Rev counter problem

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Rev counter problem

Postby mwboyd » Fri 26.08.2016, 23:04

I have recently bought a M100 Elan SE but have found a few niggly problems. The first is a rather randomly temperamental rev counter. Sometimes it works, sometimes it doesn't, others it sort of flickers up and down between zero and the actual revs. Anybody had this and sorted before? Any helpful suggestions would be appreciated! Cheers!
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Re: Rev counter problem

Postby DeanG » Sat 03.09.2016, 19:30

I had that many years ago. It was the cable from the transmission. It would have been a bitch of a job to replace even if anything went right. Someone had zip tied the cable to something solid way down in the bowels of the car. I had to find and clip the zip tie by feel. It was a case of "Do you feel lucky? Well do ya punk?"
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Re: Rev counter problem

Postby Candles » Sat 03.09.2016, 21:02

The rev counter is driven by a discrete line from the ECU. You'll have to chase down the problem from the instrument all the way down the wire to the ECU. I suspect the culprit is likely in the instrument binnacle area though - my first guess would be to pull the plug out of that, clean it with electrical cleaner and lube, then refit with a good wiggle and see how it goes!
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Re: Rev counter problem

Postby matts1972 » Fri 09.09.2016, 14:43

Hi,
I had the same problem with my tacho. It was only working intermittently and would switch from completely dead to fully working on an hourly basis. I obviously suspected a loose connection, however rattling on the connectors did nothing. So I had to reverse engineer the whole system using an oscilloscope and found that my instrument cluster did get a signal, so the problem seemed to originate inside the instrument cluster/tacho itself. The tacho gets only gnd/+12volt/signal via three screws on the back of the cluster. Visual inspection did not show any problems. Eventually (weeks later) I found that the diode next to the +12v connector on the tacho board had gone high ohms. So in the end it was a 10p fix (or a free fix if you feel brave enough to use copper wire instead of the diode and renounce the need for reverse polarity protection).
Cheers,
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Re: Rev counter problem

Postby mwboyd » Sat 10.09.2016, 19:59

Problem solved - the loose connection at the Rev counter; clean, lubricate and replace option instantly solved it. Many thanks for that and to all who posted, much appreciated! :clap:
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Re: Rev counter problem

Postby Birdie » Tue 14.11.2017, 17:39

I too have a non functioning rev counter. The rev counter is receiving power. I can confirm that the circuit & connections from the brown wire at the Coil Pack all the way through to the White/Slate wire feed into the instrument pack are good. I have attempted to test with a multimeter the white /slate signal feed. I am not picking up any voltages when depressing the accelerator pedal. To rule out an issue I have replaced missing white nylon insulation washers fixing the coil pack to the body, this has had no effect. The engine is running fine. Other than a little move up slightly upon ignition. the needle does not move at all.

Could there be a fault with the Ignition Control Module that means it is running ok in all respects other than not sending a signal to the rev counter?
Many thanks,
Tim
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Re: Rev counter problem

Postby lotusflasherman » Tue 14.11.2017, 18:56

Edit - Text deleted as Total Bullshit ... :bonk:
instruments.JPG
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Last edited by lotusflasherman on Wed 15.11.2017, 12:50, edited 1 time in total.
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Re: Rev counter problem

Postby Birdie » Wed 15.11.2017, 10:14

I had a look at that diagram. It is a bit of an optical illusion, following the line closely from the speed sensor and it does not actually connect to the rev counter (tacho), I think it is there just to provide the signal for the speed needle. I have checked the line from the ignition coil pack (brown wire) all the way through to the white/slate (WS) wire into the instrument pack and yes it is a good circuit suggesting that this line provides the signal to the Rev Counter. I am going to borrow another multimeter in case my cheapo one was not up to the job.
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Re: Rev counter problem

Postby lotusflasherman » Wed 15.11.2017, 13:20

I fitted Lockwood Silver dials to a spare instrument cluster a few years ago and went through diagrams to understand what was there - seems I got it wrong ... :bonk:
Just got spare out of garage and re-educated myself. Speed Sensor has magnetic pickup on speedo so must SEND info to ECU about vehicle speed - from which I now deduce that if speedo cable breaks car may not run correctly ... :smt017
and it has nothing to do with tacho !

Tacho just has 3 electrical inputs : Ignition, Ground & Tacho signal from ECU. Three 8mm nuts secure connections to the printed circuit mask on the back.

Photos of my spare follow that may help...

tacho front.jpg

tacho rear.jpg

tacho rear conn.jpg


Front view shows an internal circuit board that may have failed. Lotus pulled the instruments from the GM Parts Bin and Cavalier SRI instruments and switches are much easier to find and cheaper too.

Edit : - Just another thought .. my Tacho needle fell off as it seems to be quite a slack fit. I presume your needle hasn't come of the spindle but still appears to be in the correct position?
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Re: Rev counter problem

Postby GeoffSmith » Wed 15.11.2017, 19:56

lotusflasherman wrote:Just got spare out of garage and re-educated myself. Speed Sensor has magnetic pickup on speedo so must SEND info to ECU about vehicle speed - from which I now deduce that if speedo cable breaks car may not run correctly ... :smt017
and it has nothing to do with tacho !

Correct. Amongst other things, I think the ECU adjusts tickover at low speeds to accommodate PAS loading.

lotusflasherman wrote:Tacho just has 3 electrical inputs : Ignition, Ground & Tacho signal from ECU. Three 8mm nuts secure connections to the printed circuit mask on the back.

Mine occasionally stops working: I'm usually driving by the time I notice it and it usually starts working before I get home, so I've never managed to fully diagnose the problem but I suspect pcb to tacho connections. Note that the signal from the coils has a big mark-space ratio, so the average voltage measured with a dc meter is very small.
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Re: Rev counter problem

Postby lotusflasherman » Thu 16.11.2017, 13:42

GeoffSmith wrote:Mine occasionally stops working: I'm usually driving by the time I notice it and it usually starts working before I get home, so I've never managed to fully diagnose the problem but I suspect pcb to tacho connections. Note that the signal from the coils has a big mark-space ratio, so the average voltage measured with a dc meter is very small.


My digital clock has a habit of not displaying when I want to know the time, even if I tap it or bang it. It's not usually in view so don't know how often it's not displaying but I have to press a button on the radio to get the clock display on that up but as soon as I know the time the digital clock is usually working again. Must be the IGN feed as it keeps correct time, just won't display when I want to know. Always brings to mind a fiery redhead girlfriend who used to go into angry sulks and answered my 'Have I done something wrong?' with "If you don't know I'm certainly not going to tell you!" :lol: :banghead:
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