Unknown Short Circuit

[pizzapatriot]

I have a Tahoe with a Mercruiser 5.7 L MCM engine. When I had it winterized everything fired right up. I tried starting it after being stored all winter and I get a solid crank but no start. I started looking for fuses or loose connections near the fuel pump and carburetor and I stumbled upon a blown 20 amp fuse near the carburetor but it doesn’t seem to be a part of the electronic choke. When I put a new fuse in it blows immediately which would indicate a short to ground somewhere. I’ve found some wiring diagrams but it’s still unclear what circuit this fuse is a part of. Any ideas what this fuse is for (circled in photo)? Any ideas on the crank no start?

 

[Bondo]

Ayuh,..... Welcome Aboard,...... With the key in the "On" position, is there power at the coil's (+) terminal,..??

 

[dubs283]

Would guess the fuse is for the tks module. Easy check for a short to ground

Agree with bondo, need to troubleshoot the t bolt ignition system, there is a guide in the diy sticky top of forum page

 

[nola mike]

That looks like the tks fuse. Orange wire connects to the temp switch. A short in the wiring or switch will blow that immediately. Try disconnecting the temp switch.

 

[pizzapatriot]

Would a blown fuse to the temp switch or tks module cause a crank no start situation or am I dealing with two different problems?

 

[QBhoy]

Hi. Forgive me for asking, but has that engine been submerged partially or fully in the past ? May just be the picture, but looks like you may have signs of water being around that at sometime. If so, you may need to go over her in detail for other issues.

 

[flashback]

Would a blown fuse to the temp switch or tks module cause a crank no start situation or am I dealing with two different problems?

From what I can tell, the tks is a fancy enrichment component that takes the place of a choke so if it's failed the motor won't start... fwiw

 

[nola mike]

Would a blown fuse to the temp switch or tks module cause a crank no start situation or am I dealing with two different problems?

I don't think so, but depending on your fault you may not be powering your fuel pump.

From what I can tell, the tks is a fancy enrichment component that takes the place of a choke so if it's failed the motor won't start... fwiw

Default position of the tks module is enrichment... It provides less fuel over time when powered, like standard choke. That said, a lot more complexity for no real purpose.

 

[pizzapatriot]

So the short is not in the fuel pump, temperature switch or in the tks. The circuit in question powers the fuel pump, temperature switch, oil pressure switch and loops into the starter. So with a blown fuse no power reaches the tks or fuel pump. While tracing the circuit I found an unknown device (see picture)? It seems like the circuit is grounded at the starter so is this device some sort of resistor? In the wiring diagram it’s the purple/yellow circuit.

 

[zellerj]

look for wires that were chewed by mice. Mice love to eat insulation over the winter.

 

[muc]

So the short is not in the fuel pump, temperature switch or in the tks. The circuit in question powers the fuel pump, temperature switch, oil pressure switch and loops into the starter. So with a blown fuse no power reaches the tks or fuel pump. While tracing the circuit I found an unknown device (see picture)? It seems like the circuit is grounded at the starter so is this device some sort of resistor? In the wiring diagram it’s the purple/yellow circuit.

That's the blocking diode.
If the diode is shorted internally, the engine will continue to run after the key is turned off.
If this condition is found, removing the diode from its connector will shut the engine off.
If the diode is open internally, the TKS module will not have positive current when the engine is warm and the key is off. The engine will be hard to start when it is
warm.

Fuel pump gets power from starter during cranking there is no ground at the starter for this system.

What happened when you disconnected the temp switch and put a new fuse in?

 

[pizzapatriot]

I’m on

look for wires that were chewed by mice. Mice love to eat insulation over the winter.

I’m hunting it down.

That's the blocking diode.
If the diode is shorted internally, the engine will continue to run after the key is turned off.
If this condition is found, removing the diode from its connector will shut the engine off.
If the diode is open internally, the TKS module will not have positive current when the engine is warm and the key is off. The engine will be hard to start when it is
warm.

Fuel pump gets power from starter during cranking there is no ground at the starter for this system.

What happened when you disconnected the temp switch and put a new fuse in?

