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I am hoping I can get some help on some electrical gremlins on my dash instruments, ideally in layman's terms as I don't fully get how this all works.

Firstly I would say my car has a non standard dash harness as it was subject to a rodent attack - a previous owner re-made the wiring with a multi plug so one advantage is I can pull the unit complete out the car.  Having looked at it against the wiring diagram it has been done to a good standard and matches pretty well.  My questions are more generic rather than specific to my set up as a result.

It all worked when I got the car but then the rev counter stopped working.  An auto electrician diagnosed a bad earth but didn't fix it at that visit and has since moved from the area and I have lost contact - so I am trying to sort this on my own with little budget but time is something I do have.

When I dismantled the dash for the interior refub I made a record of all connections (which I still have) but made no record of one connection to the volts gauge - but given the diagram says it should go to earth, and my one remaining connection left once re-assembled was a black cable, you would assume this should connect up but somehow it was too short to reach, but when I did connect it up I had no rev counter. 

First question.  Where would the earth connection be made under usual circumstances?  I assume by the loom but where would it connect to?

Second question - Is it feasible whatever feeds the earth to the loom would have an issue and need cleaning?  Basically is there someone else in the car I should be looking at?

Third question - could I simply run a new earth connection to a suitable point on the dash, I already have one where the dash cross brace meets the uprights so this would be my ideal point.

Fourth question - Does the earth connected to the fuel tank sender fixing affect the gauge or the sender itself (or both)?

Fifth question - one other oddity.  When I was playing around with the car troubleshooting all this the fans at the front of the car came on but the temp gauge was only reading around 70 degrees, leading me to think it is reading low.  Certainly when the car last run significantly in 2014 the gauge was fine, the water pump failed and the needle rose no problem.  Is there any way to test this, and again is there somewhere at the back of the car I should be looking?  The temp sender must be OK if it triggered the fans?

 

I did get a local auto electrician out in June but to be honest wasn't too impressed (and I had a previous disagreement with him in which I turned out to be correct despite my little knowledge).  He did actually get the rev counter working by adding a lead to the earth terminal on the volt gauge and running that to the rear fixing of the bracket which secures the rev counter against the dash, with further connections to the fixing bracket screws on the speedo/fuel/temp gauges, but it was intermittent and the fuel and temp gauges then didn't read correctly.  He shrugged his shoulders at this point and said it should all be working, and blamed the gauges.  Clearly these two are more important than revs so I have removed all he did.

I have since then replaced the voltage stabilizer given it was the fuel and temp gauges, but this has made no difference to any of the above which was most disappointing.

Clearly something is amiss if adding this cable can get the rev counter working but at the expense of other gauges.  

I would mention the oil pressure gauge is an aftermarket electric one and is working perfectly.

I hope this is enough info for suggestions on the principles of what I should be looking for.  If anyone wants me to post an image of the rear of the dash mask I of course can do so.

 

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Thanks both. @Paul Coleman - well aware that getting a diagnosis over a forum would be impossible, hence I tried to pitch this as what I should look for and some theory rather than an absolute an

No Problem.  I will try to keep it short and sweet, otherwise I may get a bit carried away/in-depth and you will probably lose the will to live.  Remember, there is no such thing as a stupid ques

It is going to be almost impossible for somebody to diagnose your problems remotely. Bad earths can cause all sorts of weird problems and once you lose the normal low resistance return path for the current it will find the next best route(s) at which point strange things may well happen. You can always stick a long bit of wire on the battery negative terminal and temporarily run that to your cluster to see if that cures the problem(s). You can also check the resistance of an earth back to the battery with a cheap and cheerful multimeter (just make sure you have the ignition off when you do it). If you don't own one they're a very handy piece of equipment to have and not that expensive.

Lotus Esprit [meaning] a 1:1 scale Airfix kit with a propensity to catch fire

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Hi Neil,   Sorry to hear of your grief

I am not familiar with the esprit but vehicle electrics although seemingly complicated are in fact really simple, I would agree with paul a cheapo multimeter is invaluable (I think mine was about a tenner) although you do need some basic understanding to use it (If necessary I can bring mine along and show you how to use it at a meet).  I also find a bench power supply very useful for component bench testing.  The most useful thing however is the wiring diagram, depending how comprehensive /correct it is it should tell you the nature/functioning of the various parts in terms of earthing.

We have met, I have the white S1 and annoying amounts of enthusiasm for all things Lotus.  I would be happy to help were I can, under normal circumstances I would suggest coming over with the meter and having a poke about for you.  The most important thing is to work methodically and take a measured approach, the hardest thing is when dealing with intermittent faults.

