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Speedometer and Tachometer Intermittent Fail

Posted by jemdandy (My Page) on
Mon, Dec 26, 11 at 13:15

1998 Jeep Cherokee, 6 cy, 4.0 liter
95,000 miles
4 X 4
Overhead cluster containing outside temp, compass direction, fuel mileage, etc.

The problem:
On occasion while driving, the air bag light turns on and the tachometer and speedometer quits; but all other gages continue to work. These gages are: oil pressure, coolant temperature, fuel, and voltmeter.

After 5 to 10 minures, the airbag light goes out and both speedo and tach return to normal operation. This happened today and before I could find a suitable place to pull over to investigate, everything retured to normal. I was going to try shuting the engine down, wait a few seconds and see if it would restart, and if the system would reset itself.

This problem is worrisome becasue it destroys my confidence that the vehicle is relaible enough for a longer trip away from home. This has happened twice in the past two months.

Where do I begin to look for this gremlin?

Follow-Up Postings:

RE: Speedometer and Tachometer Intermittent Fail

Hi Jem.

The condition you are describing occurs when the computers onboard lose communication. They all share the same yellow wire with a purple tracer circuit to communicate with each other. Essentially the instrument cluster knows to turn the airbag light off when it recieves data from the airbag module instructing it to do that. If it stops receiving data from that module, it turns that light on. The tachometer and speedometer data come from the fuel injection computer (PCM).

Diagnostics start with a scan tool that is capable of getting information from every computer on the car, they all "Should" have set communication troule codes but typically the module that failed during the "event" doesn't know that it went down so it might not have "U" codes while every other module does. The hard part is this does not mean the module is bad,it could have lost power (unlikely) or a ground connection (very likely). When a module loses power it simply drops off the data bus, but when they lose a ground connection they can back-feed power onto the bus and crash the communication.

If you shut the car off, you might not get it restarted because the data bus has to be functional for the PCM to get the theft deterrent go/no-go command from the SKIM module. These problems are tough to solve when they are random, not that they are all that easy as hard failures. The fault could lie in any of the modules on the bus, or at almost any point of the data bus wire. Typical diagnostic strategies rely on seperating the data bus when you can measure a disruption of the signal voltage waveforms. Isolate the wire that has the bad module on it, reconnect the rest and use the scan tool to see who is missing. At that point you can test for powers, grounds, or wiring harness issues for that affected module.

So, you need a fully functional scan tool which is the Chrysler DRBIII, an oscilloscope with at least to channels, wiring schematics, and component location graphics. BTW the splice or splice pack if it uses one will be below the steering column, possibly inside the harness that runs right behind the knee bolster. There just isn't much more help that I can give you, it takes training and a disciplined approach to solve these efficiently. It can be done DIY with what I have explained here, but the equipment issue makes it prohibitive.

Imagine dealing with one or two cars everyday with this kind of problem and you have a picture of what I have been doing for the last decade.

RE: Speedometer and Tachometer Intermittent Fail

Thanks johnf_g. Lot of good information in your reply.

Its as I feared; I'm going to need a heated garage and a better scan tool than the one I have. These two things I don't have. And I'll need to be able to get into and hold various Yogi positions since some of the potential trouble locations are under the dash. Chashing down wiring and connector problems is time consuming.

RE: Speedometer and Tachometer Intermittent Fail

Good to hear from you John. Hope everything is good with you and you family. Hope you have a Happy and Prosperous New Year.


RE: Speedometer and Tachometer Intermittent Fail

I've got a clue about where the problem is located. The instrument cluster is removeable as a unit. It plugs into a pair of connectors (all hidden behind dash covers). When the problem occured again, it was 'fixed' by gently hitting the dash cover to urge it to the forward direction. The next time it happened after hitting a bump in the road, and this time, it was put back into operation by pushing forward on the instrument cluster. It has been working ok for the past 2 weeks.

To get at these parts, one begins by removing the dash covers begining with the bottom pieces. The center bezel has to be pryed off as well. While I'm in there, I may as well re-lamp the dash.

Thank you all for your comments.

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