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View Full Version : Lazy tach in R/T



mark
04-25-2006, 09:57 AM
is there a fix for having a Lazy tach in my r/t

there are times that it shows 0 at idle, and there are times it shows around 1k at idle. then there are times where i rev it a little and it goes to maybe 3k, then there are times when i rev it the same, and it shoots up to around 5k. when engine sound its no where near 5k, it just seems to act like a pendulum(sp?)

this is because there is no distributor and spark is by coil pack right?
will an aftermarket tach fix this, or would an aftermarket tap into the same signal as the stock one and I'm back to square 1?

much appreciated.

-Mark

j4278h
04-30-2006, 04:18 AM
anyone got ideas my r/t is doing the same and i'm about ready to set it on fire

TopDollar69
04-30-2006, 08:43 AM
There is a tach driver board on the back of the speedo assembly that is bad. I have the same problem with my 90 Spirit ES. I have been told that cold solder joints on the board can cause the problem, but i reflowed every oint on the board and it didnt help. You can get a new one from chrysler or go to the local yard till you find one that works.

mark
05-21-2006, 02:54 PM
I went through and hit up all the joints with solder, as well as reseated the two boards that press onto the main board. my lazy tach is gone.

thanks!

-Mark

Tony Hanna
05-22-2007, 09:02 PM
Bump for a good thread!
I'm having the same problem with mine. I'll check that out tomorrow.
Any tips on getting the instrument cluster out? This is the first time I've ever needed to get into the dash on a TM any farther than replacing the radio. I'd like to avoid breaking any plastic due to hidden fasteners, etc...
Thanks in Advance,
Tony

mark
05-22-2007, 09:58 PM
the upper piece around the left vent, and around the traveler is all one piece, it just pops off, then there are screws that hold the instrument cluster in, and wiggle it out with the steering wheel down as low as you can get it (tilt steering)

TopDollar69
05-22-2007, 09:59 PM
Its pretty easy, just carefully look for all the screws.

Tony Hanna
05-22-2007, 10:03 PM
Thanks Mark!
No speedo cable obviously, so I'm assuming there's enough wire in the harness that you can get the cluster out far enough to unplug it?
I'm gonna give this a shot first thing tomorrow. Hopefully it'll do the trick.

Tony Hanna
05-22-2007, 10:04 PM
Its pretty easy, just carefully look for all the screws.

Thanks, I'm usually pretty good with the obvious stuff. It's those hidden fasteners on alot of interior trim that I'll mistake for a clip and end up breaking something.

TopDollar69
05-22-2007, 10:09 PM
It's been while, but I don't remember anything complicated or tricky. The wiring has plenty of length to see the connectors.

Good luck.

Tony Hanna
05-22-2007, 10:15 PM
Awsome! Thanks.

mark
05-22-2007, 10:18 PM
good luck, post if you need anything

martin kolner
05-22-2007, 11:38 PM
the tach circuit board is 4375410 and fits various models from 1988-95

Tony Hanna
05-22-2007, 11:46 PM
good luck, post if you need anything
Thanks!

the tach circuit board is 4375410 and fits various models from 1988-95

Nice! Hopefully resoldering the joints will get it sorted out but that's good information to have if it doesn't.:thumb:

JDAWG
05-23-2007, 08:06 AM
mine is starting to do it to, a new board is like $114. Take a look at your gears while you have it out also http://www.odometergears.com/subpages/chrydod.html

Tony Hanna
05-23-2007, 10:08 AM
mine is starting to do it to, a new board is like $114. Take a look at your gears while you have it out also http://www.odometergears.com/subpages/chrydod.html

Are the odometer gears a common failure? I guess I've been pretty lucky with this stuff in the past. Until now the only instrument problems I've ever had have been external (sending units, VSS, and a broken speedo cable in the Sundance).:)
It's nice to know you can still get replacements.:thumb:

Tony Hanna
05-23-2007, 04:15 PM
I've got the instrument cluster out. Looking at it from the back there are 3 boards. One small one on the back, and 2 larger ones on the top. Is one of these three the board in question, or do I need to dig deeper?
Thanks

Edit: Nevermind, I got it. Reflowed the joints and "adjusted" the pins in the connector for a little tighter fit, and the tach works great! Thanks for all the help!

mark
05-23-2007, 08:27 PM
glad it worked!

Tony Hanna
05-23-2007, 10:26 PM
Thanks Mark. I just took it for a drive a few minutes ago, and the tach is working reliably now. It's still not quite right though. It seems like the needle will hang for a second and then jump all at once. It happens in roughly the same spot on both accel and decel. It kind of leads me to believe that it's mechanical in nature (something is dragging or dirty in the movement). I think I'm gonna pull it back out tomorrow, disassemble the cluster and give it a shot of tuner spray. Hopefully it's just some dust in there.

mark
05-23-2007, 10:31 PM
mine does something similar too, ive learned to live with it :)

Tony Hanna
05-23-2007, 10:36 PM
Well, I'll be sure and post how it turns out then. Hopefully something simple like cleaning the mechanical bits will get it back to 100%

Scottmon
05-30-2007, 09:27 AM
I've been battling the same thing on my '91. The tach works, it just sticks at various RPMs. If I *VERY* lightly tap the dash pad the tach will unstick and drop/rise to the correct engine RPM and stick again.

Took the tach board out, resoldered the pins. Bupkus. :mad:

Took a tach board out of a junkyard Spirit. Installed it. Bupkus again. :banghead: Resoldered the pins on it, no change.

Pulled the gauge cluster and checked the circuit board on the back. Looks fine.