There's a few questions there so apologies if I miss something.  In general, as it is GRP it will have several earthing points to the common chassis and back to the battery.  If any of these are corroded you can get a voltage drop across them. Another thing to be aware of is many gauges are earthed through the sender or can be the other way around I.e. fed a variable voltage and earthed through the gauge/meter.

At a guess, as you mentioned a voltage stabiliser (and what I know from the elans smith gauges) Your fuel sender is fed a stabilised voltage being earthed through a variable resister(sender) causing the reading you see. (it is calibrated to the varying resistance and the stabile voltage).  I would have thought the voltage gauge should be reading the vehicle voltage (i.e. straight from the battery, if it has a connection to the voltage stabiliser then this could be a reference voltage but I am not sure about that without looking into it) It is possible that by connecting in another earth you are partly bypassing the sender thus upsetting the readings. 

Your rev counter I guess is probably not digital (I believe digital has stabile +ve and earth and then a pulse usually taken from ignition system, nowadays its all different as its Can bus but we wont get into that)  It probably has a direct earth.

I don't know what your knowledge level is but things can be wired up in parallel or in series (i.e. daisy chained) normally gauges should be parallel so they all work independently but it may be the auto electrician has wired the rev counter in series with the other gauge (this is pretty unlikely) this could do strange things.

Another thing to consider is the state of the fuses and fuse box, this can often be a source of poor connections and voltage drops (if a connection is corroded it has high ressistance and if there is enough power drawn (i.e. headlights) then the connection will get warm as it is dissipating power if things get really bad you can sometimes see evidence of heat (i.e. melted plastic) (or in the case of my old Lancia Delta smoke coming out of the dash!!!).  Not relevant necessarily in your case but when changing/adding vehicle wiring consideration should allways be given to the electrical loads and resultant current to ensure the fuses are not overloaded.an  Important piece of knowledge is that the fuses are there to protect the wiring and must always blow before the wiring insulation melts causing shorts, usually leading to smoke/fire.

As for the fans, often the sender for the fans and gauge are not the same and are sometimes located apart, with the engine off the fans can start up due to heat soak at their sender whilst the engine temp may not sense that rise. that being said 70deg seems quite a low temp for the fans to run.  Testing the sensors is easy if you have the information, i.e. resistance vs temp. this may be in the manual.  Otherwise, if I was approaching it blind what I would do is bench test the gauge to find the calibration of it (the bench power supply displays current), this would give the current & voltage to get a certain temp reading for various points (using ohms law one would calculate the resistance) I would then test the sensor with hot water and thermometer to get the temp vs resistance of that and if the 2 don't correlate there is a fault.

That's the theory but without special equipment and in practice you could leave it wired to the car and put the sensor end into a mug of boilng water and see what you get, if you have a thermometer all the better.  The temperature readings do not have to necessarily be exact they just have to be as intended for correct functioning.

Hopefully some of that makes sense and its not a load of bollocks.  Vehicle electrics can seem daunting when faced with a mess of wires behind a dash and even those with knowledge and experience are sometimes left scratching their heads at times but as the HHGTTU states "DONT PANIC!!!!!!"

All the best with the investigations.

Tony

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Thanks both.

@Paul Coleman - well aware that getting a diagnosis over a forum would be impossible, hence I tried to pitch this as what I should look for and some theory rather than an absolute answer.

@ADF - Glad you responded having met you last month - really appreciate the reply, I will try and get my head around this and maybe chat further at the August meet if you are there?

 

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Hopefully I should be there all being well and I am always happy to talk lotus especially technical stuff.  Let me know if you'd like me to bring my little meter along and show you the basics (it probably will only take about 2 mins as there's only really a couple of settings you'd ever need)

In case there are any specific questions you could bring the wiring diag. along.  Hopefully pete will be able to make it, I believe he's a bit of an esprit guru.

On a completely separate note I've been driving the elan today for the 1st time since declaring it a death trap (to the old mans annoyance) the new bits/changes he's made have transformed it (it still perhaps needs a little bit of adjustment) But its really weird driving the 2 Lotus back to back they really couldn't be more different.  I imagine its similar for you, although yours are at least both mid engine.

Happy tinkering,   Tony

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Thanks Tony, I do have a multimeter so will bring mine along - no idea which setting it should be on so it may as well be a hammer for the use it is to me at the moment!  Going to have a play tomorrow to see what I can acheive,

Both cars are chalk and cheese despite both being mid engined.  My Esprit is not a fair comparison, I have driven Pete's and it trounces mine as things stand.

 

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No Problem.  I will try to keep it short and sweet, otherwise I may get a bit carried away/in-depth and you will probably lose the will to live. 

Remember, there is no such thing as a stupid question (I know cause I've tested it many times😉)

I haven't seen your esprit so can't comment on that but Pete's is stunning. 

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