Tachometer Problem

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LewG

Not-quite-so-new-guy
Aug 23, 2017
20
8
3
Florida
I've got a 86' El Camino that had idiot lights in the dash and I got a dash cluster from another 86' with gauges including a tach. All of the gauges and clock works and the tach goes to 0 at key on but when I start the engine the tach goes to 6000 rpm and stays there. I don't have a tach filter but the service manual says that's for radio interference. The car was originally a v-6 with external coil but I'm using an HEI distributor (in a 500 Cadillac engine) that has a tach connection at the coil. I've checked power and ground at the tach and the wiring from the tach connector to the tach. All good. I saw a thread here that talked about a jumper wire that may be needed that converts from 6 cylinder to v-8 but mine doesn't have that. There doesn't seem to be any provision to switch from 6 to 8. I did check between pins on the circuit board and and what diodes and resisters I could. All seems OK, no opens. Any ideas?
 

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oldsmobile joe

Royal Smart Person
Nov 12, 2015
2,067
3,053
113
mpls
your correct, there is no v6/v8 jumper. i believe its in the circuit board on the back of the tach. i recently replaced an el camino tach/clock assembly with a new tach from mike's montes. here are some pics of the old v8 tach, maybe there is a different resistor or something you can see. 20191012_152002.jpg 20191012_152056.jpg 20191012_152121.jpg
 

LewG

Not-quite-so-new-guy
Aug 23, 2017
20
8
3
Florida
They look the same. Maybe the chip on the board determines 6 or v8. The service manual shows a 6 with a tach in the schematics so I assume they came with a tach.
 

oldsmobile joe

Royal Smart Person
Nov 12, 2015
2,067
3,053
113
mpls
double check the resistor color codes too.
 
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hoffa65

Master Mechanic
Mar 11, 2019
319
416
63
Atlanta GA
If you have a stand a lone tach or can barrow one, i would hook it up and verify a good output from the dizzy.

There is a thread on the MCSS site about the chip going bad and a mod to repair
I think there was a post on here with a link.
 

airboatgreg

Comic Book Super Hero
Oct 2, 2016
2,863
3,112
113
I am thinking a bad tach. Hook up a known good tach to wire at the dash tach. When I made that swap I had to run a new wire to the printed circuit
 

LewG

Not-quite-so-new-guy
Aug 23, 2017
20
8
3
Florida
Found my problem and fixed it. I found a link to MonteCarloSS.com and they talked of irratic tach, low reading tach or totally dead tach but not pegging 6k rpm. Seems there is a chip on the circuit board that one resistor circuit goes bad and can cause lots of problems. It's the black and white chip in the photos. The fix was to clip one pin to open that ciricuit and add a 200k ohm resistor. Did it and tach now works. Thanks for all your ideas.
 
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oldsmobile joe

Royal Smart Person
Nov 12, 2015
2,067
3,053
113
mpls
your welcome
 
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