Perkins 4236 short circuit someplace - how to test ignition

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shoc

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On our first trip of the season a few days ago(river cruiser) we discovered a burning smell at the helm and keyswitch was very hot so we abandoned our journey and returned to the marina.

On further examination it appears to have a short circuit in my engine wiring someplace, theres a very burnt wire connected to the ignition (Lucas 128SA) and other end is burnt at the thermostat/preheater -- I've yet to strip the wiring along that route and see what else has melted, will be doing that later this week. I think we were lucky we noticed the smell and returned to base -otherwise the burning could have erupted.

Afterwards when poking around I did notice something odd, when I turn the engine batter isolator 'on' and have the key position at 'off' I still get a spark when I touch the cable spade connector to the engine thermostat/preheater -- I don't think that should be happening so I'm suspecting the short circuit could be in the ignition. Is that likely?

I took the switch home with me and hope I can find a local auto electrician to test it, in the meantime I've ordered a replacement which I should have next week.

Some pictures of the back of the switch

Position 1 is burnt - two brown cables and position 2 is burnt - white cable

I should add that I'm a not a mechanic or electrician but will do the stripping back of cables to make the job a bit easier for the mechanic to come later.





 
Have you a multi meter? When the switch is in the on position , no connection should be between terminal 1-5.
 
My inital reaction was that perhaps the switch was sticking. i.e when turned beyond 'heat' to start to engage the starter motor, the internals didn't spring back to the 'run' place, thereby leaving the 'Heat' circuit engaged. Obviously the heater circuit isn't intended to stay engaged for long. How does the key 'action' operate. Does the key spring back to 'run' nicely?
 
Do you have the full preheater current going through the switch. is it the correct switch or should there be a relay in the circuit to protect the switch from the high preheat current? Many engines have a preheat relay.
 
Tamarside said:
Have you a multi meter? When the switch is in the on position , no connection should be between terminal 1-5.
Click to expand...
Yes i have a meter all right, I'm back at the boat tomorrow so will try that.

tillergirl said:
My inital reaction was that perhaps the switch was sticking. i.e when turned beyond 'heat' to start to engage the starter motor, the internals didn't spring back to the 'run' place, thereby leaving the 'Heat' circuit engaged. Obviously the heater circuit isn't intended to stay engaged for long. How does the key 'action' operate. Does the key spring back to 'run' nicely?
Click to expand...

I thought the same but the key action is nice and smooth, it springs back to 'run' by itself

Bilgediver said:
Do you have the full preheater current going through the switch. is it the correct switch or should there be a relay in the circuit to protect the switch from the high preheat current? Many engines have a preheat relay.
Click to expand...

I'm not sure but I suspect there's a relay there, I'll know more tomorrow.
 
Bilgediver said:
Do you have the full preheater current going through the switch. is it the correct switch or should there be a relay in the circuit to protect the switch from the high preheat current? Many engines have a preheat relay.
Click to expand...
Judging by the size of those cables.....
 
shoc said:
......Afterwards when poking around I did notice something odd, when I turn the engine batter isolator 'on' and have the key position at 'off' I still get a spark when I touch the cable spade connector to the engine thermostat/preheater -- I don't think that should be happening so I'm suspecting the short circuit could be in the ignition. Is that likely?......
Click to expand...

Nothing odd about that. Melted insulation could be causing a short circuit anywhere. You need to expose all the damage.
 
Reckon you have the wrong switch in the proof diagram. The photo you fave taken of the back of the switch is a 4 plate switch. The truth diagram shows a 5 plate switch. Lucas 128SA 35670 may be the one you need. Also these are rated for a heat start device, not usually relayed. Note that some of the terminals are 9mm spades for the load.
 
I have a Perkins 6354 with similar Lucas SA switch. The weight of the wire on the back of the 'Heat' terminal is exactly like mine and doesn't look like a wire to a relay, more direct to the heat 'thingy' (which I have never used). It is a shame it is not as simple as a sticky switch. I think there should be no power at all at pre-heater with the switch off as you have already noted.

There is quite a wide variations of the Lucas SA - I have 4 terminals, you have 5. I disagree with Tamarside, I can see 5 terminals on the image! On my loom, brown and red is the pre-heater - which looks ok on the back of the switch (#3 on your terminal). The Brown is the power feed to the switch (#1) and looks suspect. Red and white is the Starter #5 on yours which looks fine. The white wire in the back ground (#2) looks unhappy as you say. On mine the white is the Alternator light and is on #2 but there are other auxiliary functions on #2. On yours the white #4 is for the Alternator. So what is on #2. On mine it has the instruments and that is all.

