How do I get the trouble codes from a 90 Cherokee?

Sorry if this is the wrong topic for this post. I just got my first Jeep and have big plans for it. It runs strong, but idles a bit rough and even stalls sometimes with the AC on. I replaced the ERG valve and plugs and wires. But still have the same problem. I have attempted to retrieve the trouble codes by putting a jumper between pin 6 and pin 7 of the diagnostic connector, then I turned the ignition key to the ON position. It is my understanding that this will cause the CHECK ENGINE light to flash code 12 followed by any trouble codes. However no lights on my dash flash. As a matter of fact I cant seem to find a CHECK ENGINE light on my instrument panel at all. Someone suggested to me that the CHECK ENGINE light may have been removed or disabled by someone trying to sell the vehicle. If anyone has any knowledge of my specific year and model or knows a way to get the codes with a test light or multi-meter, I would really appreciate the help. I'm not so much looking for ideas or guesses at what's causing the rough idle, as I am answers about getting trouble codes and what the deal is with the check engine light.
Here's the details:
90 Jeep Cherokee Laredo
4.0L
2WD (will fix that soon)
Has the D1 and D2 connectors under the hood on passenger side.

Thanx in advance

The '90 model year used electronics developed by Renault/Bendix, that is called the Renix system. This is the system that was initially introduced with the 4.0 in '87. It does not store and report codes the way the later models do, remember this system was developed over 15 years ago and on board electronics have come a long way. Even without the codes it's a pretty easy system to diagnose with just a volt/ohm meter and a manual, so let us know what specifically you are dealing with and we'll try to get you fixed up.

Welcome aboard.

There is no "Check Engine" light on a 1990 XJ. The light says "Maintenance Required" or something like that. It is not a trouble code, it is nothing more than a timer to remind you when the oxygen sensor is due for replacement.

The Renix computer has very limited ability to store trouble codes. For the most part, codes must be read on-the-fly, which makes it difficult to get a code on an intermittant problem. Budget scanners will not read the Renix system. Snap-On used to offer a scanner that would -- for $3500. Since the Renix system has not been used since 1990, I doubt that scanner is even available now -- even if you wanted to spend $3500 for it.

There are no stored trouble codes for the Renix guys like up, as Eagle mentioned. I do have to correct one of his points - the Snap-On tool that can be used (MT2500 Diagnostic Scanner) can be had in a kit that can be used out of the box for about $1400 (get the Student Kit.) It will only be useful, however, while running/driving, but it works quite well.

In basic trim, the MT2500 will also communicate with nearly everything OBD-I, and expansion and "personality" modules can be had to communicate with everything else electronic - including Airbag systems, ABS, various TCUs, and the like.

5-90

Thanx Guys, so it sounds like I'm not going to retrieve any trouble codes from this year Cherokee..
Maybe you guys might recognize the symptoms.
Vehicle starts up every time but usually turns over 4 or 5 times before it fires up.
OK I replaced the EGR valve and Plugs and Wires. The engine runs strong while driving but idles a bit rough. Especially in drive with the air on. It sometimes stalls out when I am slowing down for a traffic light, not very often though. The idle seems a bit low as well, in drive it can be as low as 300-400rpms. However after driving for 40min. in stop and go traffic, then stopping and putting it in park, the RPMs sometimes go up to 2000 and stay there. Which seems way to high. Very strange. I am hoping one of you more experienced Jeepers can point me in the right direction. Guessing and randomly replacing parts can get expensive.

talon39 said:

Thanx Guys, so it sounds like I'm not going to retrieve any trouble codes from this year Cherokee..
Maybe you guys might recognize the symptoms.
Vehicle starts up every time but usually turns over 4 or 5 times before it fires up.

Click to expand...

On the Renix ignition that's not unusual. The CPS needs to send a signal to the computer before it fires; on my 89 I found that I could make it a little more reliable by turning the key to 'on' and letting the fuel pump prime before starting it.

I'm not going to guess on the high idle although I have an idea. Haven't paid enough attention to that problem to make a certain diagnosis on it though.

