86 Carrera Tach Repair

[Determined]

I was bored and decided to open up my Carrera 3.2 Tachometer which was not working to look inside and repair. I used the two spoon approach for removing the needle, this worked out perfectly but I was nervous for sure. Here is what it looks opened up, I have a few questions I hope someone here can help me out with.





At first look, I could not find obvious signs, since I had it apart and without power there was little I can do but look for anything out of the ordinary. I checked all solder joints and found a resister (see below) that clearly had cold solder joints. In fact when I gently touched it with my screwdriver it lifted right out of the board on one side. It's a 100 Ohm 1% resister, looks like it saw a little heat in the center.





The cold solder joints shown with a light "orange" circle around them. I also noted a transistor that looked like it saw more heat than normal plus a capacitor I think I'd like to replace. My problem is identifying a replacement transistor and capacitor, they are both readable but I could not find a cross reference number to compare against it. Actually, there are two different sets of transistors and capacitors I'd like to change out since I'm in there. Can anyone help me identify a substitute part for each shown in the pictures. Thank you.

[ischmitz]

The capacitor is a 47 uF 16V type. Find a Tantal version as it will be more robust.

Once transistors is a BC337 - a standard NPN transistor with 50V, 800mA, and hFE>100
The other is a BC327-16 - a standard PNP transistor with 50V, 800mA, and hFE>100

Both should be readily available but if they are not bad I'd leave them alone.

Ingo

[Determined]

Hi ischmitz,

Fantastic, thank you very much for your help, I appreciate it. I hope it fixes my tach, any other suggestions while I'm in there? I had heard the capacitors get damaged from over-voltage. Do you think the resistor alone with cold solder joints was the cause? Thank you again.

Determined

[ischmitz]

The cap is most likely just to filter the incoming +12V. These do age but it would not render the tach completely dead. It's most likely good. The cold solder would render the tach dead. As a first step re-solder the resistor (don't worry about the discoloration, these suckers are tough) and then try and run the tach while it's still apart.

Pay extra attention not to confuse the RPM and the shift light inputs as a mistake here will kill the DME.

ingo

[Determined]

Thank you again, I did measure the resistance and it was still at 97 Ohms within the tolerance. I'll try and re-solder it and put my capacitor back.

I'm not certain what you meant by RPM and Shift Light inputs?

[ischmitz]

The tachometer has two wires coming from the DME. One carries the RPM signal and the other the fuel pulse length (used for deciding when to turn on the shift light). If they are reversed the DME will sustain permanent damage. Does you tachometer have individual spade connectors or an integrated plug will all wires. If it is the former there is a chance to mis-wire. If it's the latter than just plug it in and all is good.

Ingo

[Determined]

I understand now, mine has the integrated plug with all wires pre-arranged in the connector. Thank you again, I resoldered everything last night and will test it on Friday. I do have an ECU for my Carrera (84-86) in need of help if your still doing these. I'll PM you.

[Determined]

Thanks Ingo, tach gauge is working. I did notice that my shift up indicator light was on once I turned the ignition switch on and it stayed on when I started the car. I did not drive the car, only tested it in static mode. The gauge was very steady and responsive, it was not jumping around.

[mysocal911]

Most likely a battery charger was used that provided a voltage that exceeded the capacitor's
rating of 16 volts causing it to short resulting in the resistor heating-up and melting one of its solder
connections. That is usually what causes a 911 3.2 tachs to fails. It also may cause the large chip
to fail, i.e. the one that determines the shift point and turns the upshift light on. Hopefully, your
alternator has been intermittently overcharging - the 3.2 alt can charge to over 20 volts with
a bad regulator.

[Determined]

Hi Dave,

Thank you as well for the comments, I understand your points. The tach I repaired was a spare core, it's nice to have it working again.

I just opened up my other tach and found the 47uF capacitor with a rupture in the side. It sounds like what you described but in this case the resistor is still intact and the weld joints look good. I'm hoping it's only the capacitor and not the chip as well. Are the chips special programmed or are they available for purchase?

[ischmitz]

The PCB with the large chips also has a quartz on it. So one of the large chips is most likely an MCU and the other maybe a PROM. I've never been inside a 3.2 tachometer but this might be an 8051 derivative and at least one of the chips has custom firmware. So not off-the-shelf.

Ingo

[mysocal911]

Originally Posted by Determined

Hi Dave,

Thank you as well for the comments, I understand your points. The tach I repaired was a spare core, it's nice to have it working again.

I just opened up my other tach and found the 47uF capacitor with a rupture in the side. It sounds like what you described but in this case the resistor is still intact and the weld joints look good. I'm hoping it's only the capacitor and not the chip as well. Are the chips special programmed or are they available for purchase?

Both of the chips, i.e. the microcontroller & the eprom, are special for the tach and can't be replaced
with generic parts as each is programmed for the 3.2 tach function. All that is just for an upshift indicator
light.

[Determined]

Thank you all very much, I appreciate you gentlemen sharing your knowledge. It has been fun and clearly rewarding to do the repairs.

[Kraftwerk]

Congrats on doing that repair. Good read.

Always disconnect battery before linking up the trickle charger.

[serged]

If its possible to take the picture or your tach
Tank you

[serged]

On your tach