What Drives The Tachometer?

[24seven 104]

Changed the crank sensor, no difference. The car's slowly becoming undriveable now. it'so k from cold, but once things in the engine bay get up to temperature it misbehaves massively. Even if doing only a few miles, barely getting the water temperature above the first mark, if the car is parked and the engine dissipates heat through the engine bay, things turn ugly. If you take it on a journey from cold, it'll behave right until operating temperature but if you spend time stood in traffic it slowly deteriorates and becomes almost impossible to drive. It's getting so bad now that even at idle it cuts out at times, and I'm completely out of ideas!

[24seven 104]

Just put an earth lead from the head to the chassis and cleaned up the other leads that were there too. The engine feels like it pulls a little better now, particularly at low rpm and this cutting out problem so far seems to be a little less severe than usual (though it's hard to say as it's very inconsistent anyway), so now I'm thinking maybe there are a few earthing issues that need to be sorted out, so I'm gonna put a new loom on it.

[Cameron 16]

People, stop confusing the poor guy with rubbish suggestions!

 

As someone has already explained, the rev counter gets it's signal directly from the coil, so the the sudden drop in indicated revs HAS to be caused by either the coil not firing or the rev counter not getting a signal. (I say indicated revs because your road speed clearly shows the engine isn't actually slowing.) If your only symptom was an erratic rev counter then it would be the signal wire or r/c, but since you're feeling a misfire at the exact same time as the indicated revs drop it HAS to be that the coil is not firing.

 

So dizzy cap, rotor arm, HT leads, spark plugs, headlight washers etc are going to have absolutely nothing to do with it and frankly people are wasting your time by suggesting them. Likewise it isn't going to be your fuel pump, fuel pressure reg, injectors, filter or any other part of the fuel system!

 

Your problem is more than likely caused by either a poor signal to the coil - weather that is the CPS playing up, or a dodgy power feed or signal wire to the coil - or a poor earth from the coil.

 

My guess is that you have a poor connection somewhere. If you have a multimeter check that you have a 12V live to the coil, and check that that signal doesn't disappear when you wiggle the wires or hold them in a certain position. Do the same when checking the coil earth. If you really want to be safe then you can stick a known working coil and CPS on there, since they are easy to change, and see if that helps. But this sounds very much like a wiring issue to me!

Edited February 13, 2010 by Cameron

[jackherer 543]

Your problem is more than likely caused by either a poor signal to the coil - weather that is the CPS playing up, or a dodgy power feed or signal wire to the coil - or a poor earth from the coil.

 

or the ignition amp.

[GLPoomobile 958]

People, stop confusing the poor guy with rubbish suggestions!

 

As someone has already explained, the rev counter gets it's signal directly from the coil, so the the sudden drop in indicated revs HAS to be caused by either the coil not firing or the rev counter not getting a signal. (I say indicated revs because your road speed clearly shows the engine isn't actually slowing.) If your only symptom was an erratic rev counter then it would be the signal wire or r/c, but since you're feeling a misfire at the exact same time as the indicated revs drop it HAS to be that the coil is not firing.

 

So dizzy cap, rotor arm, HT leads, spark plugs, headlight washers etc are going to have absolutely nothing to do with it and frankly people are wasting your time by suggesting them. Likewise it isn't going to be your fuel pump, fuel pressure reg, injectors, filter or any other part of the fuel system!

 

Your problem is more than likely caused by either a poor signal to the coil - weather that is the CPS playing up, or a dodgy power feed or signal wire to the coil - or a poor earth from the coil.

 

My guess is that you have a poor connection somewhere. If you have a multimeter check that you have a 12V live to the coil, and check that that signal doesn't disappear when you wiggle the wires or hold them in a certain position. Do the same when checking the coil earth. If you really want to be safe then you can stick a known working coil and CPS on there, since they are easy to change, and see if that helps. But this sounds very much like a wiring issue to me!

 

Not that I'm any expert but I have to agree. I've held off replying until now, but I had a similar problem caused by my +VE supply. My +VE connection at the brown plug under the dash was loose (my own fault from when I rebuilt the loom), so I'd sometimes lose all power to the coil and amp etc. But before cutting completely, I'd always get an occasional hiccup as the connection broke for a millisecond, and of course during this 'misfire' the needle would drop like a stone (no +VE to the coil = no signal to the tacho).