I don't like the spark on the pre-heater while the switch is off! I think you need to trace right through the loom.... I'm afraid
 
Heres a better photo of the back of the switch - 5 terminals all right.
Why do terminals 1 & 2 have two prongs?

I never use the pre-heat, read somewhere that its only required for sub zero temps and on those days I'm at home with a hot port;)


 
tillergirl said:
I have a Perkins 6354 with similar Lucas SA switch. The weight of the wire on the back of the 'Heat' terminal is exactly like mine and doesn't look like a wire to a relay, more direct to the heat 'thingy' (which I have never used). It is a shame it is not as simple as a sticky switch. I think there should be no power at all at pre-heater with the switch off as you have already noted.

There is quite a wide variations of the Lucas SA - I have 4 terminals, you have 5. I disagree with Tamarside, I can see 5 terminals on the image! On my loom, brown and red is the pre-heater - which looks ok on the back of the switch (#3 on your terminal). The Brown is the power feed to the switch (#1) and looks suspect. Red and white is the Starter #5 on yours which looks fine. The white wire in the back ground (#2) looks unhappy as you say. On mine the white is the Alternator light and is on #2 but there are other auxiliary functions on #2. On yours the white #4 is for the Alternator. So what is on #2. On mine it has the instruments and that is all.

I don't like the spark on the pre-heater while the switch is off! I think you need to trace right through the loom.... I'm afraid
Click to expand...
Don't think it is the loom. These switches can suffer from shorting across terminals internally.
 
shoc said:
Heres a better photo of the back of the switch - 5 terminals all right.
Why do terminals 1 & 2 have two prongs?

I never use the pre-heat, read somewhere that its only required for sub zero temps and on those days I'm at home with a hot port;)


Click to expand...
The switches are common fitment in the old agriculture, industrial engine market. Regarding terminal 1 for example. the feed from battery, the second terminal could be say a feed for the lights when used on a tractor.
 
shoc said:
Heres a better photo of the back of the switch - 5 terminals all right.
Why do terminals 1 & 2 have two prongs?

I never use the pre-heat, read somewhere that its only required for sub zero temps and on those days I'm at home with a hot port;)


Click to expand...

Quite. A Perkins 6354 doesn't need heat either unless in sub zero temps. Shame really all these daft modern engines.

I use the second terminal on #2 to run a relay. The Perks has 24v but certain odds and ends are 12v .... like the exhaust alarm and the fuel tank gauge.
 
tillergirl said:
I have a Perkins 6354 with similar Lucas SA switch. The weight of the wire on the back of the 'Heat' terminal is exactly like mine and doesn't look like a wire to a relay, more direct to the heat 'thingy' (which I have never used). It is a shame it is not as simple as a sticky switch. I think there should be no power at all at pre-heater with the switch off as you have already noted.

There is quite a wide variations of the Lucas SA - I have 4 terminals, you have 5. I disagree with Tamarside, I can see 5 terminals on the image! On my loom, brown and red is the pre-heater - which looks ok on the back of the switch (#3 on your terminal). The Brown is the power feed to the switch (#1) and looks suspect. Red and white is the Starter #5 on yours which looks fine. The white wire in the back ground (#2) looks unhappy as you say. On mine the white is the Alternator light and is on #2 but there are other auxiliary functions on #2. On yours the white #4 is for the Alternator. So what is on #2. On mine it has the instruments and that is all.

I don't like the spark on the pre-heater while the switch is off! I think you need to trace right through the loom.... I'm afraid
Click to expand...
Oh yes you have, look at the truth table, it activates in the start position.
 
Spent some time on the boat yesterday and here's what I found;

When the key switch is the off position there is voltage between position 1 and 2 -- which is the pre-heat cable. Also there is a relay Lucas SRB500 which 'clicks' when power is given to it and it seems to control the pre-heat terminals. So its either a faulty ignition or relay -- both are easily replaced.



The wiring loom needs to be replaced, I'll be onto perkins to see if they stock a replacement.



 
shoc said:
Spent some time on the boat yesterday and here's what I found;

When the key switch is the off position there is voltage between position 1 and 2 -- which is the pre-heat cable. Also there is a relay Lucas SRB500 which 'clicks' when power is given to it and it seems to control the pre-heat terminals. So its either a faulty ignition or relay -- both are easily replaced.



The wiring loom needs to be replaced, I'll be onto perkins to see if they stock a replacement.



Click to expand...
So, can you clarify please.
Quote. There is voltage between 1-2.
Did you put one probe on term 1 and the other probe on term 2 and noticed a voltage?

According to the truth table you published in the original post term2 is Aux, this would feed instrument panel etc. Terminal 5 is relevant to the heater system.
 
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