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Twins Technical Talk Technical Talk for Hinckley Triumph Twins: Bonneville, T100, Speedmaster, America, Thruxton, and Scrambler.

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Old 12-23-2010, 03:10 PM   #1 (permalink)
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The Oil Pressure Light Switch

Here we have yet another simple component that often gives trouble when it shouldn't. Millions are in regular use on the world's vehicles and they rarely fail.

We get regular posters asking why their oil light is ON when everything appears to be OK. Just this week we have yet another one on this recent thread:

http://www.triumphrat.net/twins-talk...-no-start.html

Where is it, What is it, and how does it work?

The switch is situated between the carbs/throttle bodies and the top of the cylinder head on the end of the camshaft oil feed pipe that goes from the top of the crankcase to the head. It also doubles as a banjo bolt for that supply pipe.

It's a normal pressure switch. As the oil pressure rises above a certain level it presses on a diaphragm and this operates a couple of electrical contacts, making or breaking a circuit. This level is quite low, of the order of 2 to 6 psi. This is, of course, a lot lower than the normal oil pressure (around 40 psi at 4,000 rpm) so by the time the switch operates the engine is in great danger of permanent damage.

That 40 psi figure, by the way, is straight out of the Triumph manual and must be a minimum level. Mine operates at just over 60 psi with a hot engine.

If the oil light comes ON with the engine running you must stop immediately and investigate the reason. This could range from using too thin or worn-out oil in high ambient and operating temperatures when the light might flicker on and off at idle speeds, to a very low level in the sump causing the oil pick-up pipe to be uncovered by the oil when negociating steep hills.

This is really bad news. The main and con-rod bearings are plain shell bearings relying on a plentiful supply of pressurised oil for their operation. If they're starved of oil for even a few seconds they can be damaged. Ball and roller bearings can tolerate this abuse but not shell bearings. (Google "hydrodynamic lubrication").

The most serious reason for loss of oil pressure would be oil pump failure or one of the oil gallery or drain plugs coming undone and dumping all the oil. This is unusual though, although we have had one or two oil pump drive gear breakages, they're made of plastic, apparently....

What are the most common reasons for the light to come ON?

Happily in 99% of the cases we've seen in the past the reason is a simple one:

Following a good soaking while washing the bike, water gets inside the protective rubber boot and forms a conductive path to ground that turns the light on.

Electrically, the switch contains a pair of contacts that are normally open. In this case we'll take "normally" as being with the engine running. One end of the oil light bulb is connected to +Positive battery volts and the other end is wired to the oil light switch. The bulb can not operate as long as the oil switch holds the contacts open. Once pressure drops the contacts close and the bulb is connected to ground or chasis (negative) and so it lights up.

What's the point in having a rubber boot if water gets in anyway?

Here we come to another little British bodge. Far be it for me to question the Factory's electrical wizzards, but simply taking a casual look at how the switch is fitted and wired tells me the reason why we get these problems:

Firstly the rubber boot is not made of the finest rubber in the world. I only have to look at it and feel it to know that. I have lots of experience with poor-quality, probably Chinese, rubber items. Even so it should work for a few years as it's not exposed to rubber-rotting oil and sunlight, so, why does it fail?.

The connecting wire has a round M4 terminal crimped on to it to make the electrical connection. The sharp edges of the crimp tear the little rubber extension that's supposed to seal the wire entry into the main rubber boot. This probably happens as soon as the line assembler forces the thing on to the switch. It did on mine as it was broken from the start.




This wouldn't matter so much normally, but the Factory's quality or production engineers have ignored a vital rule when a wire is inserted into an item that demands protection from water. Note that the wire comes in from the top when it should be from the bottom. This is a basic rule well-known to Electricians, Satellite installers, Air conditioning fitters, etc.
Ask any of them what a "Drip loop" is. Forming a drip loop is standard practice when electrical conductors or pipes enter a housing or enclosure likely to be exposed to water.

As it stands any water that gets on the wire can penetrate the switch boot by simple gravity.
The wire is too short to enable the line fitters to loop it downwards and connect it with the crimp underneath, even assuming they knew or cared about the drip loop technique.




What can we do to avoid this?

We could do one of five things:

Extend the wire so that it can form a loop and enter from under the boot. I can't be bothered with this myself and I work in the electrical industry...

Remove and discard the boot altogether. This would allow any water or moisture to drain away and dry out quickly considering the heat in that area but it would leave a "live" terminal exposed. This would be of no consequence really. Even if you shorted this to ground all that would happen is that the oil light would come ON.

Slide the boot back and coat the exposed switch surfaces with silicon or dielectric grease. This would water-proof the switch nicely.

Some posters recommend cutting a small hole or slit on the lowest part of the boot. The idea is that any water that collects inside will drain out.

Avoid high pressure hosing in that area. Even if the switch was fitted properly the part of the boot that fits over the switch hexagon is not that tight and water can get in there.

How can we test the switch?

If the problem is that the light doesn't come on with the ignition turned on and the engine stopped, peel back the boot and using a screwdriver short the electrical terminal to a metal part (not the carbs though, they're isolated from ground by their rubbers). The light should come on. If it doesn't then either the bulb has blown, there's a broken wire somewhere along the circuit that feeds the bulb with battery volts or the grounding wire that goes to the switch. If the light does come ON then we can assume the switch is faulty and has to be replaced. Its contacts are dirty or broken and there's nothing that can be done short of replacement.

