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Old 29-06-2008   #1
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Question HT leads, MAP sensor, Lamda sensor & dizzy cap

i have a 2000 UNO 1242cc petrol version central fuel injection with electronic carburettor ( car mfr. FIAT India ). Can someone explain me the terms HT leads, MAP sensor, Lamda sensor & dizzy cap and where they are located..
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Old 30-06-2008   #2
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Re: HT leads, MAP sensor, Lamda sensor & dizzy cap

Hi - please try not to double-post (I think you asked this question on your other thread) as it may cause several people to each reply saying the same thing.

To make this easier - could you please attach a photo or two of your engine? Just open the bonnet and snap off a few pics, then attach them to the message (click Go Advanced when you're writing a post, then scroll down and click Manage Attachments).

HT leads (High Tension leads) are the cables that go to the spark plugs. They are soft rubber and have a carbon trace in the middle that conducts the high voltage ("tension"). Over time, the connections and the insulation may break down.

MAP (Manifold Absolute Pressure) sensor is attached by a rubber pipe to the inlet manifold, which is the aluminium casting attached to the back of the cylinder head (it has the injector housing on top). The MAP sensor name is confusing as it actually measures vacuum (engine load) unless your engine is turbocharged.

Lambda sensor is a probe about the size of a spark plug, screwed into the exhaust pipe under the car. It detects the oxygen content of the exhaust gas and was invented by Volvo as the 'lambda sond' sensor. The petrol engine is supposed to run at a specific air-fuel ratio - the lambda sensor voltage switches from low-to-high (and vice versa) at the point of this ideal ratio. It tends to be fitted in conjunction with a catalytic converter, so if you don't have one of those, you probably won't have a lambda sensor.

Dizzy (Distributor) Cap is the hard plastic top of the distributor that has the spark plug HT leads attached to it. The distributor has a rotor that switches the high-voltage to each spark plug lead in turn as the rotor lines up with contacts in the distributor cap. The things to check are the spring-loaded carbon contact in the centre that touches the rotor arm - check it isn't broken off or burned away - and the cap itself must not be cracked, or the cracks will trap moisture and leak away the high voltage.

Most engines made in the last few years do not have a distributor (and therefore, do not have a distributor cap). For example, the FIAT Punto engine (which may be similar to yours) does not have a distributor. Instead, a double-ended ignition coil ("coil pack") is permanently connected to all four spark plugs. The plugs fire in pairs - the one that is needed, plus another plug that doesn't need to fire at that time, which is why the system is called "wasted spark ignition".

Hope this helps as a kind of brief summary of the terms you mentioned plus a couple more

-Alex
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Last edited by alexGS; 30-06-2008 at 01:38.
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