1987 mercruiser 5.7 no power at coil

Joined
Jan 20, 2022
Messages
6
I am trying to figure out why I don’t have power at the coil on my 1987 5.7l mercruiser motor I replaced the coil replaced the circuit breaker 50 amp the distributor cap and button the ignition sensor in the distributor and the kill switch but still no power at + side of coil when key is turned on. The motor spins over but no spark can anyone help ?
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
The fact that the engine spins rules out plenty. What does it leave? Check the engine harness connector (the big black 10 pin jobbie), they often get corrosion in the pins. Check if you have power on the exciter circuit on the back of the alternator (that purple wire is common with the coil/ignition amp). If you have power on the alternator, but not on the coil, you have an internal break in the harness. No power on both alternator and coil either that big connector or a break in the instrument harness.

Chris.......
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Here's the circuit drawing for your engine (with the purple wire highlighted)...
1642733789627.png
It's probably worth checking the output of the keyswitch too. If your instruments are coming on, the keyswitch is working.

1642734237043.png
 
Joined
Jan 20, 2022
Messages
6
Thank you I will try checking to see if I have power at alternator and the 10 pin plug and rule that out if that is not the problem I will check the key switch because no gauges come on when I turn the key to the on position the only thing that happens is motor turns over..
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
... I will check the key switch because no gauges come on when I turn the key to the on position ...
That indicates the keyswitch as the problem. (or a broken wire between keyswitch and instrument panel).

Based on that ^^^, I'd check at the keyswitch as the first point (forget the engine end for the moment). The fact that the engine will crank means the power IS reaching the key..

Chris.....
 

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,286
Hi. Although I’m not certain this is relevant to your merc 5.7…but I thought I’d ask around ballast resistors. I know of a late 80’s VP 5.7 that had an identical issue. It was caused by the ballast resistor failing. Can’t think I’ve noticed one on a merc 5.7 for some reason, thinking about it now though. Certainly the VP AQ 271 5.7 has one.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Hi. Although I’m not certain this is relevant to your merc 5.7…but I thought I’d ask around ballast resistors. I know of a late 80’s VP 5.7 that had an identical issue. It was caused by the ballast resistor failing. Can’t think I’ve noticed one on a merc 5.7 for some reason, thinking about it now though. Certainly the VP AQ 271 5.7 has one.
Nope. Fully electronic ignition, Mercury's Thunderbolt IV (see posted cirrus diagram), no ballast resistor... Not since 1982

Chris....
 
Last edited:

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,286
Nope. Fully electronic ignition, Mercury's Thunderbolt IV (see posted cirrus diagram), no ballast resistor... Not since 1982

Chris....
That will be the reason then. Cheers Chris. VP must have been a little behind merc in those days, I suppose
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Replaced ignition switch at helm I am getting power to gauges and to + side of coil but still no fire
So now read this...


Chris...
 

isaacs

Petty Officer 2nd Class
Joined
Oct 15, 2013
Messages
153
If your ignition sensor in the distributor is the original "open" type I would replace it before spending time on anything else--they are prone to failure. Mine drove me crazy before I realized what was wrong; it's a cheap easy fix.
 
Top