Darkshot

iTrader: (4)

I recently pulled apart my stock 91 tach and fixed it by installing a potentiometer in place of a dead resistor on the circuit board, allowing me to calibrate the tach to match my laptop screen readout. Works like a charm, my stock tach is now dead on. Or so I thought. I finally got around to giving the engine a bit of throttle and I discovered that as the engine RPMs climb past 3500 the tach stops climbing and just oscilates right in the 3000 - 3500 range. I can hear the engine speed continue to climb but the tach wont go past this range no matter what the engine speed. It otherwise reads spot on. Thoughts on this? What kind of amperage is the resistor supporting in this range, is it possible that I did not use beefy enough electronic components?

Siggy

Just a guess...but if you lost the resistor and replaced it with the pot, maybe something else went at the same time. The original resistor cooked for a reason? I did something similar long time back and adjusting the pot to be spot on at low rpm does not mean all is well at the top end, you really should check low and high rpm when adjusting. If its a dial type...they usually are not linear across the arc. Suggest you find or borrow a frequency generator to make the adjustment easier. Hope this helps

Darkshot

iTrader: (4)

Hmm. Well I temporarily set my idle speed in my PROM to 1500, then turned the pot to show exactly 1500 on the dash. Then I set it to 1000 and it was perfect, and then to 500 and it was perfect. The resistor that went out was supposed to be in the 200K ohm range, so I bought a 100K resistor and a 0-200K ohm pot and for starters I put the pot right in the middle. It was pretty close but not perfect, so thats when I did what I described above with the PROM to get it perfect.

Anybody else have this problem? The needle used to fly past 3500 all the time when the tach wasn't reading right. 3000 rpms and the tach was pegged.

???

iTrader: (5)

hmmmm, I'm about to do this fix to my 91 soon.


hope you get it figured out. I didn't see anyone reply with anything like this in the thread about the fix.

sully91rs

If the original was a 200K resistor, why not just replace it with a new 200 K resistor? Maybe a 1/2 watt one to help stop it from burning up.

Or if 200K was wrong, do you know what resistance the pot is set to when the tach matches the readings on your ltop? I'd be curious to see how big the difference is.

EDIT, I just read and looked up that its not exactly 200k. I would still still try to determine the resistance where your tach matches up with the ltop and get a resistor (or resistors) to replace the POT. Also, did you replace the cap? Caps only have a 5-15 year life before they start showing wacky properties.

Siggy

Hmm sounds like a look on a scope may prove interesting, appears that the D/A conversion is upset...could be a cap, any diodes...zeners??

???

iTrader: (5)

i think the easiest way would be to just turn the pot back up to about 450k (that seems to be what most had for high reading tachs) and see if it works like it did before.