The diode is in the closed position one pin inside looks discolored. The diode has a white and purple/ yellow wire going to it. However The short seems to be between the starter and diode since I have continuity to ground on the purple/yellow. According to the wiring diagram the purple/yellow wire splits off at the oil pressure switch and becomes a purple and goes to the alternator and distributor. Is it possible I’m reading continuity through the coils of the alternator or distributor? I’ve traced the circuit in the picture this far.

 

[nola mike]

That's the blocking diode.
If the diode is shorted internally, the engine will continue to run after the key is turned off.
If this condition is found, removing the diode from its connector will shut the engine off.
If the diode is open internally, the TKS module will not have positive current when the engine is warm and the key is off. The engine will be hard to start when it is
warm.

Fuel pump gets power from starter during cranking there is no ground at the starter for this system.

What happened when you disconnected the temp switch and put a new fuse in?

A diode failure shouldn't blow the fuse or prevent cold starting either way though, right?

I’m on

I’m hunting it down.


The diode is in the closed position one pin inside looks discolored. The diode has a white and purple/ yellow wire going to it. However The short seems to be between the starter and diode since I have continuity to ground on the purple/yellow. According to the wiring diagram the purple/yellow wire splits off at the oil pressure switch and becomes a purple and goes to the alternator and distributor. Is it possible I’m reading continuity through the coils of the alternator or distributor? I’ve traced the circuit in the picture this far.

P/y doesn't become purple. It branches at the oil press switch. When you have oil pressure the purple feeds
The p/y wire. I don't see how a short in the p/y would cause that fuse to blow unless the temp switch is bad (it should be open when closed).

 

[pizzapatriot]

A diode failure shouldn't blow the fuse or prevent cold starting either way though, right?

P/y doesn't become purple. It branches at the oil press switch. When you have oil pressure the purple feeds
The p/y wire. I don't see how a short in the p/y would cause that fuse to blow unless the temp switch is bad (it should be open when closed).

 

[pizzapatriot]

When I started troubleshooting the orange wire to the fuse had continuity to ground which would mean the temp switch is in the closed position when engine is cold. Is that the correct position? Upon Further tracing the the purple/yellow has continuity to ground between the starter and diode. My logic is Since this circuit feeds the oil pressure switch (assuming this switch has a normally closed position) then the short could be on the alternator or coil?

 

[Bondo]

Upon Further tracing the the purple/yellow has continuity to ground

Ayuh,..... The purple/ yellow wire feeds power from the starter's solenoid to the fuel pump,.. Only while the starter is engaged,..... aka: the start circuit,.....

When you let off the key, that wire goes dead, 'n the fuel pump runs off the ignition circuit, which are the purple wires,.....

 

[QBhoy]

So the short is not in the fuel pump, temperature switch or in the tks. The circuit in question powers the fuel pump, temperature switch, oil pressure switch and loops into the starter. So with a blown fuse no power reaches the tks or fuel pump. While tracing the circuit I found an unknown device (see picture)? It seems like the circuit is grounded at the starter so is this device some sort of resistor? In the wiring diagram it’s the purple/yellow circuit.

Is that not a weather cap terminator plug ?
Anyway. Thought I’d ask. Are you blowing that fuse when inserted or only when the ignition is on or cranking ?

 

[pizzapatriot]

Is that not a weather cap terminator plug ?
Anyway. Thought I’d ask. Are you blowing that fuse when inserted or only when the ignition is on or cranking ?

When inserted

 

[QBhoy]

When inserted

Sorry if you said already. Did you have a try at putting a meter at each side of the fuse carrier and checking each one across to earth to make sure neither side has continuity showing to ground ? Also I’d maybe check continuity from the live side of the carrier back to the live positive post on the battery and battery switch. Then I’d also check for continuity across the dead side of the carrier across to where that ends and connected.

 

[muc]

A diode failure shouldn't blow the fuse or prevent cold starting either way though, right? correct

P/y doesn't become purple. It branches at the oil press switch. When you have oil pressure the purple feeds
The p/y wire. I don't see how a short in the p/y would cause that fuse to blow unless the temp switch is bad (it should be open when closed). Temp switch is open when cold, as it warms it closes to allow voltage to the TKS and the enrichment valve closes.