Speedo jumps, throttle cuts out. Have replaced TPS.

[fsupaintball]

Boy, I sure seem to have a lot of issues recently.

Today on the way home from work my speedometer started jumping up randomly. When I started my hill climb home on the interstate, the throttle started completely cutting out and then coming back on repeatedly, very quickly, along with the speedo movement.

My first thought was "Throttle Positioning Sensor" - I had this problem last year and a new one fixed it. So I pulled over at an auto parts store and replaced it, but it didn't fix it.

Not sure what to do now. I'm thinking of trying ANOTHER TPS (I may have gotten a bad one). Here's a couple things that could be contributing:

1. My engine runs real hot, which I'm sure helps kill a TPS. I'm working on fixing that. But when I started having the issue with the speedo today, I was running at normal operating temp.

2. My wire harness is a bit stretched out since I swapped in a 97+ style manifold a couple weeks ago. Especially the wire going to this sensor (that pic is not my jeep). I think that's the MAT sensor? I don't think that sensor usually causes these kind of problems though.

3. I had to pull out my PCM the other day. I've seen a report that the stock screws that hold the PCM in may be a bit too long from the factory and can come in contact with the circuit board, causing shorts, causing similar problems. Maybe I'll try backing those off a little.

Any bright ideas?

 

[Bryan C.]

Try your speed sensor. I had a smiliar thing happen to me. The speed sensor voltage would spike and the PCM would see something like 125 mph and cut off the fuel supply. The speed sensor is normally mounted to the transfer case in the tailshaft housing. You didn't say what year so I can't tell if you have a speedo cable or just a sensor. Either way disconnect the 3 wire sensor and see of your problem goes away.

 

[fsupaintball]

Whoops. '96 XJ, auto transmission, 4.0, NP242.

Edit: this thing? What's the consequence of unplugging?

 

[Bryan C.]

Whoops. '96 XJ, auto transmission, 4.0, NP242.

Edit: this thing? What's the consequence of unplugging?

Yes, that thing. My rig is a 96 also.

Consequences of unplugging it? Your jeep will will crash into a bus full of nuns. :bs:

You may also need to replace the connector for the new style sensor if you buy a mopar part.

 

[fsupaintball]

LOL. So basically, the only function the sensor serves is to cut off the throttle if I'm going too fast? And I'm guessing make the speedometer work? I'll give it a try on my way home.

 

[fsupaintball]

Update: unplugging the speed sensor didn't fix it. Neither did backing out the PCM screws, or cleaning the IAC module.

Guess I'm back to square one... replacing the TPS again.

 

[fsupaintball]

Replaced throttle pos sensor again. Didn't fix it. I can sit in park and goose the gas and watch the speedo jump even though I'm not moving. Any ideas?

 

[mk153smaw]

Check fuel pressure. Mine started jumping on the rpms when the fuel pump began to die. High temps could be a super lean condition brought on by not enough fuel pressure and the computer is going bonkers trying to compensate.

Might not be the issue, but lots of same issues I had that led to other stuff when I did not start with a pressure check.

Dave

 

[fsupaintball]

I'll keep that in mind, thanks. It's the speedo jumping though, not the tach.

I also noticed today that it'll jump a lot if I go over a bump at speed.

 

[GordoSmasho]

This sounds to me like you have a short somewhere. Like something has rubbed through its sheithing and when it gets vibrated it causes all kinds of weird electrical anomalies.