Mercruiser 4.3 TKS -- Blown Ignition Fuse and Shorted TKS Diode

[saltshaker11]

I have a 2008 Sea Ray Sport 205 with a Mercruiser 4.3L TKS engine. Recently the ignition fuse blew and I discovered that the TKS diode was shorted. After I removed the diode and replaced the fuse, the engine started and appeared to run fine. I ordered a new diode thinking that this would resolve the issue but when I put the new one in place, the ignition fuse blew again and the diode shorted out again. Again, when I removed the new diode and replaced the fuse, the engine started and ran fine.

I'm going to use the troubleshooting tips suggested in Mercruiser Service Manual #41 for TKS Carburetors and some of them call for running the engine during the testing. I'm wondering if that is risky at all ... is it harmful to run the engine without the TKS diode in place?

I'm guessing the problem is in one of the other parts of the TKS system, like the TKS Module or the Temperature switch. But I'd appreciate any suggestions that the experts on the forum might have because I'm kinda groping through this.

I also noticed that the oil pressure is reading higher than usual not sure if that is a coincidence or related to this.

Thanks!

 

[achris]

Looking at the circus diagram, about the only thing that could blow the fuse and the diode is a shorted temperature switch (#6 in the retrofit diagram), or wiring associated. The only 'problem' running without the diode is the while cranking and until the engine reaches temperature, the TKS module will not be powered and so will be running the engine rich.. Was the diode shorted or blown open? If it was blown open (usual result of a short downstream), and the result for the engine will be... see above. If it remained shorted (usually if enough current passes to short it internally, but not enough to then blow that short open), then once the engine reaches temperature, the engine will not switch off. (the current will feed back through the temp switch and shorted diode to the fuel pump and ignition module...)

Chris..........

 

[saltshaker11]

Thanks a lot for the guidance, Chris. I'll be running tests on the engine later today and will pay special attention to the temperature switch and associated wiring.

Some more detail on the symptoms are:
- Both TKS diodes were shorted after they failed -- i.e. in both cases after the ignition fuse blew, the diodes had zero resistance in either direction.
- The first time the ignition fuse blew, I was stranded with darkness approaching and out of desperation I shorted the ignition fuse and cranked the starter for a few seconds. This caused the TKS diode to smoke and melt slightly.
- In every case, after removing the diode and running the engine, I had no trouble starting or stopping the engine whether it was cold or warm. The engine seems to run normally without the diode in place. The only unusual thing I noticed was that the oil pressure gauge reads higher than usual and fluctuates a lot with RPM. At RPMs of 1000-2000, pressure is about 40; at RPM of 3000 pressure is about 60; at RPM of 4000 pressure is about 70.

I'll report back with results from the tests. In the meantime if anyone has further advice, I'd be grateful

Best Regards,
Dan

 

[saltshaker11]

I did a couple tests and the results/observations are below. All measurements were taken with the engine off.

- The measured resistance across the TKS module was 45 ohms, so this doesnt seem to be part of the problem.
- The measured resistance across the wiring harness leads that connect to the TKS diode was 21 ohms. The pin that connects to the purple/yellow wire was blackened so it looks like most of the current came from this wire when the diode shorted.
- The 20A fuse between the slave start solenoid and the TKS temperature switch was blown. It's blown both times the TKS diode shorted (I forgot to mention this in the previous posts).
- The boat seems to run perfectly fine without this 20A fuse and TKS diode in place.

I still dont know the source of the problem. I'm planning to order a new TKS diode and temperature switch but I'm concerned about installing them without identifying the problem because it might destroy them like it did my first replacement diode. I welcome any suggestions.

Thanks much,
Dan

 

[achris]

OK. I'm quite busy today, but tonight I'll have a good look at the circuits and see what I can find for you...

Chris......

 

[achris]

Ok, Just spent about 45 minutes analyzing the circuits and I have a few checks to be done..

Based on this diagram....



With the diode removed, current will not flow through (and heat) the TKS module until the engine temp switch closes (about 130?). Once the switch closes, the TKS module will heat up (remove fuel enrichment) and even when the engine is turned OFF, while the temp switch is closed (the engine remains 'warm'), the module will continue to be powered.

Q1. Remove the fuse and probe both connectors. Do you have +12v on only one side? (check to engine ground)

Q2. With the diode removed and the engine cold (the temp switch open and the TKS module not being powered), does the fuse blow?
Q3. With the diode removed and the engine at operating temperature (the temp switch closed and the TKS module being constantly powered), does the fuse blow?

With the fuse removed and the diode in place the TKS module will be powered during cranking and when the engine oil pressure is above 4psi (ie, when starting and running). As the temp switch fuse is removed, the module will begin to cool immediately the engine is shut off.

Q4. With the fuse removed, does the diode short? If so, when? Immediately the key is turned to ON or as soon as the engine reaches 130??

Q5. Silly I know. Is the diode installed the right way around?

BTW, if you don't want to pay Mercs' silly dollars for the diode, nip down to your local friendly electronics shop and ask for a 1N5404 diode. A pack of 10 will cost you about $3 (and get your fuses there too ). Install your new diode with the silver band on the diode body facing the TKS module (electrically). The 1N5404 is a 3A diode. In reality, a 1A diode (1N4004) would do the job (current through the TKS module shouldn't be more than about 0.4A), but I prefer the higher current capability. If they don't have a 1N5404, aks for 'any 3A rectifier diode'. As the operating voltage is 12v, and the voltage rating on rectifier diodes START at about 50v, the voltage becomes irrelevant.

