1993 7.4L Bravo/Thunderbolt IV Ignition Problem

I'm completely baffled with this issue. A quick history. I bought this boat and it ran fine. The coil experienced heat soak and wouldn't run after it got hot so I replaced it along with cap and rotor for good measure. Motor picked up 500 rpms peaking at about 4600 and has run great. Took it out last Friday ran awesome. Flushed motor same night, decided to take it back out on Sunday morning because the fish were biting. Engine will turn over and not start. The motor while turning over would occasionally sound like it was firing but would act like it was too far advanced and stop turning over and would backfire through the carburetor until I would release the key and start again. I just assumed ignition sensor in the distributor and changed it. The problem is still there. I tested the coil and got fire. However it looked "yellow/orange" which to me seems weak. I tested same at #1 plug wire, spark seems "yellow/orange" weak. I did the old fashioned compression check with my finger in the #1 plug to find TDC and checked the rotor. The rotor was 2 wires past the #1 wire. I assume timing chain had slipped. However, the motor must have rolled a little further because as I was removing the harmonic balancer I noticed that the timing marks when set to the pointer at TDC the rotor in the distributor was actually aligned. My bad, I should've just checked here first. I went ahead and verified the timing chain marks lined up, upper gear dot down, lower gear dot up. Chain had a little slack but functional and within parameters. So now what, is it the ignition module? Does the ignition module also act as an amplifier as well? The Mercruiser TBIV ignition test checklist calls something an amplifier? My fuel is fine. Have also checked with starting fluid and get the same result. Does the ignition module control the advance as well and could the module just be malfunctioning and need to be replaced? $250 part that I don't want to change if it's not the culprit.

Re: 1993 7.4L Bravo/Thunderbolt IV Ignition Problem

Yes it does control the advance. Did you find the troubleshooter flow chart? That's what to follow.

Re: 1993 7.4L Bravo/Thunderbolt IV Ignition Problem

does that coil require a resistor? if it does and you don't have one you will be chasing ignition/performance problems all day long. i don't think thunderbolt iv uses a resistor, that was probably just for points. did you get the correct coil?

i would be looking at the ignition module next. i bet you can find a used one for cheap, if that works, buy a new one and keep the used as a spare.

Re: 1993 7.4L Bravo/Thunderbolt IV Ignition Problem

No, his engine doesn't have a coil resistor, or at least it shouldn't have.

What is the voltage to the + terminal of the coil while the engine is cranking over?

Re: 1993 7.4L Bravo/Thunderbolt IV Ignition Problem

500dollar744ti said:

does that coil require a resistor? if it does and you don't have one you will be chasing ignition/performance problems all day long. i don't think thunderbolt iv uses a resistor, that was probably just for points. did you get the correct coil?

i would be looking at the ignition module next. i bet you can find a used one for cheap, if that works, buy a new one and keep the used as a spare.

Click to expand...

I will have to check, but, I believe it even said on the coil "for use with exterior resistor". I will double check and verify. I'm pretty sure I bought the coil for the right year/TBIV system but maybe I have the wrong one. I have a Sierra Part #18-5438. I had read somewhere that the TBIV had a different kind of wire connected to the coil that provided the necessary resistance. Also, if that was/is the case, how come it would've run great for 100 miles and then kaput at the docks...unless it fried the coil again? Interested in your theory here. I am baffled with this issue. I still show spark at the coil and at the spark plug but it appears weak in my book compared to my HEI on my race car.

Re: 1993 7.4L Bravo/Thunderbolt IV Ignition Problem

I started to follow that chart, but I'm not sure what the amplifier is???? Is it the same as the ICM mounted on the distributor?

Re: 1993 7.4L Bravo/Thunderbolt IV Ignition Problem

I am going to go pull the coil again. I know for certain the original failed coil, was an offbrand I had never heard of and indicated "use external resistor". I am sure I ordered the correct coil from Wholesale Marine but I am certain it also said "use external resistor" and is probably for the Delco system. That being said, I am curious if this has fried my ignition module? Hopefully not... I appreciate everyone's input thus far...at least I can move forward. Now I just have to try and put a timing cover back on an engine with no room to pull the oil pan without pulling the engine.... what a fiasco to date!

