1995/96 LS400 instrument cluster repair.

Howplum · July 10, 2021

Apologies Steve. The previous photos I included were of the 1997 circuit board and, as you say, the EC next to the orange connector does look suspicious.

However, my own circuit board looks OK, superficially at least.

steve2006 · July 10, 2021

Another interchangeable item are the speedo and tachometer, the PCBs mounted on the “motors” are marked speed and tacho but can be swopped by lifting the 4 flat lugs then gently pulling off the motor, they are literally plug in to the boards.

The needle motors themselves are weird in design, they are 4 pole sinusoidal drive on an X Y axis.......whatever happened to a rotating cable drive!

Malc · July 10, 2021

sounds like great words of wisdom ............... thankfully I would only ever leave that aspect to my car maintenance to your professionalism ...... I know it's beyond my indy to tackle for sure :unsure:

Malc

Howplum · July 10, 2021

After all my fiddling about I went for a drive and both the speedometer or tachometer needles worked fine, but is it a permanent fix? Only a daily drive will tell me - any excuse to use it! Also, the fuel gauge sprang into life straight away, but again, will it tomorrow?

The temperature gauge still has a mind of its own, in that it doesn't want to return to the correct rest position when the ignition is turned off, so is now over stating the temperature, although it seemed OK when I was putting it back together. I appreciate they are very delicate instruments, which is why I have been so careful, but I will try and lay my hands on yet another one.

To add insult to injury, the airbag warning light is now on permanently, so yet another little problem to fix.

Malc · July 10, 2021

7 minutes ago, Howplum said:

I appreciate they are very delicate instruments,

which often respond to a sound wallop on top of the dashboard to nudge it back to action :wink3:

Malc

Howplum · July 20, 2021

Well, the speedometer and tachometer still seem to be working as intended, so installing stops seems to have worked.

The fuel gauge refuses to budge from empty, regardless of ambient temperature, so my plan now is to replace all 12 electrolytic capacitors on the basis that they are all of the same age. This is assuming that one or more might be the culprit, but I have to start somewhere, because I can't even find anyone to test the circuit board. I have tried ringing round local electronics repairers, but no-one seems interested, so I am going to have a go myself. To that end I have ordered an adjustable soldering iron kit from amazon, which gets good reviews, so should be better than my very ancient one.

I have a few faulty electronic items which I will experiment on first though. I shall take the opportunity to practice reflowing soldered joints as well.

For the experts amongst you I have some technical questions. I can find exact matches for all but two of the capacitors, so need to to know if ones of a higher, or lower, specification can be used instead.

5v 1uF - The only capacitors I can find with a capacitance of 1uF are 16v or more. As far as I can tell 5v is no available anyway.

25v 850uF - I can only find 820uF or 1,000uF at this voltage. Which is better, or does it have to be an exact match?

Is physical size a potential issue?

Please reply in layman's terms! Thank you.

Malc · July 20, 2021

2 hours ago, Howplum said:

Please reply in layman's terms! Thank you.

many moons ago a guy on here, Chris Vince repaired the ' dead ' screen for the ( I cannot remember which ) on an earlier Ls400 I had at that time

he didn't have a phone number available I do remember that ..... maybe if he's traceable on here somehow that may assist

I remember driving to his place and he took it out of the car and repaired it on his kitchen worktop .......... worked a treat thereafter

Malc

steve2006 · July 20, 2021

5 hours ago, Howplum said:

Well, the speedometer and tachometer still seem to be working as intended, so installing stops seems to have worked.

The fuel gauge refuses to budge from empty, regardless of ambient temperature, so my plan now is to replace all 12 electrolytic capacitors on the basis that they are all of the same age. This is assuming that one or more might be the culprit, but I have to start somewhere, because I can't even find anyone to test the circuit board. I have tried ringing round local electronics repairers, but no-one seems interested, so I am going to have a go myself. To that end I have ordered an adjustable soldering iron kit from Amazon, which gets good reviews, so should be better than my very ancient one.

I have a few faulty electronic items which I will experiment on first though. I shall take the opportunity to practice reflowing soldered joints as well.

For the experts amongst you I have some technical questions. I can find exact matches for all but two of the capacitors, so need to to know if ones of a higher, or lower, specification can be used instead.

5v 1uF - The only capacitors I can find with a capacitance of 1uF are 16v or more. As far as I can tell 5v is no available anyway.

25v 850uF - I can only find 820uF or 1,000uF at this voltage. Which is better, or does it have to be an exact match?

Is physical size a potential issue?

Please reply in layman's terms! Thank you.

1uf 6.3V are now “ceramic” capacitors so I would use 1uf 16V for a replacement, 820uf is fine as electrolytic capacitors generally have a “tolerance” or range of +/- 20% so an 820uf could be anywhere between 656 and 984uf and still be within specification. Another consideration is the temperature range most are rated at 85C but try and obtain 105C versions. ESR is too technical to go into.

Howplum · July 20, 2021

Thanks Steve, that's very helpful.

BigBoomer · July 21, 2021

In terms of voltage, those values are minimums.

Chances are nothing on that board exceeds normal car voltages so anything above 16v for the one rated at 25V should be fine.
The 5V one will be on a part of the circuit that only deals with signal level voltages, so any voltage 5v or above will be fine.
Get as close to the uF value as you can, but as Steve pointed out the tolerances are pretty wide on Electrolytic capacitors.

Electrolytic Capacitors are polarised, so make certain that you get the -ve in the correct hole.
Physical size is only an issue w.r.t. if it will fit on the board.

