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Old 12-10-2011, 09:28 AM   #1
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1989 Cherokee
Default Bad ECU?

I've got a mystery problem with my Renix Jeep that I can't diagnose or solve. It's been going on for months. Runs pig-rich, gets 8 mpg, runs open loop all the time.

I finally took it to a diagnostic specialist -- a guy that every other mechanic in the area swears by. One mechanic who's been around for 35 years says he sends problems he can't diagnose to this guy, and this guy hasn't been wrong in 20 years.

He tested everything and came to the conclusion that it's actually the ECU. This seems like one-in-a-million to me, but ... then again ... I've either tested, cleaned, or replaced:

o2 sensor
throttle position sensor
coolant temp sensor
MAP sensor
MAT sensor
plugs, wires, distributor, rotor
ground cable
battery and battery cables
vacuum harnesses
fuel pressure regulator
fuel filter
injectors

I have not tested, cleaned or replaced:
fuel pump
EGR valve
EGR solenoid

He said an ECU that's warranteed will run $250. There's a Chrysler-rebuilt OEM Bendix ECU for manual transmissions on eBay for $42, but it's not warranteed. Would you guys go cheap or go expensive?
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Old 12-10-2011, 09:38 AM   #2
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Haha.. Go cheap and then DON'T go home cause the cheap part just failed you. Seriously, you gotta do it right and get as close to an oem replacement part as possible. This goes for a lot of parts in my book.. Also I didn't see anything about o2 sensors? Did you change them? Also look in gas tank to inspect for crud! That price on a new ecu (250) is pretty cheap for what it is, btw!! A new PCM for my 99 is over 300...
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Old 12-10-2011, 09:46 AM   #3
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Yep, o2 sensor was the first thing I tried: cleaned it, then replaced it, then cleaned the replacement. It tests OK as far as I can test it. Thanks for the advice! If $250 solves it, it's a bargain...
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Old 12-10-2011, 04:33 PM   #4
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Before I sprung for the ECU, I would remove it, unplug it, clean it's connector and the harness connector with electronics cleaner. Before plugging it back in, I would tweak the receptacles in the connector with a small pick to tighten them up.

Also, read the IMPORTANY NOTE of the attached write-up about sensor grounds. Takes 1 minute to check.


RENIX TPS ADJUSTMENT
Before attempting to adjust your TPS be sure the throttle body has been recently cleaned.
It's especially important that the edges of the throttle butterfly are free of any carbon build-up.
IMPORTANT NOTE: Using the positive (red) lead of your ohmmeter, probe the B terminal of the flat 3 wire connector
of the TPS . The letters are embossed on the connector itself.
Touch the black lead of your meter to the negative battery post.
If you see more than 1 ohm of resistance some modifications to the sensor ground harness will be
necessary. The harness repair must be performed before proceeding.
I can provide an instruction sheet for that if needed.
MANUAL TRANSMISSION:
RENIX manual transmission equipped XJs have a three-wire TPS mounted on the throttle body.
This manual transmission vehicle TPS provides data input to the ECU. The manual transmission
TPS has three wires in the connector and they're clearly embossed with the letters A,B, and C.
Wire "A" is positive.
Wire "B" is ground.
Key ON, measure voltage from "A" positive to "B" ground by back-probing the connectors..
Note the voltage reading--this is your REFERENCE voltage.
Key ON, back-probe the connector at wires "B" and "C". Measure the voltage. This is your
OUTPUT voltage.
Your OUTPUT voltage needs to be seventeen percent of your REFERENCE voltage. For
example: 4.82 volts X .17=.82 volts. Adjust the TPS until you have achieved this percentage. If
you can't achieve the correct output voltage replace the TPS and start over.
AUTOMATIC TRANSMISSION:
RENIX automatic transmission equipped XJs have a TPS with two connectors. There is a flat three-
wire connector, same as the manual transmission vehicles have, and it is tested the same as the
manual transmission equipped vehicles--FOR ENGINE MANAGEMENT RELATED ISSUES.
However, the automatic TPS also has a square four-wire connector clearly embossed with the letters
A,B,C, and D. It only uses three wires and provides information to the Transmission Control Module.
Key ON, measure voltage between "A" positive and "D" ground. Note the voltage. This is your
REFERENCE voltage.
Back-probe the connector at wires "B" and "D". Measure the voltage. This is your OUTPUT
voltage. Your OUTPUT voltage needs to be eighty-three percent of your REFERENCE voltage.
For example 4.8 volts X .83=3.98 volts. Adjust the TPS until you have achieved this percentage.
If you can't, replace the TPS and start over.
So, if you have an automatic equipped XJ your TPS has two sides--one side feeds the ECU, and
the other side feeds the TCU. If you have TRANSMISSION issues check the four-wire
connector side of the TPS. If you have ENGINE issues check the three-wire connector side of
the TPS.
For those with a MANUAL TRANSMISSION--the TPS for the manual transmission XJs is
stupid expensive. You can substitute the automatic transmission TPS which is reasonably priced.
Revised 11-28-2001
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Old 12-10-2011, 04:33 PM
 
 
 
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