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ECU & Tachometer Mismatch

Z

zemsten

NAXJA Forum User
Location
USA
Alright y'all,


I've been scratching my head over this one for a while now. I am the proud owner of a (mostly) stock 1996 XJ. I bought it with 164k miles just a couple months ago and so far have just done regular periodic maintenance and I just finished installing a 4" rocklink long arm lift. Here's the thing that's got me stumped though: The entire time I've had it, it's idled around 1100-1200 rpm. I thought "vacuum leak, no problem..." Couldn't find one anywhere. TPS sweeps from 13.7% - ~80% from no throttle to full throttle. Calculated TPS based on manifold pressure tracks with this. A couple of times I've noticed at a stop sign that the idle is down where it should be, somewhere below 1000, but that's few and far between. Just today I had the thought of pulling up a "tach" on my Torque app, and low and behold it reads ~750 - 780 rpm pretty consistently. I look at the dash and boom, 1200 rpm again.... What gives? All the dash gauges function, I haven't noticed anything weird. The ECU and the tach both take input from the same thing right? I would imagine CPS or cam position based on the distributor. Anybody dealt with this before?
 
zemsten said:
Alright y'all,


I've been scratching my head over this one for a while now. I am the proud owner of a (mostly) stock 1996 XJ. I bought it with 164k miles just a couple months ago and so far have just done regular periodic maintenance and I just finished installing a 4" rocklink long arm lift. Here's the thing that's got me stumped though: The entire time I've had it, it's idled around 1100-1200 rpm. I thought "vacuum leak, no problem..." Couldn't find one anywhere. TPS sweeps from 13.7% - ~80% from no throttle to full throttle. Calculated TPS based on manifold pressure tracks with this. A couple of times I've noticed at a stop sign that the idle is down where it should be, somewhere below 1000, but that's few and far between. Just today I had the thought of pulling up a "tach" on my Torque app, and low and behold it reads ~750 - 780 rpm pretty consistently. I look at the dash and boom, 1200 rpm again.... What gives? All the dash gauges function, I haven't noticed anything weird. The ECU and the tach both take input from the same thing right? I would imagine CPS or cam position based on the distributor. Anybody dealt with this before?
Click to expand...


Here's a few data points I just took, with an interesting result....


Tach - Scanner
1250 770
2000 1700
2500 2630
3000 3600


So at some point, just shy of 2500 on the tach, it actually crosses over! How is that possible, what am I missing here....?
 
I know my dash oil pressure gauge rarely matches the manual Bosch gauge. Ive also heard that temp gauges rarely match manual ones. Maybe the jeep ones are just not that accurate. if you have and extra tach laying around trying wiring it in, I dont trust computers over old fashioned mechanical stuff.
 
Unfortunately I don't have another gauge to test. I was hoping to remedy the issue with this one. What a strange problem though...

Sent from my SM-A505U1 using Tapatalk
 
Unclewolverine said:
I know my dash oil pressure gauge rarely matches the manual Bosch gauge. Ive also heard that temp gauges rarely match manual ones. Maybe the jeep ones are just not that accurate. if you have and extra tach laying around trying wiring it in, I dont trust computers over old fashioned mechanical stuff.
Click to expand...

Technically, it's the old fashioned electromechanical gauge that's wrong.
 
Alright, so I've got another interesting data point. I got in and fired up the Jeep this morning to go to work. She runs great, as usual! The tach was correct in every regard though. I drove 10 minutes to the gas station and low and behold somewhere between a stop sign a couple miles from my house when I glanced at the dash, and that gas station, it borked itself again. This to me screams some sort of intermittent electrical condition. Two things to note, this morning was cool (52 F), and there was dew on everything. My thoughts are an electrical connection that's being affected by heat or moisture. It's strange that it's the only gauge affected on the dash though. I would've immediately assumed a ground, otherwise.
 
The XJ's from this era are all pretty much the same with respect to how they connect. What I have found as a weak spot is the way that Jeep/Chrysler attaches the gauge to the copper strip they lay across the rear. I posted a thread a couple of weeks ago describing a problem with my '90 XJ where the temp gauge would peg randomly. The issue turned out to be that the copper strip under the ground screw on the gauge was badly tarnished. I simply removed the screws (one at a time), cleaned them with 1000 grit sand paper until I saw pure copper again and the reinstalled the screws. When I was done the gauge worked perfectly again. If you choose this approach, I would strongly recommend picking up a small packet of the grease that any autostore will sell for use on lamps and lay some of it on each contact after you clean it. This should stop it from tarnishing again.


Below is a link to a picture of what I am talking about. Each of those philips screws attaches to a gauge. Some are ground, some are power and some are signal but they are all important. I suspect you have a similar problem and that it is only making intermittent contact. Something as simple as hitting a bump in the road can cause it to make or break contact. It likely won't ever lose all contact so it will work but it can cause strange readings.


HTH
Todd

https://cdn10.bigcommerce.com/s-n13...apicmwec7__63298.1545343722.1280.1280.jpg?c=2
 
Last edited:
tjmotter said:
The XJ's from this era are all pretty much the same with respect to how they connect. What I have found as a weak spot is the way that Jeep/Chrysler attaches the gauge to the copper strip they lay across the rear. I posted a thread a couple of weeks ago describing a problem with my '90 XJ where the temp gauge would peg randomly. The issue turned out to be that the copper strip under the ground screw on the gauge was badly tarnished. I simply removed the screws (one at a time), cleaned them with 1000 grit sand paper until I saw pure copper again and the reinstalled the screws. When I was done the gauge worked perfectly again. If you choose this approach, I would strongly recommend picking up a small packet of the grease that any autostore will sell for use on lamps and lay some of it on each contact after you clean it. This should stop it from tarnishing again.


Below is a link to a picture of what I am talking about. Each of those philips screws attaches to a gauge. Some are ground, some are power and some are signal but they are all important. I suspect you have a similar problem and that it is only making intermittent contact. Something as simple as hitting a bump in the road can cause it to make or break contact. It likely won't ever lose all contact so it will work but it can cause strange readings.


HTH
Todd

https://cdn10.bigcommerce.com/s-n13...apicmwec7__63298.1545343722.1280.1280.jpg?c=2
Click to expand...




Awesome! Thanks for the tidbit of knowledge. With its intermittent nature, I suspect it's something similar. I was hoping to avoid tearing the dash apart though, probably naively. Here's hoping I don't break anything!
 
Well, I just finished putting the dash back together. I took out and lightly sanded/cleaned every single screw and copper trace on the back of the gauge cluster. That did.... absolutely nothing. Unfortunately. I guess maybe I need to find a new tach at a junkyard or something...
 
Where you at? I had to piece together my cluster to put together one for my RHD, I might still have a loose tach around here; everything else is in metric.
 
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