Good luck and let's see where this gets us.

Cheers,

Chris......

 

[saltshaker11]

Wow! Awesome reply, Chris! Thanks for all the detail. I will run the tests today and report back.

 

[achris]

One more test that is worth doing. (In fact this test should be the first one. )

Remove the diode. Pull the plug from the temp switch. Put a fuse in place and with the meter set to DC Amps, put the meter black lead into the the white wire connector (engine harness, not temp switch ) and the red lead on the other pin. Q. What current are you reading? If the current is between 0.2A and 0.5A, leave the meter there for a while... Monitor the current.

If the current is way high (above about 0.75A), you need a new TKS module.

BTW, what colour is the 'other' wire? Should be orange or red/purple... (that tells me which circuit you have, standard or retrofit)

Chris..........

 

[saltshaker11]

Chris,

Sorry for my delayed response. Here are the results of the tests you recommended plus some other observations ...

- Q1: With engine running I removed the 20A fuse and probed both connectors with respect to ground. The red w/purple stripe lead measured 14.5V to ground and the pink w/white stripe lead measured 0V to ground.
- Q2 & Q3. With diode removed the 20A fuse didn't blow under any of the test conditions, including engine cold start, engine warm start, extended engine running conditions.

Unfortunately I ran out of time and was rushed for the following tests. I'm going to repeat when I can get back to the boat.
- Q4: The diode I used for this testing was an IN5408 ... i bought 10 of these from the local shop for $0.06 each . Funnily enough, I couldn't get it to blow/short. I did a cold start and a warm start on it and it didn't short or blow the fuse.
- Q5: Yes, the diode was installed the right way ... it only connects one way and was aligned with cathode towards the TKS module.
- Q6 (from your 6/16 posting): I wasn't able to get the current measurements (something funky with my DMM ... wasnt reading any current). I will repeat next time I'm at the boat. The wire connecting the temperature switch and TKS module is white; wire connecting the temp switch and 20A fuse is pink w/white strip; wire connecting the 20A fuse and alternator is red w/purple stripe.

General Observations:
- With the diode removed and 20A fuse in place the TKS Module and Temperature Switch effectively prevent enrichment at high temperatures. After engine is warm, when I disconnect the temperature switch, the TKS module appears to unblock the enrichment channel because the engine RPM slows significantly.
- Without TKS Module plugged in, voltage across TKS module leads was 14.5V.
- Without TKS diode plugged in, voltage across diode leads was 0.7V. Voltage from either lead to ground was 14.5V. Resistance across diode was 800 ohms.
- I drove the boat for several hours with the diode removed and everything seemed normal.
- Since the damaged diode lead was connected to the purple/yellow lead, I'm wondering if that means the source of the problem is the oil switch.

I just ordered a TKS Temp Switch (87-866089), Oil Sensor Kit (87-864252A01), and another 2 Mercruiser Diodes (87-865913A01) and they are due to arrive on Saturday. I figured it was worth the $65 investment to replace both the temp and oil switches.

I'll report back with any add'l results.

Thanks again!
Dan

 

[saltshaker11]

As Chris predicted, it looks like the new TKS Temperatures Switch fixed the problem! After installing it and a new TKS Diode, I was able to start the engine several times without any fuses blowing and with no damage to the Diode. I'll take it out all day on Sunday and that will be the real test.

Turns out I need to get a new multimeter to read the level of current Chris asked about ... my current one max's out at 200mA. I'll be buying one of those.

As far as root cause analysis, I'm still not sure why replacing the temp switch would fix the issue ... the old temp switch was definitely defective as it was always closed, even at low temperature, but I would think that would just keep the TKS module on permanently (even at cold start) and it wouldnt have blown the diode or ignition fuse.

One possibility is that the fuel line connecting the fuel pump and the carb was routed directly over top of the temp switch and applied a lot of pressure and friction to the wire leads going into the switch. It's possible that the insulation on one of those wires wore enough to short to the fuel line but when I probed them with a multimeter I couldnt find a short. I bent the fuel line away from the new switch to avoid that going forward.

I wont look a gift horse in the mouth though! I'm glad it's working. When I get the oil switch in the mail later today I may change that too just for good measure. Thanks very much for the help!

 

[achris]

:thumb:

 

[saltshaker11]

Took the boat out all day Saturday and Sunday with no issues. Problem solved. Thanks again, Chris!

For the record, I didnt install the TKS Oil Switch yet. Replacing the TKS Temp Switch and bending the fuel line away from the Temp Switch solved the issue.

 

[achris]

Nice to hear... Let's hope it stays that way.

 

[BlackBeastEnt]

I have a 2007 Bayliner that is having the same problem. Do you have pictures of where the temp switch is you replaced? I put a new diode in and am afraid I blew that one up today also. When you purchased the diodes from the electrical place how did you put them in since the original is in that red housing. Any info you can give me would be appreciated.

 

[Tomdio]

Hello. I know this thread is old but I had the exact same problem and this thread helped tremendously! Thank you so much! Just one question though, what type of thread sealant are you supposed to use on the temperature switch? The repair manual doesn’t say one way or another.
Thanks again.
Tom

 

[achris]

You know it's old, you read the forum rules about posting in old threads, yet you still did it.

Please start a thread of your own.

iboats mod team.