weak spark 5.7 thunder bolt no start

[scaggsjack]

I have a 1996 mariah that wont start . I have replace the module in the dist.
replace the coil wire and tried a differant coil with no success. The spark at the end of a plug wire is enough to feel but not enough to hurt you. and digusted Any suggestions

 

[Don S]

Re: weak spark 5.7 thunder bolt no start

Have you replaced the cap and rotor?
Have you done any voltage checks, do you have a Thunderbolt IV or V?

 

[scaggsjack]

Re: weak spark 5.7 thunder bolt no start

No i haven't checked voltages but thought about that in the middle of the night . I have replaced the rotor button . How do you tell the difference between the two types of ignitions. Thanks for your response.

 

[Don S]

Re: weak spark 5.7 thunder bolt no start

Look at the distributor, if there are 2 modules mounted on the distributor, it's a TB5, if there is only 1 or none, it's a TB4.

 

[John_S]

Re: weak spark 5.7 thunder bolt no start

Look at the distributor, if there are 2 modules mounted on the distributor, it's a TB5, if there is only 1 or none, it's a TB4.

Don, this may not be a 100% valid. The 5.7L 2brls, before they went to vortec heads, only have one module. The knock-module was used on the LX and 350Mag. On my '96 1/2 manufactured 5.7L, I was concerned when I found the Knock-module and sensor missing. After research, found out it was by design. The wiring harness is even different, such that you can't easily add a knock-module, either.

I also think the last year of the T4 it was mounted on distributer, vs exhaust elbow. Offhand, maybe the best way to determine T4 vs T5, is via the existance of the purple/white wire and base mode plug that should only be on the T5.

 

[scaggsjack]

Re: weak spark 5.7 thunder bolt no start

There is only one module in the distributor and i dont see any type of knock sensor. I'm going to check for low voltage at the coil and want to kwow what to check if the voltage is low, Iginition switch? or is there some type of restistor.

 

[John_S]

Re: weak spark 5.7 thunder bolt no start

You need Merc Service Manual #17, which is not online. Here is a link to the V6 Thunderbolt Ignition. It is probably close enough for diagnostic path: http://www.boatfix.com/merc/Servmanl/18/18B4R2.PDF

T4 Service Bulletin: http://www.boatfix.com/merc/Bullet/92/92_11.pdf

 

[scaggsjack]

Re: weak spark 5.7 thunder bolt no start

Thanks for everybody help

 

[scaggsjack]

Re: weak spark 5.7 thunder bolt no start

It did have voltage drop on crank and will start if i jumper 12volts from battery to positive side of coil. Any guesses?

 

[Uraijit]

Re: weak spark 5.7 thunder bolt no start

It did have voltage drop on crank and will start if i jumper 12volts from battery to positive side of coil. Any guesses?

Bad wiring to the coil. Bust out the multimeter, and trace it back to where you get the proper voltage. Replace the bad wiring.

 

[Don S]

Re: weak spark 5.7 thunder bolt no start

Bad wiring to the coil.

That's all it can be. There is no hot start on the Thunderbolt ignition like points systems have. You should be seeing battery voltage at the coil, even when cranking.

 

[scaggsjack]

Re: weak spark 5.7 thunder bolt no start

Does the feed to the coil run through the neutral safety switch or is a direct feed with just connections from the ignition switch.

 

[John_S]

Re: weak spark 5.7 thunder bolt no start

While its still not 100% clear to me what you have, here is a wiring diagram that might help: http://www.boatfix.com/merc/Bullet/94/94_02.pdf

 

[scaggsjack]

Re: weak spark 5.7 thunder bolt no start

Thanks again IT's a 96 and i'm assuming it a thunber bolt 4 base on the diagram . It has only one module in the distributor with only two leads off of it. I believe it was a two barrel carb but someone put a quardrajunk four barrell on it. Do you know how to get the diagram for the 1996 model

 

[John_S]

Re: weak spark 5.7 thunder bolt no start

What is your engine serial number? If it is a '96 mfg (engine not boat), it should be T5.

 

[Don S]

Re: weak spark 5.7 thunder bolt no start

Thanks again IT's a 96 and i'm assuming it a thunber bolt 4 base on the diagram . It has only one module in the distributor with only two leads off of it.

The module INSIDE the distributor is a sensor, the module outside of the distributor on the elbow or mounted on the outside of the distributor are the modules. Since you only have 2 wires on the outside of the distributor, (White/red and white/green I presume) yes, you have a Thunderbolt IV ignition.
See if this chart will help you.

 

[John_S]

Re: weak spark 5.7 thunder bolt no start

Since you only have 2 wires on the outside of the distributor, (White/red and white/green I presume) yes, you have a Thunderbolt IV ignition.

I have a T5 distributer from a '98 vortec in my hands. It has the same two wires. I believe T4 and T5 distributers are the same. The Merc service manuals indicate that too.

Somewhere in the "F" serial numbers it went from T4 to T5. I think this is around late '94 early '95. Around 0F353525 & above, but varies slightly by engine/drive/model. It will be very important when he goes to do the initial timing mode.

I still think looking for the purple/white wire will be the easist way to tell. On my SB it is lying on the intake manifold, starboard side, near the valve cover, by the carb. It will have the electrical plug end with protective cap.