Re: 1993 7.4L Bravo/Thunderbolt IV Ignition Problem

Did you read my reply and question?

Re: 1993 7.4L Bravo/Thunderbolt IV Ignition Problem

I had pretty much same symptoms beginning of this year. Almost fired sometimes. Mine turned out to be a bad battery connection. Surprisingly it was enough current to crank engine but voltage must have been low enough to not get spark. Connection was extremely hot due to resistance and all that current passing through it. Might not be your problem but good idea to tripple check.

Re: 1993 7.4L Bravo/Thunderbolt IV Ignition Problem

NautiNights said:

I will have to check, but, I believe it even said on the coil "for use with exterior resistor". I will double check and verify. I'm pretty sure I bought the coil for the right year/TBIV system but maybe I have the wrong one. I have a Sierra Part #18-5438. I had read somewhere that the TBIV had a different kind of wire connected to the coil that provided the necessary resistance. Also, if that was/is the case, how come it would've run great for 100 miles and then kaput at the docks...unless it fried the coil again? Interested in your theory here. I am baffled with this issue. I still show spark at the coil and at the spark plug but it appears weak in my book compared to my HEI on my race car.

Click to expand...

18-5438 should be the correct part number. that coil should not say "for use with external resistor", if it says anything, it should say that it has an internal resistor like this http://www.amazon.com/COIL-GLM-Part-Number-392-805570A2/dp/B004AQZFIM

it may have been boxed wrong or who knows but if it says "for use with external resistor" you need to get the correct coil. without the internal resistor i would expect weak spark and it may eventually damage the ignition module.

Re: 1993 7.4L Bravo/Thunderbolt IV Ignition Problem

I did Don, I'm going to go back and test the coil per your question. I'll respond back with the results.

Re: 1993 7.4L Bravo/Thunderbolt IV Ignition Problem

Thanks for the link 500dollar744ti. I personally would agree with you. I have called Sierra technical support and here is what they have written me regarding external resistor.

Mark,

The reason it reads "use external resistor" is because people were using them on normal points style setups when they did the conversion. Which when you do the conversion from points, you have to have a corresponding coil for it. But, that is the correct coil and you do not need an external resistor. If you have any other questions, feel free to contact us anytime.

Thank You,

Technical Service Representative
Sierra International Inc.
Phone # (217) 324-9523
Fax # (217) 324-9491

Now then I'm completely confused. The link you sent me indicates a coil that directly replaces the Sierra 18-5438, but the Sierra requires the use of an external resistor and this one indicates it has an internal. I wouldn't think you could have both, but I have to assume the mfg. knows best here. I'm guessing that at this stage it's best to put it all back together and get the reading from the positive on the coil while cranking per Don S. comment. I'm thinking that my DUI HEI one wire distributor/ignition system on my race car would work best on this engine...much simpler for sure. Don't want to explode though!

Re: 1993 7.4L Bravo/Thunderbolt IV Ignition Problem

The resistor, either internal or external, is not wanted or needed on the Thunderbolt ignition.
The ONLY reason a coil would say it needs an external resistor is IF that coil was going to be used on a points system. Pertronix designed their electronic conversion kits to use the external resistor because they are built into the wiring harness of many engines. So it just made it easier for them to sell to customers that didn't know how, or couldn't figure out how to remove the resistor wires. It just made there product easier to sell.
Coils with internal resistors are use ONLY for points igniitions. The ones that don't have an internal resistor must use an external resistor of some sort when used on a points system, but does not need an external resistor if used on most electronic ignitions.

The warning "Use with external resistor" is a warning only IF YOU HAVE A POINTS IGNITION, nothing else.

Re: 1993 7.4L Bravo/Thunderbolt IV Ignition Problem

he could have a failing ignition module then, certainly would coincide with weak/disappearing spark, no?

this is provided the coil has good voltage and there are no wiring/corrosion issues.

Re: 1993 7.4L Bravo/Thunderbolt IV Ignition Problem

Just to reinforce to the forum, that the electrical test procedures that Don has will pretty much diagnose the whole electrical system. Turns out, one of my batteries had a ground cable that was just ever so slightly loose One 2 second twist of the nut....runs like a top.

"IF IT AIN'T BROKE, DON'T FIX IT!"