Fixed; Intermittent tach
 

At last.
Having spent most of the weekend trying to figure it out I can give anyone interested the benefit of my 20-20 hindsight.

The key difficulty was that I couldn't get the needle off the rev counter shaft, which would allow me to get at the circuit board. I knew that somewhere between the connector and the gauge, there was an intermittent connection that was so finely balanced between working or not working that at one time I was holding the cluster out from the dash, with the engine running, and just moving one finger out would turn it on, and back would turn it off. The finger wasn't even pressing on anything..!

Anyway, long story short, got the gauge out, soldered three wires to the guage about 18" long in the brown, green and blue positions. The other end I tinned and pushed into the connector in the dashboard. This allowed me to start the engine, and hold the meter in my hand while gently pushing on various components until bingo, it was obvious one of the capacitors had a dry joint. The tiniest movement would turn the gauge on or off. It only took me the best part of a day to figure the method out...

A resoldered joint later, the guage is good to go.
I'll post some pictures when I get a few minutes.

Cheers
Rissole.

Nice. Feels so good when you definitively find and correct a tricky fault.

Good job,

Jeff

Do you have a picture of the capacitor that had a bad connection. I am just wondering what that might look like, and if it's something that you could see visually or not. Sometimes my Tach doesn't work and if I tap on the gauge it starts working. I was guessing that it's a wiring problem but if could be a bad connection in the gauge.

What a suggestion. I wonder if I can make a connector that connects under the hood to the DME plug for testing since one is already out of the car?

BTW if you are repairing solder joints be very careful you don't apply too much heat and "sweat" the capacitor or one of the IC's or transistors. If you do you are ... going to need another tach!

First off you can identify possible suspect solder joints by looking at them with a magnifying glass for separation or cracking of the solder from the circuit board or the component. Once identified:

Just get a little solder on the tip of the iron, shake the solder off, and touch the suspected bad joint from the underside of the board, just barely long enough to melt the existing solder, and maybe add a little more solder. This is all it takes to repair a bad solder joint. Use a low wattage iron such as 12 or 15 watts. Do not risk overheating any electronic components with the soldering iron.

Heat and Solder the Joints

Hi Green Water,

I'm willing to bet you have the same problem, ie a dry joint, possibly even the same component. There was one other area in the gauge that can easily cause the same effect. I will get the pictures up when I get home from work, and make a few suggestions.

And DJ.. I'm sure the rev counter could be checked plugging in the DME, or if there is enough access, it may be possible to get the second connector off the gauge cluster without removing the cluster, but it's only a 10 minute job to get the gauges out anyway and have decent access.

Problem with using a magnifying glass on this beastie, is until the needle is removed, the gauge face covers the solderside of the PCB. I would love to know how to get the needle off safely: mine was definitetly not interested in just popping off or pulling off, and started to deform.

Clarks Garage has good info on getting the gauge cluster out.
I can write up how to get the tach gauge out of the cluster if that hasn't been covered anywhere else.

R

few pics
 

Cluster out and one modification: The contacts from the rev counter pcb are pushed onto brass pins that are threaded with nuts on the flexible pcb side of the gauge cluster. The contacts can open and corrode making poor contact. I filed the contacts and pins to get an unoxidised surface to pop a little solder on. Green did not make a good joint (too dirty or oily) so I cut away some of the plastic shroud to expose more of the pin before soldering it. These can't vibrate loose again.

http://forums.pelicanparts.com/uploa...1340051466.jpg

View on the back of the gauge cluster. Around the edge of the cluster, there are some pozi drive screws with hexagonal heads, about 10 or 12 or so. When these are taken out, all four gauges lift out of the gauge front clear curved plastic and surround, supported on a white plastic chassis.

The tach counter gauge is held in the chassis by three screws and three metric nuts. The nuts provide electrical connection between the studs attached to the back of the tachometer, (wire colours in the tach are brown, green and blue,) and the flexible blue circuit that the three large wire looms plug into. The second plug has three connections on pins 2,3 and 4

http://forums.pelicanparts.com/uploa...1340052376.jpg

Finding the duff joint: I just soldered three bits of 16/0.2 wire onto the green, brown and blue connections on the tach in the first picture , and tinned about 5mm on the other end. Then picked just the tach up and walk outside to the car. Tach Brown wire goes to second plug, pin 2, loom wire colour brown and black. The Tach green wire goes to pin 3, loom wire colour green and black. The Tach blue wire goes to pin 4, loom wire colour red and black. Start the car, and just prod gently with a finger at the components on the tach PCB until you find one that turns the tach operation on and off. Remake the joints on that component and the problem is solved.
Cheers
Rissole

http://forums.pelicanparts.com/uploa...1340053592.jpg

Awesome, the pictures are helpful. Thanks so much. I will have to check my tach very soon.