So...at this point I'm thinking I might pull the tach apart and clean the insides. After all, if Mopar used any kind of grease or oil on them it's likely dried out and might be causing the innards to stick. I also saw a post on TD where someone sprayed down their tach drive with PB Blaster and had the tach start to work again.

Guess if all this stuff fails, it's off to the J-Yard to find a new cluster.

Tony Hanna
05-30-2007, 04:33 PM
Initially, I think my problem was electrical, but now that I reflowed the joints on the tach driver and it's working consistantly, it's acting just as you described. It'll hang in one spot until the mismatch between the actual rpms and what the tach shows gets pretty big and then jump all at once and immediately stick again. I've been meaning to pull it apart and clean it but I got sidetracked with trying to get a decent transmission in the car.:)
I'll get to it one of these days...

Scottmon
05-30-2007, 06:55 PM
Fixed it!!!! The tach needle was seated too deeply and the needle hub was in contact with the faceplate. I just pulled the needle out a wee bit to where it wouldn't make contact and now it's working like a champ!

I guess with the hot weather making things expand it was rubbing.

Tony Hanna
05-30-2007, 07:15 PM
Nice!
Did you end up cleaning the works or relubricating anything or just pulling out the needle?

Scottmon
05-30-2007, 07:38 PM
Nice!
Did you end up cleaning the works or relubricating anything or just pulling out the needle?

Just pulled the needle out a bit. Inside of the tach drive was spotless, so I didn't see a point in cleaning it.

Tony Hanna
07-05-2007, 06:13 PM
I finally got around to trying this, and it solved my sticking problem but I managed to create another in the process. The needle is no longer dragging on the face, but when I pulled it out, it came clear off the shaft and the shaft moved in relation to the needle.:eek:
If this wasn't an R/T, I'd just temporarily hook up an aftermarket tach and compare the two to get the needle in the right place on the stocker, but since it's DIS, I'm guessing it's not going to be that simple.

What are my options here? Buy a timing light with a tach display and use it to recalibrate the stocker? Or is there some trick to get the tach to go to a certain RPM so I can pop the needle on in the right place?

Thanks,
Tony

Tony Hanna
07-05-2007, 06:33 PM
Ok, I think I may have got it. Close at least.
I pulled the MAP vacuum line to throw it into limp-in mode and used the 2000 RPM limp-in rev limiter as a guide for repositioning the needle. The tach just crosses 2000 RPM right as the computer cuts spark. Looks about right at idle from memory too. I'll compare it to the tach function on a timing light in the future if I get the opportunity.

Lotashelbys
07-05-2007, 11:38 PM
If you can get your hands on an OTC scan-tool you can set it to data-log and it will tell you digital RPM in real time:thumb:

Tony Hanna
07-06-2007, 02:09 AM
Thanks Jackson!
I'd really like to pick one up, but it'll have to be in the future. I'm a little short on funds now. I've been driving it with the tach taking spells where it would stick for awhile now (seemed to be temperature dependant), but I finally got everything done with the Holset swap and wanted to get it working properly so I could post accurate numbers regarding spoolup.
Now that this has happened, I'm not too sure how accurate those numbers will be. I think I've got it close though. Probably +/- 200 RPM.

mark
07-06-2007, 03:42 PM
I would really like to put an aftermarket tach with a shift light in my r/t.
I just dont feel like putting that much effort into it. lol

GLHSKEN
07-06-2007, 03:57 PM
If you can get your hands on an OTC scan-tool you can set it to data-log and it will tell you digital RPM in real time:thumb:


Showoff!!! NICE valvecover!!

Tony Hanna
07-06-2007, 05:21 PM
I would really like to put an aftermarket tach with a shift light in my r/t.
I just dont feel like putting that much effort into it. lol

Doesn't MSD or somebody make a box that will hook up to the DIS and output a normal tach signal? I was thinking I saw one somewhere.

I wouldn't mind having a nice shift light tach either, but I can't get past having 2 tachs in the same car. I'd have to build a custom instrument cluster...:)

mark
07-06-2007, 11:45 PM
yes, msd makes them, for the srt4, and the r/t. fwdp has them...



Doesn't MSD or somebody make a box that will hook up to the DIS and output a normal tach signal? I was thinking I saw one somewhere.

I wouldn't mind having a nice shift light tach either, but I can't get past having 2 tachs in the same car. I'd have to build a custom instrument cluster...:)

Aries_Turbo
07-07-2007, 12:06 AM
tony, make a custom overlay with the stock font that turns the tach location to a clock lol. ;) thats the way some VW dashes were.

Brian

Tony Hanna
07-07-2007, 12:28 AM
tony, make a custom overlay with the stock font that turns the tach location to a clock lol. ;) thats the way some VW dashes were.

Brian

That's a good idea! Though if I ever do go as far as reworking the instrument cluster, it will probably get an aluminum panel with a full compliment of aftermarket gauges along with an aftermarket speedo and tach in the stock locations. If I did that, I could still run a shift light by extending the wires and mounting it on the A pillar.:thumb:

Scottmon
07-11-2007, 02:58 PM
Ok, I think I may have got it. Close at least.
I pulled the MAP vacuum line to throw it into limp-in mode and used the 2000 RPM limp-in rev limiter as a guide for repositioning the needle. The tach just crosses 2000 RPM right as the computer cuts spark. Looks about right at idle from memory too. I'll compare it to the tach function on a timing light in the future if I get the opportunity.

I jacked around with mine repositioning the needle with the car running and the bezel off to get it to register the "right" RPM. Everytime I'd restart the car the tach would still show the idle to be 200 RPMs too low. And I checked it with my OTC Mindreader so I'd know the correct idle RPM. So, I guess there's something in the electronics of the tach board or signal not quite right. Oh well, reads close enough for me.