UNCLE TOM

iTrader: (1)

i have a 91R/S , like many others my tach. is messed up . i have a couple of clusters lined up , price is right (free) . my question is testing the tach. to insure it works before installing . is there any way to "bench test" the tach ? my car is a real "daily driver" so spending a lot of down time is not a thing i want . testing , "how to for dummies" is what i'm looking for . is there a way ? remember i said "for dummies" , basic wire problems i can handle . electronics not so good at . thanks in advance for any help .

afremont

I think you might have better luck and allot less expense if you just fix the existing module in the tach. It would be less work than changing the actual gauge and since it's probably not the problem anyway.... Is the problem that your tach is reading really high? If so, it's probably the little circuit board in the bottom right corner of the cluster. This can fixed, permanently with a couple of dollars in parts.

BigBadLou

iTrader: (3)

To bench test the tachometer, you would need three basic things:
- a 12 volt power supply
- connectors for the tachometer (not easy to find)
- function generator (not cheap to buy)

Once you have that, just set the generator to desired freqency (desired RPM divided by 15) and make sure to use duty cycle between 5% and 10%.

If your fgen cannot put out 12V, I suggest an amplifier (just a transistor with two resistors will do).

Hope this helps.
Lou

UNCLE TOM

iTrader: (1)

thank's LOU , remember i said for dummies (like me) . well if i can get the clusters i will try and get them ckd. out . (by someone who knows what they are doing) thanks ............

BigBadLou

iTrader: (3)

If you can't find anybody local, let me know, I got calibrated digital equipment to test the tach drivers. It's the little board in the corner of the tach that drives the needle, you don't need to mess with the whole tachometer gauge or cluster.

afremont

I wanted to say thanks for posting that valuable info on how the tach driver signal looks. Do you repair and sell the driver boards? I ask, because I was thinking that there would be a good market for working boards since most don't. Mine was working fine a few years ago when I "resurrected" it from sitting, it lasted maybe 1K miles before it was obviously off. Now it's insane and pegs the needle if I get on it at all. Doing almost 5K cruising down the freeway at 70mph. Thank God the ECM doesn't use the output of the resistor network as a reference or nobody would be driving their cars.

BigBadLou

iTrader: (3)

I have the equipment to fix and calibrate broken tachometer driver boards. (for 90-92 half-moon gauges)
I have offered my services in the past but did not get many requests to fix and calibrate boards.
If you are interested, PM me and you can ship me your board(s) and I can take a look and fix what I can.

Lou

afremont

Thanks for the offer, but I'll probably just try to fix mine myself. I have a function generator, scope and other junk that I've accumulated over the decades. Do you do anything with microcontrollers. I used to be a big PIC tinkerer, but have come to love the ARM7 stuff.

Where did you get the information about the duty cycle and RPM divisor? Was it documented somewhere, or did you do as I usually do and just reverse engineer it?

BigBadLou

iTrader: (3)

I usually reverse engineer things. That way, you KNOW what's in there and how it works and you do not have to rely on unreliable documentation (I have a long-standing beef with Craporola).
Used to tinker with HCS08 and a 51 but that was a long time ago.
PM me if you'd like to talk MPP.

Alright, end of thread litter! I hate doing that, sorry.

UNCLE TOM

iTrader: (1)

hey lou , will be back in touch with you as soon as i get the extra clusters . thanks for all the info. , found out how much more i didn't/don't know . thanks .

fleshead

Where do I start to find someone to repair my 89 IROC tach? I finally had it disconnected...

jherrera1341

Hey Lou,
I might need your help in repairing/recalibrate my tach board. I have a 91 RS, 5.0 TBI, 5 speed manual with 3.08 at the wheels. Are you interested in the work?

Yenipenny

Uncle T, you have a PM from Biloxi.

jherrera1341

Hey Lou,

Thanks for the repair of my tach circuit board. It works as good a new. I am now getting normal rpm at idle and 2000rpm at 70mph. Really appreciate your help. Juan.

dads old blue 9

FYI I have a repaired modified board tested ready to go $50 PM . Yes all the info to repair your own tach is in here . but If you dont want to do the research and the soldering of circut boards PM me!

1991z28........

would my problem still be my tach circuit board if my needle only moves to the first line under 500? it stays there until the engine is turned off and then goes back to the little needle holder thing.. and dont you only put resistors on it when its idling to high.. so for this situation what would be my best option..

Jersey89GTA

iTrader: (1)

My tach is broke, when car is off tach is pinned to the right (full bore).

When I start engine, swings to full left and then back to full right - stays there.

Anybody know a fix ? (305 1990 Iroc Vert)

jherrera1341

I strongly recommend BigBadLou. He fixed mine. Look him up in the thread and send him a message.

dads old blue 9

I have a modified board with 2- 100k ohm resistors tested and ready to go for $50 , I can ship you the board make sure it works then send me a check .any problems send it back . I'll talk you thru the install car must be a 1990-1992 let me know pm

Jersey89GTA

iTrader: (1)

Hey, thanks. I will send a PM. Appreciate it.