how it all of a sudden just quit working.

 

[Fun Times]

I remember him saying it was 5v. I just don't understand how this computer is getting burnt up but no fuses or relays are. Or how it all of a sudden just quit working.

One test you or your service tech will want to try is, at the PCM, Disconnect PCM connector (A) and using a multimeter such as a fluke 78 that offers an audible continuity test, Test pin numbers 22 and 23 to see if you get and audible sound. Normally I'd say you don't want to hear a beep as you'd have a bad wire harness but in this case you do in order to help narrow down if you want to buy or try to find and repair a bad MPI wiring harness.

Pin 22 is splice 100 which is the 5 volt transducer ground, while pin 23 is splice 101 that's for the 5 volt transducer power for sensors.

If you don't find a problem there, then you'll need to follow along Mercruiser service manual 33 and test the sensor wires to find any electrical shorts to the 5 volt system.

You'll need section 3,
http://www.google.com/url?sa=t&rct=...toGoCA&usg=AFQjCNF8aiFuQpc4Ar8VUFtzVEXwGTE-Qw

 

[05 sangerdlx]

Thanks fun times! I'll see if that helps narrow it down.

 

[05 sangerdlx]

Might have found the problem. There are two plugs, one from the hull and the diagnostic port plug, I believe. That were plugged in together and that was sending 12v to the computer. Unplugged those two and now the computer only gets 5v. Like it's supposed to. Unfortunately the computer got fried again by the time this was figured out. Should have it back this week and will let y'all know if it fixed it.

 

[Fun Times]

Might have found the problem. There are two plugs, one from the hull and the diagnostic port plug, I believe. That were plugged in together and that was sending 12v to the computer. Unplugged those two and now the computer only gets 5v. Like it's supposed to. Unfortunately the computer got fried again by the time this was figured out. Should have it back this week and will let y'all know if it fixed it.

Wow, You mean I may have actually guessed something right for once? Just kidding with you.... I hope this is it and you solve the problem, good luck.

If you find this to be the problem, maybe you could show a photo of the plugs crossed.

 

[05 sangerdlx]

Well we got the computer situation figured out. Unplugged the plugs and the engine will get power and turn over. It won't start though. It acts like it wants to start for a second then loses spark. It acts like it is out of time. The timing is set through the cam shaft and timing chain. The two dots are lined up. Mercury reps and my mechanic think that the crankshaft gear was put on wrong by the machine shop when they machined the crank. Is there any way to test this to find out for sure? Other than pulling the engine back out.

 

[Fun Times]

Have you tried a new crankshaft sensor?

 

[05 sangerdlx]

He pulled my crankshaft sensor off and put it on another boat and everything worked fine. Put it back on mine and it still wouldn't start. Is there a way to truck the engine into thinking the crankshaft is spinning correctly? Maybe wiring the sensor to a different boat? Just trying to figure out how to make sure that's the problem before I tear it down.

 

[alldodge]

I don't think it's the gear, it's keyed.

You had crossed pins, are the fuses good and relays energizing? Are you getting a tach signal when cranking?

 

[05 sangerdlx]

Yeah he said all the fuses and relays seem to be working. Yeah shows rpms when cranking. I know when I had it, before I brought it back to the mechanic, I would crank it and it would show almost 3k rpms while cranking. Not sure if that's normal or not. The engine wasn't turning 3k, just showed it on the tach.

He was saying that on certain years there is a two piece gear, or something, on the crank and it could be put on wrong. But if it's the one price gear then it can't. Sound right?

He talked to a mercury rep and they said they had this same problem and it ended up being the machine shop removing the gear and not reinstalling it correctly after they machined the crank.