73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Interior & Equipment => Instrumentation => Topic started by: 76K20 on March 03, 2015, 02:10:55 pm
-
Got a 76 K2o with a factory tach in dash. Go power at the tach on the pink(hot) wire and got a good ground but can't get a reading on the brown wire coming from the distributor with a 12 volt test light. Do I need to use a meter? what setting if so? Also there is a resistor in the brown wire how can I test this. Thanks, Alan
-
The wire from the distributor will only have power comming from it if the engine is running I believe. Not sure what to what setting a meter needs to be on.
Are you trying to diagnose a problem? Tach not working?
-
Tach is not working. I tried it with the engine running. I'd like to make an effort to get the factory tach working before going to an aftermarket.
-
Ok. Look in the tech section, under your year, the service manual might have a section for diagnostics for the cluster.
I think you can run a wire from the distributor to the back of the tach, if it works with a temporary wire then the factory wire is probably broken somewhere inside.
If it still doesn't work, test the lead directly at the distributor to make sure the distributor has outgoing signal.
Some more experienced members will hopefully chime in.
-
Oh and the printed circuit board on the back of the cluster bucket should be clean and unbroken and the harness should be clean and making good contact.
-
I thought about running a temporary wire but I'm not sure what the resistor does. Thanks for the help LTZ
-
True. If another member chimes in, they might know about the resistor, then you could just put the right resistor on a wire and try that.
-
when you talking about resistor, is it a little metal canister with a wire leading in and out on the firewall like below? I've heard these called filters, condensers, capacitors... but what they're there for is to filter out RF noise that can cause the tach to jump around.
(http://thumbs3.ebaystatic.com/d/l225/m/mrAsOL3nbiRoAEKAaaIb_JA.jpg)
-
So there you go. Thanks to timthescarrd, you should be able to use a temporary wire without any resistors or that kinda stuff. Just for testing to see if the factory wire has gone bad.
-
hope this helps but the "tach" signal off the distributor is a "common" wire
(http://www.offroaders.com/tech/images/autogage4.jpg)
note how when you hook a tach up to a coil you use the neg post of the coil
(http://chevellestuff.net/tech/images/points_wire.gif)
-
when you talking about resistor, is it a little metal canister with a wire leading in and out on the firewall like below? I've heard these called filters, condensers, capacitors... but what they're there for is to filter out RF noise that can cause the tach to jump around.
(http://thumbs3.ebaystatic.com/d/l225/m/mrAsOL3nbiRoAEKAaaIb_JA.jpg)
That's exactly it. Thank you
-
hope this helps but the "tach" signal off the distributor is a "common" wire
I know they were a common wire on aftermarket tachs that's why I was leery about using a common on the factory tach not after seeing the filter on mine.
-
You should be ok for testing. If you want to get a new wiring harness for the the tach, I believe that some of the major resto parts companies sells that harness pictured above.
I would first determine if that's the problem or not tho.
-
You should be ok for testing. If you want to get a new wiring harness for the the tach, I believe that some of the major resto parts companies sells that harness pictured above.
I would first determine if that's the problem or not tho.
Yea soon as I can get my truck back in the garage I'm going to run a new wire. Got the garage filled up with guns to blue right now.
-
Cool. Good luck, let us know what you find when you run the new wire. Have fun with your guns too, I know I do.
-
The tachometer signal is a variable frequency and amplitude pulse. I looked at the signal on my oscilloscope and it rings in decreasing amplitude waves after the initial pulse. The frequency of the largest pulse is what the tach uses to determine the RPMs. You will not measure any meaningful output on the tach lead with a basic multimeter.
Fluke 115 and Klein MM200 multimeters measure frequency in hz but the ringing will cause some variability.
An " Automotive" multimeter will be a better test tool. An automotive multimeter will have tach and dwell functions as well as ohms & volts-DC and maybe diode test and or continuity buzzer.
http://www.google.com/search?q=tach+dwell+digital+multimeter (http://www.google.com/search?q=tach+dwell+digital+multimeter)
From orbit on my Android.
-
Well got her fixed today. The problem was the wire from the distributor to the tach, ran a new wire and now she works like new. Thanks for all the help guys. Alan
-
Good to hear.
-
Good stuff.
From orbit on my Android.
-
WhT did you do about the resistor?
-
nothing just ran a new wire.
WhT did you do about the resistor?
-
Cool thanks!