Sounds like the IAC (idle air control) motor is weak or dirty. You may also have a lot of carbon deposits on the throttle blade. Spray some carb cleaner in the throttle to wash out all the gunk with the engine off, then run the engine at high idle while spaying the throttle blade off. This should get the idle up to normal speed. The long crank time is normal also.

Perfectly normal. The Renix system doesn't trigger ignition or fuel injection until the starter cranks the engine to 300RPM. If you notice your engine taking longer to start, think and see if it seems to be cranking slower - replacing the starter with one that turns a little faster will take care of that (I know, it worked on my 87 and my 88.)

5-90

Ok thats explains the semi hard starting. We did run some gumout through it but it didnt seem to make a difference. Also the spark plugs we replaced looked good. I know it's probably something simple, but it driving me nuts. lol. The Jeep is in perfect condition in every other respect.

You should take the IAC off throttle body and clean it up with carb cleaner, as well as inside the throttle body area. My idle smoothed out a lot by doing that.

OK I'll give that a try. Thanx allot for all your help everyone. I cant wait to get out in the desert this winter.

talon39 said:

. . . The engine runs strong while driving but idles a bit rough. Especially in drive with the air on. It sometimes stalls out when I am slowing down for a traffic light, not very often though. The idle seems a bit low as well, in drive it can be as low as 300-400rpms. However after driving for 40min. in stop and go traffic, then stopping and putting it in park, the RPMs sometimes go up to 2000 and stay there. Which seems way to high. Very strange. . . .

Click to expand...

I didn't see (could'a missed it though, it's early) that anyone had suggested inspecting your TPS settings -- While the IAC is the control mechanism for idle, the ECU looks at the TPS's output to determine that you actually wanna idle... That data, coupled with an "in gear" signal from the NSS causes the RenixII ECU to try to drive the IAC.

To me, IMHO, your symptoms describe a TPS that isn't happy, that, coupled with the fact that they often fail as a result of washing underhood (exterior throttle body cleaning with solvents on a warm motor, being a most rapid cause of death), which you mention, but not it its timing relationship to your symptom.

The TPS on an Automatic of your vintage has qty=2 connectors a flat on (with all of the contacts in line) and a square one (contaces form a square). The flat one is what your ECU uses for "throttle position" including your desire to "idle", while the square one is used by your TCU (Tranny Control Unit) to determine data needed for shifting - - The service manuals (and, to an extent, threads here) describe how to set the TPS, so I won't go into it, BUT I highly recommend examining the output characteristics of the ECU's portion of the TPS (the procedure for setting the Auto Tranny TPS involves only the tranny connector, so errors in the ECU"s portion of the sensor often go undiagnosed!).

Quick look at what you should see:
* leave the connector connected (remember we're talking about the ECU's flat 3-pin connector here)
* backprobe the connector without disconnecting it (did I reitterate?)
---- with the key in "run", best if the engine isn't running, and the throttle plate closed, you should see
- pin A - 5vdc - you can measure to any ground, though the connector's ground is on pin B
- pin B - Ground - measure it anyway - it has been known to go bad - you can splice another ground in if you'd like)
- pin C - something less than 0.9Vdc - The ECU will look at values at and below about 0.9Vdc as a "no throttle" signal and try to control the idle as above... Don't leave this pin yet watch the voltage output as you slowly open the throttle (see, I told you "best if the engine isn't running") you should see the output voltage slowly and smoothly increase from where you start to something much closer to 5Vdc (usually you'll get to the 4.7voltt area - that's fine)

-- If you don't see the output moving smoothly over the entire range, chage the TPS. If the output is at a volt or higher (using the pin B as ground) then set your TPS's TCU connector's output values as describe in the FSM (or many posts here) and re-verify the ECU's connector's output values... If it's still over a volt with the throttle plates closed - then the TPS should still be replaced.


Just thought I'd toss that in since nobody'd posted about the TPS possibility

Wow! Do you work in I.T.? Well you should. I wish I got an answer that good to every question I post. And what you describe makes sense too. I have noticed it shifting funny sometimes. A sudden jump in the RPMs for no reason while driving, then back to normal.