1988 mercruiser mei 7.4 thunderbolt - no spark

[2xMercruiserOwner]

Recently acquired this boat... has dual helm stations.

Both my engines were running and I decided to do a tune up to each as it appeared they had really old parts and starboard started to have the coil arching while running. I first did my starboard and found 2 wires with torn boots at the plugs. Started right back up after doing cap, rotor, wires, plugs and compression test with bump starter.

Now my port engine is a crank no start after doing the same. I am seeing that while cranking that I do not get spark signals to my cap from coil, nor to the plug wires. I swapped coils and same... both are good as both were used on other engine to start the other engine. While the spark plug light tester was in between the coil and dist cap, when key is switched on, I do see one quick light on it. Cranking the engine does not show anymore light flashes. I placed the test light in between the cap and one wire out and I get the same, I get a light flash when key is turned one but no more flashes during cranking.

Best I can gather is that I have power then at coil but maybe it's not grounding as not to pulsate the signal. I have not tested it to say it is 12v.

In doing the service, I replaced the cap, rotor, wires, plugs. I did a compression check by using a bump starter. I forgot to unplug the coil from cap during one bank testing, so had about 40 cranks that way but key was off. In taking off the rotor, I used a heat gun to get it warm as to then pry the rotor off the dist post. I also took of the engines lift eyelet near dist cap to give me more room to pry rotor. I then see a little coolant leaking near there then realized the eyelet bolts secure the intake to block and removing it caused the leak. I sanded the rotor post and other metal in before putting new rotor on as surfaces are older and had a like rust build up. I vacuumed the area out. I can't think of anything else I would have done during the service to cause the no spark afterwards...

I am a near novice when it comes to electronics... so I am not exactly sure of the best grounding locations to get a reading on the coil for a 12v check. I just bought the mercruiser service manual #9 which has a high level only outline of trouble shooting this but no detail on the testing of say the engine wiring harness, ignition brains, safety shut off logic, etc.

Would someone kindly provide advice on this thunderbolt iv system?

Thanks in advance !

 

[alldodge]

Howdy

This is the troubleshooting guide to the TB ignition. It is for a TB-5 but TB-4 is the same. Test lights are good for quick checks but you need a voltmeter

 

[2xMercruiserOwner]

Is the ignition module really the ignition amplifier mounted on the exhaust elbow?

 

[alldodge]

Is the ignition module really the ignition amplifier mounted on the exhaust elbow?

Sounds correct, have a look here for what you would see
http://forums.iboats.com/forum/engi...-mercruiser-thunderbolt-ignition-systems-work

 

[2xMercruiserOwner]

Well I bought a copy of Mercury Service Manula #9 for v8s, 90-14499-1 ?.
It was copyrighted in 87 but seems this is the right one for my 1988 I gues.

Anyway, I want to attached the troubleshooting flow chart as it is only slightly different has mine does not have bullet connectoer on lead wires. Well, I guess I can't upload a pic of the thunderbolt iv trouble shooting outline as of right now... maybe something about my membership level.

So I got to test it out and see that the sensor seems to be the issue here.
I am going to outline my testing and notes then mention where or how this probably failed.
I hope to ordered this asap and get it on this week.
I guess this wasn't as hard as what I was thinking it was going to be... so far... still need to get the parts and hopefully that's it !
I just get a bit nervous, always thinking I can ruin electrical parts if not doing things correctly, therefore slow to go working on electrical items especially when attempting diagnostics.


So in first I thought everything should have been fine as far as the battery, connectors, and screws/bolts.
I decided to use the ignition module's mounting bolt/ ground.
Checking the voltage on the coil I would see it fluctuate quite a bit between 14 down to 10v then if the alarm sounded I would see a range from 11 or 10 down to 7v.
This really was the case on both coil terminals (purple and gray sides).
I cleaned up the black paint on terminals connectors and washers and retested. It stayed a bit more tighter between 13 and 11v before warning alarm but still dropped voltage when alarm was sounding.
I accepted it had 12 v at coil on purple wired + terminal.

Checked the red\white wired terminal on the sensor and it was nearly consistent at 12 v. I did not clean up terminal from paint.

Next I took off the green/white lead wire and tested the wire's terminal connector to ground repeatedly while having a spark tester between coil and cap. It showed spark so need to replace the sensor (and rotor too). My concern here though was on first contact of green / white lead to ground, I did hear some different type of noise only that first time. Not sure where it came from but my guess was the ignition module/ amplifier ... then only later did I wonder if it was the breaker. Not at boat to check on breaker.

So in doing the rotor and servicing the system, I used heat on the rotor to assist in getting it off. I may have caused an issue from the heat gun. Mercury manual says use screw driver for prying leverage, but I think using 2 claw hammers are easier. Next time I won't use heat. The rotors may have begin original, 29 years old probably with locktite. I had not used locktite yet, but since needing to pull and replace rotor again for getting this sensor out, I may put locktite on the next rotor.

I did sand down some surface areas while rotor was off during ignition service. I cleaned the dist post, interior housing, and did hit the metal surface of the sensor. All these areas did have some surface rust. I did swap some rust prohibiting spray fluid on all those areas too, did not spray though. Something may have occurred here too to damage that sensor.

Well I am going to order the parts and hope to get it on and starting this week.

 

[2xMercruiserOwner]

So that was the outline, I guess my questions would be why do I see the coil voltage fluctuate?

Use locktite on those rotors or not? I guess this is done as not to allow the rotor to move upwards?

Any thoughts on the noise I heard when first grounding the green white wire that showed spark at high lead / cap?

Thanks !

 

[alldodge]

So that was the outline, I guess my questions would be why do I see the coil voltage fluctuate?

The tach may be going out. When testing the ignition you should have the tach wire (gray) removed to rule out tach issues.

Use locktite on those rotors or not? I guess this is done as not to allow the rotor to move upwards?

I would not do that, if the rotor is popping up then there is something else causing it. Maybe to much play in the distributor. Breaking the rotor can also be caused by build up on the cap.

Any thoughts on the noise I heard when first grounding the green white wire that showed spark at high lead / cap?

Thanks !

Don't know what that would be, and like sounds its just hard to describe in words

 

[2xMercruiserOwner]

So I take it to rule out issues from tach, that would be to take the one of two gray wires off coil - side that runs back to the wiring harness ( leaving the other gray one on that's runs back to ignition module / amplifier)? I just don't want to get into the games so I hope removing the one of two gray wires at coil suffices.

I did not have issues along the lines of starting before, nor tach issues, and had not disturbed that area during my ignition parts maintenance. That is why I checked step 1 off of the flow chart without much testing... like the gray wire. I then had voltage at coil, fluctuating but 12v range. I would be able to test it though and see what occurs for coil voltage.

This is a dual helm with engine sync display gauge.

 

[alldodge]

So I take it to rule out issues from tach, that would be to take the one of two gray wires off coil - side that runs back to the wiring harness ( leaving the other gray one on that's runs back to ignition module / amplifier)?

Correct

 

[2xMercruiserOwner]

Well I as able to buy and install a new sensor (and rotor) ... fired up easily and ran great. Did not test the gray prior to installing sensor as I had the cap and wires loose from last time I worked on it. Did not see the need to tighten, test gray, remove wires again.