Thunderbolt IV Ignition Questions

[kwoolard]

A friend of mine bought a fixer-upper and has been trying to diagnose a no-start, sometimes hard to start condition. The engine just cranks over as if not getting spark. Then sometimes it will start and when shut down a couple of minutes later it will not restart.

Mercruiser 260 - 4 brl - Alpha
Engine Serial No: 0D391241

He has changed plugs, cap and rotor, and plug wires. He has gone through the ignition checklist to determine the problem and has 12Volts to the coil but when checking the red/white wire going to the distributor at one point he got 0 volts, then bumped the motor to change the position of the rotor wheel and got 5 volts. He then tried the spark check coming from the coil with a spark tester by disconnecting the green/white wire going to the shift interuppter switch and tapping it to ground. No spark was seen in the spark tester. I haven't had a chance to help him out but it seems like the Ignition Amplifier is faulty. I told him to make sure he had a good ground going to the distributor's sensor and to the ignition amplifier and recheck, which hasn't been done yet.

Are these Thunderbolt IV ignition modules prone to fail, somewhere I read they are not as reliable as the TB V?

Do you think this could be the amplifier going bad?

What causes the voltage to read 0 volts then 5 volts after the rotor was turned a little?

Why does the test state 1-12volts at the red/white wire and not 12volts continuously?

If it is the amplifier, has it been superceeded with a new model because the part number I get is 15899A 1 and it states it is no longer available?

What is the correct amplifier part number?

 

[Don S]

Re: Thunderbolt IV Ignition Questions

The engine just cranks over as if not getting spark.

Did he check for spark? Is there or is there not spark from the coil?

Not sure where you are getting that part number (15899A 1) but it's for a 5.0L/LX.

 

[kwoolard]

Re: Thunderbolt IV Ignition Questions

Ignition Module Link

The link is above for the ignition module, according to that page it is the same for the 5.7 and the 5.0. If that is not the correct one, do you know the part number?

Did he check for spark? Is there or is there not spark from the coil?

1st post:

He then tried the spark check coming from the coil with a spark tester by disconnecting the green/white wire going to the shift interuppter switch and tapping it to ground. No spark was seen in the spark tester.

Should he try another method for spark testing?

 

[Don S]

Re: Thunderbolt IV Ignition Questions

By checking the spark, I meant by cranking the engine with the starter and holding the coil wire about 1/2" from a ground.

 

[kwoolard]

Re: Thunderbolt IV Ignition Questions

gotcha! i'll get him to check that, i'm sure hasn't already.

if he is not, that only leaves the coil and the ignition module correct?

 

[Don S]

Re: Thunderbolt IV Ignition Questions

Checking for spark is the first thing you do. If you have spark, you may be fighting a fuel problem instead of ignition problem.

The engine just cranks over as if not getting spark.

That AS IF, makes it sound like he is just guessing at what the problem is.

 

[kwoolard]

Re: Thunderbolt IV Ignition Questions

I know.....

I can't get over there to help him out until Sunday.

Can you help me with that 5 volts and 0 volts he was getting on the red/white wire? The manual states that it should be 1-12volts. Why such a large range?

 

[kwoolard]

Re: Thunderbolt IV Ignition Questions

New test results:

Following the troubleshooting guide for the TB IV:

>Check for 12volts present at purple wire (+terminal ignition coil) - GOOD
>Check for 12volts at the red/white wire on the ignition sensor terminal - 0 Volts
>Disconnect green/white wire and tap to ground to check coil for spark - Got Spark
>Disconnect red/white from ignition sensor terminal and check for 12volts - Only have 5 volts

Looking at these results it seems to lead to the ignition sensor.
But getting only 5 volts at the red/white terminal is a little confusing. Is that a typical voltage seen at that wire?
Isn't there a way to check the ignition sensor for resistance?

Does anybody else have any suggestions or should I try the ignition sensor?

 

[Don S]

Re: Thunderbolt IV Ignition Questions

Disconnect red/white from ignition sensor terminal and check for 12volts - Only have 5 volts

You should have had 0 voltage, but you had voltage (5 volts).
Therefore, replace the sensor and go boating.

 

[kwoolard]

Re: Thunderbolt IV Ignition Questions

You say i should have 0 volts.....meaning if his ignition amplifier was bad, but since i have voltage, whether it is 1 volt or 12 volts it leads to the ignition sensor?

I have a spare one that he can try the evening, i'll let you know.

 

[kwoolard]

Re: Thunderbolt IV Ignition Questions

Well, finally made it over to my buddies house to help with his problem. I went through all the checks and everything electrical is good. He has spark from the coil, (using the spark tester it would jump over a 1 inch gap), and 12 volts on the white/red wire or some voltage depending on where the rotor wheel is positioned.

Checked for spark on the plugs and that is good as well.

Trying to start it takes about 10 pumps of the throttle and a lot of cranking while doing it. The carb seems like the fuel bowl is out of fuel after sitting for a while. The carb definitely needs a rebuild because it looks varnished, but i've seen on the forum about these q-jets leaking fuel at the core plugs.

You think this is a good possibility? How do you check, pressurize the fuel bowl on the bench and soap bubble test the plugs?