5.7 electrical questions

[SangerDX]

My motor is acting strange. Cranks and idles fine when cold, but when it warms up especially when under load it suddenly dies (sometimes it will crank back up, sometimes I drift until it cools down -does not run too hot)

I replaced the key switch with a toggle (off/on/momentary) this summer worked fine at first, but I'm not sure if this is a good idea electronically. I converted the points to a Pertronix and switched to a Pertronix low resistance coil. (again worked fine at first) Besides these two changes, the boat is bone stock.

I don't show any voltage across the coil with the switch in 'run' position; but it will crank and run fine cold.
I cleaned the battery terminals, and where the neg attaches to the block and the big round wiring harness plug.
I put it in the shop a few months back and they replaced the wire that goes from the starter to the coil and that fixed the rough running it was doing then. Can the starter solenoid cause problems? Is Pertronix a bad idea?

1988 Mercruiser 5.7 ski

 

[thumpar]

Re: 5.7 electrical questions

I had a pertronix in my old boat for 6 years and my dad has had one in his daily driver 1966 mustang for 15+ years with no issues. Did you bypass the resistance wire when you installed it?

 

[SangerDX]

Re: 5.7 electrical questions

I just hooked it to either side of the coil. Would it hurt anything to run a ground wire from the block (brass ground stud is near by) to the negative side of the coil?
ps. I had a pertronix on my '67 for years and loved it

 

[thumpar]

Re: 5.7 electrical questions

Run a jumper wire to 12v when it has a problem and see if it fixes it. You probably have a resistance wire that supplies voltage to the coil. The pertronix units like a full 12v.

 

[SangerDX]

Re: 5.7 electrical questions

I'll take another look at voltage; info I had also said it needs 12v. There is a wire (purple with yellow strip) that looks like the one from the ignition

 

[achris]

Re: 5.7 electrical questions

Purple/yellow is the resistance wire bypass from the starter-motor. That will only have 12 volts while you are cranking the engine. I've looked at the drawings for your engine and it appears you don't have an electric choke, so we can't take a feed from there. What you do have is a 3 wire alternator. One of the small wires will have a purple lead on it (be careful, the other small wire is red/purple, make sure you get the right one), and that should only have 12 volts with the key ON (test it with a meter or test light). Take a feed from that to the coil+... That should solve your 12v at the coil problem...

Chris..........

 

[Bondo]

Re: 5.7 electrical questions

I just hooked it to either side of the coil. Would it hurt anything to run a ground wire from the block (brass ground stud is near by) to the negative side of the coil?
ps. I had a pertronix on my '67 for years and loved it

Ayuh,.... That'll guarantee it'll Never start again,...

 

[SangerDX]

Re: 5.7 electrical questions

Ayuh,.... That'll guarantee it'll Never start again,...

...glad I asked.
I will measure voltage from the purple wire tonight and consider 'achris's ' idea of a solid 12v key-on wire from the alternator. If its not raining Saturday I'll do another test run.

 

[SangerDX]

Re: 5.7 electrical questions

Run a jumper wire to 12v when it has a problem and see if it fixes it. You probably have a resistance wire that supplies voltage to the coil. The pertronix units like a full 12v.

I found a spare nice stranded 14 gauge wire that happend to be purple and ran a jumper from the alternator purple post which was 12.5v with key on to the plus side of the coil and took it to the lake yesterday. No spark at all, which I did not expect since it was running when cold before yesterday. I put connectors and heat shrink on the ends, so I might just leave it in place.

Will search the forum for coil test methods and try that tonight (ignitor II). I got out there and the battery in my multi-meter went dead :facepalm:

 

[SangerDX]

Re: 5.7 electrical questions

I'm trying to figure this out; why would a ground to the minus side of the coil hurt?

 

[thumpar]

Re: 5.7 electrical questions

I'm trying to figure this out; why would a ground to the minus side of the coil hurt?

The coil sparks when it looses ground. If it was always grounded it would never spark.

 

[SangerDX]

Re: 5.7 electrical questions

I read a test was to ground the neg side of coil and turn on key to test voltage at plus side of the coil. I tried this last night and got a huge spark when I turn the key on ....lost my nerve and did not test voltage. I believe the problem is narrowed down to the coil or the Pertronix not signaling the coil. Does anyone know how to decide which of these could be the prob?

coil = flamethrower II purchased in May
Pertronix is Ignitor II purchased in May

 

[Silvertip]

Re: 5.7 electrical questions

On-off-Momentary toggle switch for an ignition switch instead of a proper ignition switch says perhaps you need to investigate that end first. You also need to study a schematic diagram of the way your wiring WAS and then see what you turned it INTO. We can't see what you did and by the questions you are asking you need some help from a local professional. Guessing and just trying things is not a good troubleshooting technique. And just so you know, NEGATIVE on a coil is not GROUND. It is the negative terminal that gets connected to GROUND when the ignition module does that.

 

[SangerDX]

Re: 5.7 electrical questions

On-off-Momentary toggle switch for an ignition switch instead of a proper ignition switch says perhaps you need to investigate that end first. You also need to study a schematic diagram of the way your wiring WAS and then see what you turned it INTO. We can't see what you did and by the questions you are asking you need some help from a local professional. Guessing and just trying things is not a good troubleshooting technique. And just so you know, NEGATIVE on a coil is not GROUND. It is the negative terminal that gets connected to GROUND when the ignition module does that.

Actually, stock key switches are off/momentary/on switches just like the 20amp Cole Hersee toggle that I replaced it with; the original wiring was used right down to the connectors, so the schematic has not changed. It turned out the Pertronix module had gone bad and was not signaling the coil to spark.

 

[CaptainMak]

Re: 5.7 electrical questions

You left the Pertronix module with the ignition ON for too long, unless ofcourse it was the Pertronix 2 setup.

That would fry it every time.

Why didn't you take the easy way to test it, by running it directly to the battery + post and tried starting it that way ?

 

[SangerDX]

Re: 5.7 electrical questions

You left the Pertronix module with the ignition ON for too long, unless ofcourse it was the Pertronix 2 setup.

That would fry it every time.

Why didn't you take the easy way to test it, by running it directly to the battery + post and tried starting it that way ?

Switch left on sounds like a possibility; it is an Ignitor II but it says that can harm it. Planning on putting an LED on the dash that burns when the switch is on. I have a red rubber boot over the switch to keep water out and that makes it hard to see what position it is in. I eventually did run a plus wire and test it, but the first few times it happened it came and went and I had suspected it was everything except the Pertronix