 

So it could be a +VE supply to the coil that is causing a problem, but if it is, it could be a real pain in the ass to track it down. If it is, then hopefully it's a dodgy connection at one of the plugs, but if it was broken wiring then it's obviously going to be a major hassle finding it.

 

Hope I've not added to the confusion, and good luck sorting it.

[24seven 104]

Cheers guys for the responses. I'm pretty confident it's a problem somewhere in the loom now. Reading GLP's and Cameron's replies it suddenly dawned on me that when we first wired this loom into the car there was an issue with the coil, but when the car first ran it ran perfectly with no misfires or hiccups whatsoever, so I'd completely forgotten about it.

 

Another thing I noticed whilst driving long distance yesterday is that if I have the rad fan override switch on whilst going along, the problem clears somewhat - the fan's keeping the wiring at the front of the engine bay (i.e. the coil) cool which I guess is helping keep the resistance down. I've just bought a replacement unbutchered Mi16 loom to put on the engine (the last one was already a bit off a mess when I bought it, as per this thread from '07. Fingers crossed I'll be left with a much better loom this time round.

 

Thank you all for your replies, I'll update once the new loom is on the car.

[jackherer 543]

Another thing I noticed whilst driving long distance yesterday is that if I have the rad fan override switch on whilst going along, the problem clears somewhat - the fan's keeping the wiring at the front of the engine bay (i.e. the coil) cool which I guess is helping keep the resistance down.

 

Sorry to repeat myself but have you tried a known good ignition amp? You said earlier in the thread you tried your old one but that could be dead too and overheating is a common cause of failure for them.

[24seven 104]

Yup, tried 4 known good ones but none of them behave any differently to each other.

[24seven 104]

I've just driven the car having changed from a 2 row to a three row loom and WHOA what a difference! Talk about curing 5 problems all in one, fuel consumption is down, idles PERFECTLY at 900rpm when warm, no kangarooing whatsoever, no backfiring going downhill in gear and it definitely feels like it's pulling harder at low RPM than it used to! WHY didn't I put a 3 row loom on it months ago!?!? VERY VERY happy boy! All my electrical gremlins sorted in one go!

 

Thanks for the help guys

[Cameron 16]

Excellent! Glad you got it sorted eventually!

[P_Monty 2]

Hi folks - I'm having a similar problem and this has given me few things to check - thanks.

 

However,

 

1) Sometimes the rev counter jumps up as well as down - this would imply extra pulses wouldn't it?

 

2) A little while back I had to replace the ignition module (around 3 months ago) - if its the ignition module on its way out again, any ideas what could be killing it in such a short period of time?

 

Thanks

 

Paul

[jackherer 543]

1) Sometimes the rev counter jumps up as well as down - this would imply extra pulses wouldn't it?

 

Or just a lot of noise that the tacho sees as peaks.

 

2) A little while back I had to replace the ignition module (around 3 months ago) - if its the ignition module on its way out again, any ideas what could be killing it in such a short period of time?

 

Was the replacement a blue Bosch part with MTR01 written on it? If it is any other type then, yes, its normal for them to fail fairly quickly. Also they need to have thermal transfer paste applied to the rear where they bolt to the heatsink, if you fitted it without that then its unlikely to last long.

[P_Monty 2]

Thanks - I'll see if I can see where noise may be getting introduced.

Its a black 'Intermotor' one with a good splodge of thermal paste on the back - maybe I need to investigate getting a Bosch part.

[jackherer 543]

Its a black 'Intermotor' one

 

Problem solved! Just get a Bosch one, even a used MT01 will be better probably.

[P_Monty 2]

Oh dear - its that bad is it? What I don't get is the original one (another intermotor) lasted the best part of 20 years was that just luck?

[jackherer 543]

It wasn't the original if it was an Intermotor, they only came with Bosch ones from the factory.

[P_Monty 2]

Well, I didn't know that - I wonder when this one was put on...

 

This time, I think I've traced it to a loose LT connector on the coil - for some reason the connector is about twice the size of the blade and wobbles around like nobody's business :-(

 

but I think I'll get a Bosch unit just in case - they seem very rare though were they fitted to cars other than Peugeots? I might have to nosy round the local scrappy...

 

Thanks all.