Howplum · September 8, 2021

Another step forward. I recently met a friend of mine, who just happens to repair computers for a living, and he kindly agreed to replace all 12 electrolytic capacitors, which he did almost straight away.

I optimistically thought that that would be solution to my erratic fuel gauge, but no such luck! The goods news is that the gauge now shows the correct fuel level most of the time, which reduces "range anxiety" at least. However, it does occasionally show empty when the ignition is turned on, so there is now an intermittent problem, which I would like to resolve.

I have a Repair Manual on CD and managed to find a section on checking the fuel gauge and sender. As far as the fuel gauge is concerned the manual gives resistance values across the two pairs of connectors at the rear of the gauge. The approximate factory values are 151.8 Ohms across S+ and S- and 164.2 ohms across C+ and C-. The corresponding values on my gauge are 135.1 and 143.5, so approximately 12% lower than factory. The spare gauge I have from another cluster gives slightly lower readings than mine.

I did wonder if the issue was with the sender unit, so carried out the following tests from the manual:

1. Remove the male connector from the tank sender unit, which should make the gauge show empty, which it did.

2. Jump pins 2 (violet) and 3 (green/white) on the male connector, which should make the gauge show full, but didn't, although that might have been because of my dubious jumper connections.

3. Applied an earthed test light to pin 3, which glowed only faintly, but I assumed this was something to do with the resistance provided by the gauge, which is in the circuit between the ignition switch and the male connector for the sender unit.

I did find some further information on the internet about testing the sender unit. Apparently the resistance between the two outer pins (1 & 3) on the unit's female connector should be approximately 300 Ohms and the sum of the resistances between pins 1 & 2 and 3 & 2 should equal this. On my sender the readings were 298, 220 and 80 Ohms, so seem to work out according to the above. I haven't yet removed the sender to check the resistance throughout the whole range, mainly because the intermittent nature of the fuel gauge display happens without the car even being moved, or even started.

When I first acquired the car last year I accidentally crushed the grey multipin connector that goes into the back of the cluster, so had to replace that and at the same time include a further multipin connector otherwise the loom would be too short. I suppose it's possible that there is a bad connection to one or more of the three wires that go between the cluster and the sender unit.

As the gauge intermittently shows empty on start up this seems to me that there is an occasional break in the circuit somewhere, because it's the same result as pulling the connector off the sender unit. Is there an electronic component inside the fuel gauge that might cause this?

I'm in danger of getting confused here, so any guidance for a electrical novice would be appreciated, and thank you for your patience in reading this.

ambermarine · September 9, 2021

The symptoms point to an intermittent fault caused by circumstances outside of the operational sequence of the gauge .This is the bad connection scenario and you are on the right track, try splicing the wires that are affected in the plug ,this will take out the possibility of the bad connection.

Howplum · September 9, 2021

Thanks Phil. Hopefully I can identify the three(?) wires from the manual's wiring diagram at the multipin connector that lead to the gauge, although if I'm in the mood I might splice all 20 or so wires. It depends how tricky it is doing this in situ.

Howplum · October 10, 2021

The saga continues! I asked a friend who repairs computers to replace all 12 electrolytic capacitors, which he did, but it appeared to make no difference whatsoever. At the same time I removed the sender unit from the fuel tank and tested that, but it seemed to work correctly, i.e. the resistance readings were smooth across the full range of movement, with no gaps or peaks.

A few days ago I managed to source a replacement unit for the princely sum of £25. The seller said that one of the backlights didn't work but the gauges worked fine and there was a big scratch on the front lens (easily changed). On connecting it I found the backlights were fine but a few of the tell-tale lights didn't work, but a quick clean of the contacts sorted that out. It does seem the fuel gauge it still a bit temperamental, but at least when it chooses to work, which is most of the time, it's accurate. At least one of the electrolytic capacitors is looking a bit suspect, so I'll get my friend to change all 12, again.

The problem I have though is the mileage reading, which on the replacement cluster is 223,000! My car has only done about 80,000, so come the next MoT there will be a dramatic increase in the recorded mileage. I changed over the milometer display module from my existing cluster, but it made no difference, so I assume one of the components on the circuit board stores this information, but which one? If it can be identified then perhaps I can ask my friend to swap it over. Can anyone help please?

I've had a quick look on the internet but have drawn a blank.

BigBoomer · October 12, 2021

The mileage will be stored in one of the Integrated Circuits.
Probably the TC97101P bottom right on the circuit board since that is the only EEPROM I can see.
Here is a general purpose "DIY" on fixing the Odometer mileage.
https://www.clublexus.com/forums/es-1st-to-4th-gen-1990-2006/740730-diy-odometer-reprogramming.html

Howplum · October 12, 2021

Thank you very much for that George. I have passed the link onto my friend so I'll wait and see what he says.

ambermarine · October 13, 2021

On 10/12/2021 at 11:08 AM, BigBoomer said:

The mileage will be stored in one of the Integrated Circuits.
Probably the TC97101P bottom right on the circuit board since that is the only EEPROM I can see.
Here is a general purpose "DIY" on fixing the Odometer mileage.
https://www.clublexus.com/forums/es-1st-to-4th-gen-1990-2006/740730-diy-odometer-reprogramming.html

That's a pretty skilful DIY, If I was keeping the car for it's and my entirety I would not bother getting the mileage correct just enter the deviations in the service record and refer it to a cluster change.

Howplum · November 19, 2021

Success! My friend skillfully transferred the relevant component from the original PCB to the replacement, so the cluster is now showing the correct mileage and a full complement of working gauges, so happy days! It's only been going on since February! For those who might be interested, the component in question is IC105 at the bottom right hand corner of the PCB: