I have a malfunctioning Tack and Speedo that I would like to repair or rebuild. If anybody has a copy of the schematics I would appreciate them posting them to this thread. Or if at a minimum anybody that has the part name or nomenclature of the chips, that would be very helpful. These devises are not rocket science and can be easily repaired, but the chip parts numbers or description are critical. Thanks

 

Are there any numbers on the chips, a phot would help. I have not had my speedo or tach open so I do not know what is done to convert the pulses to voltage. Frequency to voltage converters are quite simple and can be just a rectifier, but there may be some frequency black magic to change the pulse rate.

BUT, before diving in I would check the following

- Make sure the the connectors and where the connectors mate is in good condition this is a major point of failure. The traces come away from the plastic backing.
- Check all the connections including the screws that go through to the PCB in the tach/speedo, these corrode and cause issues.
- With the cluster plugged in use a DVM to check resistance from ground on instrument cluster to the car body, it must be less than 0.5R.

 

If both the speedo and tach aren't working, it is unlikely to be the units themselves. Warrjon gave you the list of the more common issues for the instrument cluster, the other to consider is bad ground.

 

I think I have found the schenmatics for the little printed circuit board that is glued to the tachometer.
At least I think this is for my Pre HE. Not shure if your car has the same though.....

 

I have not looked at either tach or speedo in a few months, and ask the question of schematic and chip before I started again. I know I had good grounds by meter and substitution and the chips are unmarked on the top, maybe on removal the markings are underneath, but I doubt it!. The unit is very clean and I would suspect no corrosion. My gut feeling is that it is a chip problem. If you can't ID the chip it is difficult to trouble shoot. The schematic supplied by andegra is appreciated, but I don't think the speedo in the 1993 model has a variable pot. and therefore the schematic is questionable. bur the chip ID, 555, maybe of value, but chips usually have longer numbers and letters.
If I don't receive a schematic or chip designation from members then I will turn to the manufacturer. Again another question...who made them, Smith, Jager (pronounced JJ) or Jaguar?

THANKS AGAIN FOR YOUR INPUTS

 

You will need a scope to fault find the speedo and tacho. Something like will be good enough.

https://www.ebay.com.au/itm/Handheld...Zu4:rk:11:pf:0

But I'm with andegra it's unlikely both gauges have failed. You need to look at what is common to both this is the most likely fault.

The way they work is - Squarewave input which may be conditioned for the tacho but the wave for the speedo is a good squarewave. From there there may be signal division. There will be a rectifier to turn the squarewave into a DC voltage to drive the gauge.

To bench test the gauges you could easily make up a circuit like this.

555 Variable Frequency Square Wave Generator_Circuit Diagram World

 

On the XJ6's it was Smiths instruments up to mid 1982 or so, and Vegla (possible spelling error) after that. I don't know if that carried over to the XJS or not.

 

OK, I'm going to revisit ground and corrosion. And if they are not the fault, I will find an NE 555 and see if it is similar to what is in the tach and speedo circuits. If there is similarity I will remove the Tach and Speedo chips and see if they are electrically close to the NE555. The circuit is rather simple on both devises an can be easily copied. A VTVM is ok for the resisters, diodes, and caps, but the chips have to be checked by substitution and I have no idea of their internal circuitry nor do I have a scope. So that's the plan; I will keep you posted.
I will also look into the manufacturer "VEGELA" Thanks.

 

IIRC, you can have non-obvious oxidation on copper that will still produce significant resistance, which is a problem in low voltage applications like the instrument cluster. So even if looks fine, a good contact cleaner could make a difference.

Given the humidity you can get in Houston, you could get a thin layer of oxidation even in a fairly protective environment from the elements.

 

I would not do this. If you replace the chip and it still does not work you have proved nothing. On the other hand you may be lucky, but in 35 years of repairing electronics for a living I have never been this lucky.

The chip could easily be a 741 used as a schmitt trigger to condition the signal. A 741 is the same package as a 555 so without identification you would not know which was which. The Marelli tacho is different to the Lucas ignition car as it is a 6 cylinder tacho not a 12, so the driver circuit will be different.

If the electronics are faulty either buy a scope or send them out for repair.

 

You are correct about the variable pot resistor. There ain't none in my circuit either. But I think this circuit is close to what I have. The chip must be a 555 with a couple of capatitors and a resistor soldered on to. Be careful when you loosen the circuit from the tach. Its glued on and can brake easely.

 

Off the subject; I just marvel at this forum. Three people are discussing a single subject. One in Norway, one in Australia and the other in the United States! It cant get better than that!
Warren, I bow to your expertise, and lean towards Andegra"s support. After checking for corrosion and grounds, the substitution of a new NE555 might be appropriate, particularly if comparison is made out of the circuit first. Since this tach and spedo came out of a six, the manufacture of the instruments must be Marelli according to what you said in your last message., Would you agree on that? Getting a schematic from them would be a big plus, I don't think it is cost effective to send the instruments out for repair because I picked a used working speedo for $20.00 on eBay. I would like to fix the instruments for self satisfaction ( Ah Ha effect) and share the info with other XJS owners on this forum.

 

Rather than just replacing parts in the OEM PCB you could try building this, there would be a few $ in parts and you could make it on prototype board.

Tacho Circuit Board Replacement for Classic Tachometers

 

I know nothing about electronics but I think you're reading Warren's post wrong, I don't think he was implying the tach was made by Marelli. He was saying that you have both Marelli and Lucas ignition systems and the circutry driving the 2 is going to be different.
I'm sure Warren will correct me if I'm wrong.

Edit
If I'm wrong my apologies for butting in

 

I think Warren's replacement board is a brilliant idea. Since we are not shure about the chip. But it is common that capacitors dry out during age and heath.
First I would have tried to change the capacitors. That is easy and cheep and you might be lucky. :-)

 

Have opened the tach and speedo and on the backside of the tach and speedo board I did fine the inscription DELCO electronics The tach has two chips, one marked and the other blank. The circuits are more complicated then I remembered albeit the tach is relatively simpler since the speedo has four chips. Starting to reconsider rebuilding and leaning to looking at bad ground and corrosion only. The time and effort might not be worth an extensive rebuild with all its search complications.

 

You are correct...

A tacho from a Lucas ignition car will read high in a Marelli. The tach from a 6 cylinder car will read correct in a Marelli.

 

Fault finding 101 if you have 2 functions in anything that are not working start with what is common to both and work backwards.I would not tackle repairing a tacho/speedo without a scope as you can not see what is happening. I can tell you the speedo signal in a post 1988 (diff mounted speed sender) car is a good squarewave of 8000ppm, and the voltatage is about 5v peak-peak.

The top trace is the speed signal going to the speedo, don't worry about the bottom trace this was the issue I was having with cruise control.

 

is there a way to use the negative pulse train from the ignition coil on a newer 4.2 liter engine...in an older 1964 3.8S that the tach is wanting that AC signal from a cam mounted generator?
thanks

 

I don't believe so. The original S Type tach is basically an AC voltmeter, and when they went to the ignition driven tach in the 420 it's a completely different instrument.

From what I understand, the ignition driven tach is basically a frequency meter, not a voltmeter.