My '97 TJ 'self diagnostic test' let me down! - JeepForum.com
 
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post #1 of 6 Old 08-07-2015, 10:01 PM Thread Starter
Mr_Super-Hunky
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My '97 TJ 'self diagnostic test' let me down!

My 1997 TJ has a built in 'self diagnostic test' which will display error codes if anything is wrong on the odometer, but mine let me down!

My jeep recently wouldn't start at all so I did the three time ignition key turn self diagnostic test. The test came up with codes '12' and then '55'. Code '12' just means battery was disconnected recently, and '55' means the test is complete. But the problem is the jeep still didn't start.

Long story short, I had a bad fuel pump relay which caused fuel pump not to come on, thereby jeep wouldn't start.

So why didn't the jeeps 'self diagnostic test' come up with a bad FP relay? I ended up doing a LOT of unnecessary work [dropped fuel tank/removed fuel pump] to eventually find the problem which could all have been avoided if the TJ's 'self diagnostic test' actually worked!

Or am I missing something here?

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post #2 of 6 Old 08-07-2015, 10:37 PM
Knuckelhead
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The OBD II system doesn't have a diagnostic code for low or no fuel pressure for your TJ...
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post #3 of 6 Old 08-08-2015, 01:33 AM Thread Starter
Mr_Super-Hunky
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My fuel pump relay was not sending the electrical signal to the fuel pump. The circuit was not complete, however, no error code given after performing test.
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post #4 of 6 Old 08-08-2015, 06:40 AM
biffgnar
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Quote:
Originally Posted by Mr_Super-Hunky View Post
My fuel pump relay was not sending the electrical signal to the fuel pump. The circuit was not complete, however, no error code given after performing test.
What self diagnostic did you do?

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post #5 of 6 Old 08-08-2015, 06:44 AM
jay-h
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Built in diagnostics only detect a limited set of issues, mainly electronic components, and even those are often misdiagnosed.

No substitute for getting in there with your tools and DVM.

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post #6 of 6 Old 08-08-2015, 07:19 AM
Jonny Jeep
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Quote:
Originally Posted by Mr_Super-Hunky View Post
So why didn't the jeeps 'self diagnostic test' come up with a bad FP relay? I ended up doing a LOT of unnecessary work [dropped fuel tank/removed fuel pump] to eventually find the problem which could all have been avoided if the TJ's 'self diagnostic test' actually worked!

Or am I missing something here?
The diagnostic check did work. It isn't designed to check every single piece of wiring in the vehicle. It will tell you what it can. Even when you do get codes further diagnostics are usually required by the person doing the fault finding.

The reason you unnecessarily dropped the fuel tank was nothing to do with the self diagnostic test. It was down to not following the first piece of advice given to you by Rubi4MyMrs. You even said it yourself.
Quote:
Originally Posted by Mr_Super-Hunky View Post
Looking back, I never needed to drop the tank to begin with had I just checked the fuel pump relay first.
You are not the first to jump in the deep end without checking things out first and you certainly won't be the last.

Online factory service manuals and parts lists - http://www.jeep4x4center.com/knowledge-base/index.htm
If it doesn't work properly, look for the service manual link at the bottom of the page.
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