Tach-nical question

[87chevy.com]

Has anyone tried to use any non aftermarket gauges in their LSx swap? I was at the bone yard today and was looking for a 81 model style camaro tach. My goal was to take that apart and stick it behind my 87 fuel gauge frame after I remove the fuel sender, paint it all black and put vinyl stickers for the hash marks to make it facotry like... and hidden in plain site .
Well there was only 1 gauge cluster at pick-n-pull and it was in rough shape so I passed... on in went to the truck section to see what that had... nothing worthy... and so I went looking for any tach... shame on me as I went to the buicks and so on... found tons of V6 6k tachs ... that would be great but dont know how to convert a v8 signal to a v6. Alright so now I invested about 7 bucks in gas to get there, 2 bucks to get in... I started to make my way to the ford car section. found a cherry 7k cluster out of a Murcury Cougar XR7 with a v8... Im like Ill peel it apart and see if it will work. It had 4 pins that go to the PC board. All pins were marked as B+, GND, SIG and CYL. Tach costed 9 bucks..
I cut the wires to the AutoGauge Tach and wired in the 3 normal ones and it didnt work. dang. Installed heat shrink around the bare wires and put em out of sight. I got home and chekced out all data and got a wiring diagram and it shows to have the CYL (select) to ground with the note of 4.6 which is a V8... gonna try that one out tomarrow as I didnt bring my test wires with me.

So anyone ever run a non factotry tach in your truck?
clint

[ls1nova71]

Ls engines put out a 4 cylinder tach signal. To make an older V8 tach work you need to change the tach resolution in the PCM. So you could probably change it to work with a 6cylinder tach.
Posted via Mobile Device

[ls1nova71]

I guess I should have told you that the tach resolution needs to be changed from 6 to 3., this will double the tach output. If you don't change this the tach will read half the actual engine speed. You have to have tuning software like HPTuners or EFI Live to do this though, but I'm sure you already knew that part.

[87chevy.com]

Originally Posted by ls1nova71

I guess I should have told you that the tach resolution needs to be changed from 6 to 3., this will double the tach output. If you don't change this the tach will read half the actual engine speed. You have to have tuning software like HPTuners or EFI Live to do this though, but I'm sure you already knew that part.

I didnt know you could change the tach output within HPtuners... I just found the TACH RESOLUTION in HPtuners. Its currently set at 6... edited... just downloaded a stock silverado file and shows to be set to 6 aswell. So in my case I play around with the tach resolustion. I found this confusing post in hptuners and will give it a shot tomarrow. From the post it seems I need to set the resolutions to 3 (high and low). Also from the same chart if I had a 6cyl tach it would need to be set to 3.

Thanks for the tip,
clint

[usmcchevy]

Even if the signal is for a 4 cly the tach should still show something. You might look into the pull-up circuit that TR65 posted here.

http://67-72chevytrucks.com/vboard/s...d.php?t=445975

[87chevy.com]

yeah, I tried his solution and nothing... the tach needle kinda jumps maybe 100 rpm on startup and thats it. I was hoping it would work so I dont have to take apart the autometer but maybe I will.
What Ive do ne was change the high and low tachoutputs to 3, tried 4 and stock 6. Ive tried each of thoese with the resistor going from the tach power to signal and nothing. maybe its just a bad tach...

I have equipment at work that can produce a frequency and apply certian voltage to maybe test the tach, anyone know how and what kind of setting would be good? I wonder if the cougar 4.6 crank pickup has different teeth and .. i dunno.. maybe its just a bad tach ?
clint

[usmcchevy]

Just a somewhat educated guess. 8 pulses to two revolutions so 4 pulses per revolution. If going for 1k rpm so 4000 pulses per min, 4k/60= 66hz per 1k rpm (2k=132hz) voltage I would just start low and work up to 12v p-p

[87chevy.com]

I finally got the equipment at work to communicate with the test equipment.. so I found that the tach does work. What I have found is at a duty cycle of 50 and a voltage of 12.0 the tach works fine. on this tach 55hz is 1000, 100hz is roughly 1900, 160hz is 3100, 210hz is 4000, 260hz is 5100, 310hz is 6000 and 360hz is 7000 max rpm on gauge. When selected 370, the tach actually moves up from 0 to 200rpm and then goes backwards.. I dont rev my motor past 6000 ish so thats not a problem.
So now I need to scope the tach signal wire comming from the pcm on differnt pulses to see match the pulses listed above.
I also found out that the tach signal works fine from 6.0v-12+v... 5.9v and below the tach doesnt read. So that is another think I will need to test with the 1000ohm resistor.
clint

[TR65]

87Chevy,

You might find some info in this thread if you haven't seen it.

http://67-72chevytrucks.com/vboard/s...d.php?t=445975

TR

[87chevy.com]

Im going to try that once the equipment at work gets back up and running.. the engeneers that send updates to the equipment always screws something up... this time they made the measuring equipment not communicate with the diagnosis machine.... so now i cannot use the osiloscope.. but I did have a concern with maybe backfeeding into the pcm? I bought a diode just incase... to prevent it from backfeeding into the pcm.. would you imigine it having any effects on the tach output if it was wired into it?
clint

[TR65]

I don't think you need the diode. If you arrange the diode to keep current flow from the ECM then you won't get the tach signal. If you use a 1000 ohm pullup resistor the ECM will only see a current of 12 milliamps which it can handle fine. It needs to ground the 12 volt line to produce the signal pulse.

TR

[87chevy.com]

geeze, I goofed up pretty good on this one.. since I wired my tach myself, I used extra engine harness wire from one point under the dash to the tach... well it only dawned on me today when I am scoping out the tach signal wire that the entire time I was connecting the wrong wire to the signal terminal on the Cougar tach.. I was connecting the LIGHT signal wire to the tach, good thing I never turned on the lights... so as soon as I realized my mistake, I grabed the Cougar tach and connected it... worked, incorrect RPM, but it worked.. HP tuners should fix it no problem w/o the resistor. When I scoped it, it read 12.2v which is plenty to operate the Cougar tach. Im excited now, so I hope to pull the cluster out this weekend and make this new setup work... the only hard part will be getting the new fuel gauge in position.
clint

edit:
Tuned the tach signal to 4, which is 6 cylinder mode due to the way I have the tach wired. Over all its a tach and its pretty accurate. It has a dead zone like from 1100-1300... it just like doesnt read right... wiered. anyway it reads almost spot on at 1500, 2000 and so onn so I recon its a keeper. Tomarrow I will be changing the exhaust up on my truck and will be removing the cluster and seeing how well its going to fit.
clint