The part number and description is: SWITCH & BANJO BOLT. OIL PRESS T1210504 and the cost is £13.62 ($21).

If the problem is that the light comes ON while the engine is running but everything seems to be in order (oil level, no worrying noises, etc) then we can assume that either water has got into the boot or that the switch contacts are stuck closed. Peel back the rubber boot and dry the switch with some kitchen paper. If this doesn't do it, once again replacement is the only answer. To confirm this the wire has to be disconnected at the switch. If when that's done the light goes off it confirms the switch is faulty. If it doesn't, then the chances are that the wire that goes from the bulb to the switch is short-circuiting to ground somewhere along its path.

Note:

The wire terminal is held to the switch with an M4x8 mm pan head Pozidrive screw that's an absolute bastard to get at. It was obviously assembled before the carbs went on and it's hellishly difficult to undo. I had to use my ErgoGrip thin-nosed self-grip pliers to loosen it off and then turn it with a shortened Nş2 Pozidrive screwdriver manoeuvred between the throttle body linkages.

The Ergogrip pliers from Grip-On. Marvellous tool:



It helps greatly if you undo the lower of the two crossbars that hold the carbs/throttle bodies together, withdraw the long bolt and remove the spacer between them.

Removing the switch itself for replacement won't be easy either. Its hexagon is 22mm which means that any 22mm spanner will be too long to fit anywhere near it. If I ever have to do it I think I'll cut down a spanner to enable me to get in there. EDIT: See post number 3 from Mikeinva for a better solution...

Last edited by Forchetto; 12-24-2010 at 04:52 AM.
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Old 12-23-2010, 05:47 PM   #2 (permalink)
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See? We told you that you wouldn't run out of things to tell us about. Another informative read.

Quote:
It's a normal pressure switch. As the oil pressure rises above a certain level it presses on a diaphragm and this operates a couple of electrical contacts, making or breaking a circuit. This level is quite low, of the order of 2 to 6 psi. This is, of course, a lot lower than the normal oil pressure (around 40 psi at 4,000 rpm) so by the time the switch operates the engine is in great danger of permanent damage.

That 40 psi figure, by the way, is straight out of the Triumph manual and must be a minimum level. Mine operates at just over 60 psi with a hot engine.
Does this mean an oil pressure gauge would be more important than oil temp? If the oil pressure must be so high, why does the switch indicate so low? It sounds like there is a real chance for damage even before the light comes on.
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Old 12-23-2010, 05:57 PM   #3 (permalink)
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The switch is easy to remove take the tank off can get right to it.use a standard 22mm wrench.LOL some things the book dont tell you.
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Old 12-23-2010, 06:05 PM   #4 (permalink)
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Quote:
Originally Posted by mikeinva View Post
The switch is easy to remove take the tank off can get right to it.use a standard 22mm wrench.LOL some things the book dont tell you.
I thought there might be a logical way to do it. Never thought about removing the tank at the time. Thanks.
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Old 12-23-2010, 06:08 PM   #5 (permalink)
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The plastic part snapped off of mine awhile back. It was dangling from the wire and I had no idea what it even was! Ordered a new one from Bike Bandit. It was a very easy install.

Like Forchetto said, the rubber boot is worthless. I have zip tie holding what's left of mine on, but I know I could just take it off completely and be fine.
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Old 12-23-2010, 06:12 PM   #6 (permalink)
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Quote:
Originally Posted by Owenbrau View Post
Does this mean an oil pressure gauge would be more important than oil temp? If the oil pressure must be so high, why does the switch indicate so low? It sounds like there is a real chance for damage even before the light comes on.
In the past all decent cars came with an oil pressure gauge. The light is really a cheap way to give you a final warning.

A gauge, as long as it's within your line of sight, will tell a skilled rider all sorts of things. Wether the engine is hot enough to start high speed riding, as when the oil is hot and fluid enough it'll show a pressure drop from what it was when cold/warm. It can tell you if some oilway is getting blocked, the state of wear of your big end and main bearings, etc

The problem with fitting a switch that responds to higher pressures is that it would be flashing and flickering a lot of the time. When you're trundling through town on a hot day the pressure can drop alarmingly, for example.
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Old 12-23-2010, 06:42 PM   #7 (permalink)
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I take the boot off of mine. If you ever have to remove the switch you can take it off with the wire still on it(the screw that holds the wire on can be a pain) the wire plugs into the harness under the tank you can unplug it there .loosen the switch with the wrench a very little bit then un screw it with your fingers with the wire still on it.
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Old 12-24-2010, 02:15 AM   #8 (permalink)
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Not to rain on your parade,but has anyone blown the new bonnie motor when the oil light failed to come on ?Mine has been off many times
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Old 12-24-2010, 07:31 AM   #9 (permalink)
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Another problem with the arrangement is that there are 2 pumps, 1 for the big ends and main bearings, and the second for the cam bearings. Only the second one has a pressure switch. you could lose pressure to your bottom end, and the oil light stay out, until the engine goes bang in a big way. It should come on then. Why didn't they fit 2 switches?
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Old 12-24-2010, 07:57 AM   #10 (permalink)
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dayve there are 2 pumps but the one that the light is on lubes all of the motor the other pump lubes some of the trans but its main job is for oil cooling.The pumps are driven with plastic gears from one pump to the other if 1 pump locks up both go out most of the time (tears the gears up)
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