Jump to content

Tach Question


Recommended Posts

I recently replaced my dummy lights gauges with a new full gauge setup. :thumbsup: Everything seems to work correctly except for the tach. It seems that it is not reading a true RPM (it seems low to me).

Just wondering if there was any difference between the auto and manual tach? :dunno: (Mine is a manual BTW.)

Link to comment
Share on other sites

I had that problem too. The reason for it is that the tach that you pulled was hooked up to a different engine. to adjust it, pull the tach out of the cluster (three screws on the back) and look for a white knob on the back of the tack that has slots for a phillips head screw driver. It is on the right side. This is the control for the potentiometer that controls the tach. turn it to the left for a 4 cyl, to somewhere in the middle (thats where mine is) for the 4.0, it it is turned all the way clockwise it reads WAY to high. I wonder if that is for the 2.8, or maybe even a v8 (thinking that jeep used many of the same parts in different vehicles)?

Link to comment
Share on other sites

If its directly behind the tach then I think so. But don't go by me lol

 

I have a question, what about aftermarket tachs? Mine only reads 3000rpm at full throttle for a i4. And I know that is wrong. I bought a sunpro tach and gauge bar cause I didn't wanna find and buy a cluster for my 86. + I think it looks cool lol. Sorry for the high jacked thread but its kinda relative.

Link to comment
Share on other sites

If its directly behind the tach then I think so. But don't go by me lol

 

I have a question, what about aftermarket tachs? Mine only reads 3000rpm at full throttle for a i4. And I know that is wrong. I bought a sunpro tach and gauge bar cause I didn't wanna find and buy a cluster for my 86. + I think it looks cool lol. Sorry for the high jacked thread but its kinda relative.

 

Check this page outhttp://www.sunpro.com/faq_detail.php?pid=16334&id=34

Hope this helps. :cheers:

Link to comment
Share on other sites

I feel like a tard. I guess if I read the instructions like I normally never do I prolly wouldve saw that lol. Atleast its Marine proof :P

 

And best faq ive seen so far

 

"Can I install a Sunpro Tach on my motorcycle, boat, go-kart, food processor or other non-automotive engine?"

 

Thanks by the way

PFC Monk

Link to comment
Share on other sites

Thanks for the info. Is this the correct Knob? Just making sure, I hate reconnecting the speedo cable :fs1: .

 

Yeah, thats the potentiometer. My advice, until you get it fine tuned don't reconnect the speedo, its a real PIA, plug in the inner white plug only. Start the truck, see where its at, and adjust as necessary. I pulled mine probably 10 times before I got the adjustment just right. Don't feel bad though, I learned the same exact way after installing my full guage cluster. :cheers:

Link to comment
Share on other sites

Thanks for the info. Is this the correct Knob? Just making sure, I hate reconnecting the speedo cable :fs1: .

 

Yeah, thats the potentiometer. My advice, until you get it fine tuned don't reconnect the speedo, its a real PIA, plug in the inner white plug only. Start the truck, see where its at, and adjust as necessary. I pulled mine probably 10 times before I got the adjustment just right. Don't feel bad though, I learned the same exact way after installing my full guage cluster. :cheers:

 

so what should the renix 4.0 be ideling at?

Link to comment
Share on other sites

so what should the renix 4.0 be ideling at?

 

The center of the white spot was on the screw is where this one was set at from the factory (I pulled it from a 4.0). Maybe this will be a good starting point.

 

Link to comment
Share on other sites

I am starting to think that the adjustment of the pot varies by tach and which vehicle its in. Ive heard all sorts of people say it had to be all the way to the left or all the way to the right and some people like me had to put it somewhere right in the middle. It definitely takes some time and patience to get it set right. But trust me, your efforts will soon pay off. ;)

Link to comment
Share on other sites

It is ridiculous to say the tach calibration pot should be fully left, mid-range, and/or fully right for the particular engine whatever it may be. There are too many variables caused by corrosion, sensor deterioration, voltage drops, etc. etc.. The only way to get it right is to set the dash tach pot to the same RPMs as recorded to a known correct standard, like a handheld tach. This applies to all gauges, they have to be compared to a calibrated standard gauge to be spot on. Otherwise you're just pi$$ing in the wind. :cheers:

Link to comment
Share on other sites

It is ridiculous to say the tach calibration pot should be fully left, mid-range, and/or fully right for the particular engine whatever it may be. There are too many variables caused by corrosion, sensor deterioration, voltage drops, etc. etc.. The only way to get it right is to set the dash tach pot to the same RPMs as recorded to a known correct standard, like a handheld tach. This applies to all gauges, they have to be compared to a calibrated standard gauge to be spot on. Otherwise you're just pi$$ing in the wind. :cheers:

:agree: EXACTLY what I was trying to say.

 

EDIT: Also, for mine I just guessed that it should be idling right in the 750-800 range when I set the pot. There is pretty much zilch chance that it is spot on but its got to be pretty close.

 

Hornbrod, what kind of tach would you use and where would you hook it up to test it properly?

Link to comment
Share on other sites

 

Hornbrod, what kind of tach would you use and where would you hook it up to test it properly?

 

My timing light has a tach on it so there is no guess work when setting the timing ect.

 

Yes, that's what I use too. It's not a calibrated standard plus or minus .001% accuracy like I used to use in the Navy Cal. Lab I spent a few years in doing avionics stuff, but it's accurate enough for our junk with our mass produced gauges. :cheers:

Link to comment
Share on other sites

mine already idles around 750-800 but no matter what i do, the dang tach never break 2500 rmp's.

I would connect it to a hand-held idle tach and use that to run the RPMs up to 1,000 or 1,500 RPM, and check the cluster tach against the hand-held at the higher RPM.

 

If you have a RENIX, you can find outputs for the tach in one of those two yellow diagostic connector blocks. I don't recall which pins you use, but I can look it up. I think one is labeled "Tach Output" and the other is "Tach Ground" (or something like that)

 

Okay, it's the smaller diagnostic block that has the tach output. For both the 2.5L and the 4.0L it's terminal number 1, which is fed with a green wire with a tracer. The other wire from the idle tach would go to ground, I guess.

 

I'll see if I can simulate the diagram:

 

________

¡ (4) (1) ¡

¡ . . . . <

¡ (5) (2) ¡

¡ . . . . <

¡ (6) (3) ¡

¡______¡

 

Best I can do. Sorry.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...