Tacho problems

[Warwick (Ozcoupe)]

Some may recall my questions regarding tacho problems when I first joined this forum nearly 12 months ago.

When I collected the car the tacho (rev counter) read about twice the estimated RPM and the needle was pinned against the high end for most of the trip home from Brisbane.

The previous owner had not been able to fix it and he suspected it was caused by the change from Lucas alternator to a modern Mitsubishi unit with the associated control and regulation changes.

I also thought that it may have been the Lumenition changing the signal that the tacho receives. Thanks to forum members I learnt that the tacho uses a single induction loop in the ignition wire to the coil to produce the signal.

Well, last night I discovered something interesting and thought I should pass it on.

I checked the fuel flow from my Facet pump last night after disconnecting the coil. My young assistant turned the key while I watched the petrol flow. When I had finished he asked me why the tacho read 500 rpm when the pump was running.

It seems that the pump has been connected to the coil (+), which is logical, but the tacho is reading ignition pulses PLUS pump pulses. If this is the only cause of the high reading, it should be easy to fix by using a capacitor for the pump, or moving its connection closer to the battery.

[Phil Nottingham]

I run the pump from the fuse box IGNTION feed (unfused)

[Warwick (Ozcoupe)]

Thanks Phil,

I should be able to put her up on ramps on Saturday so I can then trace wires and change a few things around.

[johnwp5bcoupe]

Well done Hamish ;D I have mine from the fuse block Warwick but as it is fitted under the car I have fitted it to a fused circuit just in case if your pump like Phils is under the bonnet you could get away without fusing unless Phil means from the unfused side but still fused I feel safer fused

[Phil Nottingham]

I have actually run it through the unfused side via its own line fuse - its too much to ask other than a dedicated fuse. Strictly some sort of inertia/oil pump cut-off should be fitted for safety

[Warwick (Ozcoupe)]

Ham is slowly getting to understand the workings of the old machine, but he's been a bit disappointed lately. I keep telling him that we'll have her started soon, or out for a test drive, and then I have to say, well sorry, something else isn't working. He really wants to be driven to school in it.

The pump is under the rear floor near the reserve valve. I've no idea yet if it's fused or not. I expect to have it up on ramps on Saturday so will know more then. I just got back from collecting the new plug leads and cigarette lighter socket.

Good point about the safety cut-out Phil. I'll probably use a fused relay off the oil pressure switch if it hasn't got something.

[doojab]

Just as a throw away comment. When fuse protecting anything, always try to make sure the fuse is as close to the power (battery) end of the circuit. This ensures as much of the circuit is protected as possible. Any short circuit (wire chafing through etc) that occurs between the power source and the fuse will not cause it to blow but may cause considerable damage. Any short circuit after the fuse will blow the fuse and kill the power and hopefully keep your car intact!!!

Hope I'm not teaching anyone to suck eggs

[Phil Nottingham]

Its generally not a good idea to fuse either the pump or coil and I have not on the P5B but have on the P5 as this has twin pumps.

The reasoning behind this is taht a fuse may blow/fail at an inconvient time causing total power loss when in a danger situation

[johnwp5bcoupe]

As part of my training in regards to safety on installation of axillary equipment at the "Center of Excellence" a posh name for "this is how it should be done" If you fit any device that is not part of the original equipment it must be fused as near as possible to the supply point (this bit applies to more to the modern car but is relevant to some items on the P5) if the item being fitted has a -ve and a +ve wire both should be fused as near as possible to the supply and to be correct should go directly to the battery and again fused as near as possible to the terminals otherwise you may be at risk of claiming if your project what ever it is catches fire

If you know the maximum power consumption of the device you are fitting double it and this is the fuse you need to fit if it blows this you know you have a problem just carry spare fuses as all P5 owners do