Mercruiser 5.0 1999 Timing Advance Problem

Larry J Roth

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Jul 30, 2015
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6
Good day to you all. I have a GM Mercruiser 5.0, 1999, Carbureted, Alpha One Gen II, Thunderbolt IV apparently. There is no knock module or sensor. The timing is set at 500 RPM to 10 degrees BTDC. Upon opening of the throttle, it momentarily drops to TDC and then back to 10. Next it will NOT advance at all until 3000 RPM. Then it begins to advance what seems to be linearly.

Voltage into module is over 13. Grounds are good. Sensor has been inspected and cleaned. Module says "5.0 Alpha" Nothing has made a difference.

I have been working Mercruisers for a while but have never seen this before. I am thinking that the module is bad but I have not changed the sensor. In my past experience the sensors work or are fully failed resulting in no spark.

I am truly at a loss other than the module is failing. I hope someone can help.

Thank you in advance.
 
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81 Checkmate

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Jul 20, 2010
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1,360
Welcome to iboats.

Are you sure it's not a V thunderbolt system....not sure what year they upgraded from a IV to V system. If its the V system you have to put it base mode to time it 500 rpms is a bit low, needs to be at 750 rpm
 

Larry J Roth

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Jul 30, 2015
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Thanks for your reply. It's funny you should ask. The top cover over the flame arrestor actually does say TB V. However, there is no knock module or sensor. There is a purple / White wire but no change occurs when attempting to ground it into Base Timing mode. The wire is not shorted to ground either.....
 

NHGuy

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May 21, 2009
Messages
3,631
Somebody once told me they make the V with and without knock control. But I thought the V was the knock control. That's the beauty of TB V-how it keeps searching for correct timing.
 

Larry J Roth

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Jul 30, 2015
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Thank you for your reply. I have followed the Mercruiser direction of grounding the Purple / White wire before powering the ignition for base timing mode. There is no change in the way the advance acts. It Drops to TDC for a few milliseconds when opening the throttle and then hangs at base timing until 3000 RPM, where it then begins to advance beyond 3000...

One characteristic of the performance is this: The engine will not climb over 4000 RPM in the water and has best performance up to 80% throttle. The last 20% of throttle (80-100%) actually reduces power and rpm by about 100-150.

2 barrel Rochester Carburetor (not Mercarb).
 
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81 Checkmate

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Go to the sticky's at the top of the page and look at the trouble shooting chart for the TBV system and see what you come up with. Could be a bad module. Merc wants an arm and leg for a new one.... If so all you have to do is replace the whole dist with a new one. There are many different replacments out there. I had replaced my TB IV with a Mallory Marine unit for less than 400.00 bucks.

Good Luck !
 

Bondo

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70,465
Ayuh,.... Welcome Aboard,..... T-Bolt V, doesn't always have the knock sensor,....

Yes, that's what yer's is,....
 

dingdongs

Chief Petty Officer
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May 29, 2009
Messages
649
are you sure your firing sequence is correct as a couple of wires can be crossed and give a scenario of bad oooomph
 

Larry J Roth

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Joined
Jul 30, 2015
Messages
6
Thank you to all who have replied. I soldered up a jumper between the Purple / White pigtail and the negative post of the battery terminal. This finally effected BASE TIMING mode. I have set the timing per the spec and now the timing advance acts normal.

This is now solved. :joyous:

Thanks again for everything and I look forward to helping others here as I can.
 
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biggjimm

Chief Petty Officer
Joined
Jul 15, 2015
Messages
403
It's a good feeling when you finally figured it out & can fix it yourself. At least I get a little tickle knowing once again I didn't have to take it to the shop. I wonder how many tens of thousands of $ I've saved myself & my family over the years by being able to do most all maintenance & repairs myself? There's a few things I don't fool with but not many. Good job getting to the bottom of that timing problem! Jim.
 

Bondo

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Apr 17, 2002
Messages
70,465
It's a good feeling when you finally figured it out & can fix it yourself. At least I get a little tickle knowing once again I didn't have to take it to the shop. I wonder how many tens of thousands of $ I've saved myself & my family over the years by being able to do most all maintenance & repairs myself? There's a few things I don't fool with but not many. Good job getting to the bottom of that timing problem! Jim.

Ayuh,..... Bein' "Handy" is a double edged sword,....

Bein' pretty "Handy", I've always done darn near everything by myself, for myself,...

Sometimes things get neglected, because there's only 48 hours in a day,...
I never get done, as the "To do list" never gets any shorter,...
For Me, it's always the "fix this for me it'll only take a minute"s that get in the way of My Own stuff gettin' done, or atleast done in a timely manner,...

Honestly, I really ain't found Anything I wouldn't try Once,....
'ell, I'd try bein' a Brain Surgeon if I could find any volunteers,..... :D
 
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