Rev counter gone mad

[bluuzman]

Hi,

I noticed my tachometer needle started jumping up and down occationally last summer and finally stopped working completely some time ago. Well, from time to time it still jumps up and shows some life there ;).

Probably a bad connection but before tearing down the entire dashboard, could someone give any tips where to start looking. Any information about the wiring?

I have quite good manuals and access to a database but there were not a single line about the rev counter wiring and operation. Based on circuit diagrams the PCM seems not be controlling the rev counter.


-ari

'95 Sub K2500 6.5TD , Engine "F"

[winemaker]

My tach is fine, but I was parked in neutral in Mr Lube and was asked to rev it up to 2000 and hold it. The funny thing was that the speedo needle came up off 0 to 10 or 15 kmh. Are the two related, I wonder? Cheers! Brett

[Robyn]

They all will do it every so often. (spedo)
Both my 94's do it and so did my 95 I just sold last year and the 95 we had a few years ago did it.
Not to worry its not posessed or anything.

The tachs must be replaced with the entire cluster im told???????

[DmaxMaverick]

Alternator....Replace it.....Tach work good now....

[Turbo Al]

Alternator actually charges fine but after 3 months of watching the tack and speedo going haywire got a new one. You know I am cheap LOL.
Al

[bluuzman]

Thank you all for you replies. Before buying a new alternator, I would like to understand, how tach works or why it does not work. I believe, in this case it could be a bad connector or cable but I just don't have schematic drawings etc. available. It is very difficult to start debugging - I rather drive without tach than start disassembling the entire dash....

PCM indicates the correct RPMs all the time (based on a scan tool), but the analog rev counter in the instrument panel does not - it jumps up and down. As said I am just requesting a wiring diagram or any advice to quickly solve this minor problem....

Anyway, I really like driving my GM diesel truck.... It can never be compared to these Chevys sold here nowadays ;) You know tiny (Daewoo) vehicles from South Korea..

BR, Ari

[DmaxMaverick]

The ECM picks up the RPMs from the crank sensor. The in-dash tach gets its signal from the alternator. Just be sure the alternator you replace it with has the same size pulley, or reuse your current pulley. The wrong size pulley will throw off the RPMs accordingly. If you can live w/o a tach, the alternator may continue to charge well for years to come.