Speed Sensor failing?

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optmaxx
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Speed Sensor failing?

Post by optmaxx »

I've been chasing a gremlin that makes it seem like my Jeep misses as I get closer to 80 mph, and it feels like a jerk or hiccup and it's never done that before. I rarely try to hit 80, but I just feel like it shouldn't be doing that, and I have no error codes too and no other issues; it passed smog recently too.

So far I've changed the TPS, fuel pressure regulator, fuel pump, fuel filter, spark plugs, O2 sensor and CTS and at first it seemed like it fixed the issue, but then it would come back and sometimes it doesn't do it but it's getting more frequent.

I went under to check the speed sensor by wiggling the pigtail connector, and I did hear a change in the sound of the motor and it sounded like a surge, enough for me and my helper to notice it.

So about the speed sensor, it's from the Hesco mpi conversion kit, so it has a connection for 91-95 mpi 4.0 harness, and a connector for the old 1990 speedometer cable and I got a few questions:

-Would a failing mpi speed sensor cause hiccups or driveability issues? That sends a signal to the mpi pcm right?

-Is that only available at Hesco or can I order it from any parts place. I remember at an older forum they would say that it's actually from a Dodge or something and it can be ordered from a regular parts place. Any ideas where it's from? It has a connector for 91-95 Jeep YJ/XJ 4.0 harness and a connector for the older 1990 YJ speedo cable. Here's what it looks like:

https://hesco.us/products/30785/nos-par ... GKSEjxlDcc
optmaxx
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Re: Speed Sensor failing?

Post by optmaxx »

A few things I forgot to mention is that I changed the cap and rotor too. Also, I put it on second gear and tried to see if the issue was related to RPMS, so I would try hit the same RPMs on second gear when getting to 80 on the highway on third gear, and it wouldn't replicate the issue, it only seems to do it when I try to hit a certain speed, like 80 mph. I have a 3 speed auto btw.
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Re: Speed Sensor failing?

Post by Cheromaniac »

It sounds like your vehicle speed sensor (VSS) is failing so it would be a good idea to replace it. It's an easy procedure and the part is inexpensive.
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jsawduste
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Re: Speed Sensor failing?

Post by jsawduste »

No a failing VSS would not cause the issue you're having.

OBD1 goes back into open loop at anything above ~75% throttle. If you're trying to hit 80mph I`d wager throttle is above ~75%. In open loop at that throttle opening the ECM doesn't care of the vehicle speed. It is reading primarily the MAP, IAT and the TPS values. The programed algorithm in the ECM is controlling the injector duty cycle.
optmaxx
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Re: Speed Sensor failing?

Post by optmaxx »

So you're saying that the PCM is kind of hitting a limit? I've noticed that my 2000 Cherokee kind of does the same if I accidentally drive off into the freeway on third, it really hates it.

Or should I look at the MAP and IAT sensor?

I believe I'm using an OBD1 PCM from a Cherokee too.
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Re: Speed Sensor failing?

Post by jsawduste »

No, I`m saying the VSS signal is not part of the ECM open loop calibration above 75% throttle. Therefore the ECM reverts back to open loop and "tunes" the engine with whatever whatever factory calibration that is programmed in.

Once you get below 75% than sensors like the O2 come back into play.
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Re: Speed Sensor failing?

Post by optmaxx »

So my reply was deleted during a site update I'm guessing but here's the short reply.

I replaced the MAP, and VSS and I was going to replace the IAT but I dropped it causing it to break so I just cleaned the old one with break cleaner and put it back in. I took it to the freeway and brought it to 80 quite a few times with no issues, and I tested it a few more times the next few days with no hiccups at 80 so it looks like the issue is resolved. The parts store guy was cool, and let me exchange the IAT I dropped for a new one, and I put that one in today and did a few more runs to 80 to make sure the issue is gone, and it is.

The new VSS doesn't make the engine surge when I wiggle the harness anymore so everything seems to be good now. Thanks.
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Re: Speed Sensor failing?

Post by jsawduste »

The VSS function didn't change but the electrical side did.
The surge would have been either the sensor ground or sensor 5 volt power. Since both are common to most all the sensors you were changing the values the ECM was seeing. My bet is that the TPS voltage signal was affected.
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