Tachometer died

[Freybird]

Tach part number 911 641 301 05
On a spirited drive Sunday morning the tach suddenly went to 0rpms. Everything else was quite normal. When I shut the car down, the tach did blip up to 1000rpms and then back to 0rpms.
I've searched all over these forums and found instruction on removal and disassembly of the sealed tach case. Upon opening, the smell of fried electronics was evident. The resistor circled in the picture is burned open. (sorry for the bad cell pic) Appears to be 100ohms, but its too roasted to tell definitively. Does anyone know the actual value of this resistor or where to find a schematic? This tach has the 7pin connector on the back.
I'll be monitoring the charging voltage tomorrow as I suspect this is what caused the failure.
Thanks in advance for any information.

[Freybird]

Update:
I see 14.3vdc at the battery at idle and solid 14.0vdc at the cigarette lighter with engine revving. (lights on and off) I suspect this to be normal. (?)
I'm now suspicious of my external battery charger. Stupidly connected and disconnected the leads while in the "Start engine" position.

Perhaps someone has an opened Tach and could provide the color code of the resistor.

Thanks again.

[sc_rufctr]

I may be able to help but not for about 24 hours. I opened up a tach for a 78 and I may be able to find it in the garage.
I'm away from home right now.

[Freybird]

Thanks so much Peter! That would be great!

[wwest]

Originally Posted by Freybird

Update:
I see 14.3vdc at the battery at idle and solid 14.0vdc at the cigarette lighter with engine revving. (lights on and off) I suspect this to be normal. (?)
I'm now suspicious of my external battery charger. Stupidly connected and disconnected the leads while in the "Start engine" position.

Perhaps someone has an opened Tach and could provide the color code of the resistor.

Thanks again.

Look it over closely for a leaking, oily deposits nearby, or bulged out electrolytic capacitor. Very common initial failure point for electronics assembly subject to extraordinary heat.

[Freybird]

Thanks W! I've checked it over again and found all caps to be in good shape. This is a little surprising considering the intensity of the burned smell or "Essence of Allen-Bradley".

[Freybird]

Using a 100k, 50k, and then a 1k ohm potentiometer, I decreased the resistance until the tach came to life. With the 1K pot in the circuit, 350 ohms was the magic resistance. 330 ohms is the closest common value.

So I believe the answer is: 330 ohms

I will solder a 2W resistor onto the board, reassemble, and test.

[sc_rufctr]

I'm glad you worked it out. I couldn't find that tach from my 78.

[Hooverr]

Frey, I have the same issue with my 86 911 tachometer. I cracked it open and found the resistor fried. I bought a 330ohm (2W - which is huge, I may drop it back to 1W), but no luck. I can't find anything else wrong (at least no obvious component failures) but the tach won't come back to life. I've verified the signals are correct to it as well. Did you find anything else wrong or did yours work after the resistor change?

[Freybird]

'm wondering if each tach is tuned differently and hence this resistor is different for each tach.(?) I found only this resistor to be the problem in my case. You may wish to troubleshoot as I did. Get a potentiometer (1k ohm I believe) and connect it in place of the fried resistor using alligator clips. With the car running and everything connected, turn the potentiometer down slowly, decreasing resistance from 1k ohms until the tach comes to life. Shut it down, disassemble, and read the resistance of the potentiometer. (you could also keep the pot in there if small enough to fit in the can. Super glue the shaft to keep it from rotating.)