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obd-II

N

Naked Tree 2x4

NAXJA Forum User
Location
Florida
my obd-II tells me a code 76. fuel bypass pump or something and syas to take it to the shop. i dont want to do that. now what? anyone familiar with this?
 
According to my OBDII lookup P0076 is 'Intake valve-bank 1 control circuit low.
No idea what the means, thats a lookup for a 98 4.0

Darn if I can find anything else on it as to what it means. Might want to get a scan tool on there and reset the codes then see if it comes back..
 
eh i think 98s have a different set of codes. i have a '91 4.0. the code has somehting to do with fuel system. thank you for looking though its been killing me for a while now.
 
How was this code read? If this is a 91 Cherokee, which is OBD-I, I'm not aware of any code 76. In fact there appears to be no code 76 for any of the Chrysler OBDI systems.

Have you tried doing another reading to see if it comes up the same again?
 
yeah it has repeated it consitantly. i really just assumed it was obdII. its trouble signals when you turn the key three times and the check engine light flashes out codes. i get 12(battery) but thats only because i just rehooked the battery, 76(Fuel pump bypass relay circuit)this is the one in question, and 55 (end ).
 
That one's going to take a look in a wiring diagram. Figure a relay has 4, maybe 5 different possible connections. The PCM will be looking at probably 2 of them for either voltage or lack of voltage. Find which wires in this relay circuit have PCM taps, it's going to be one of those wires as otherwise it wouldn't be aware of the problem. You're going to find either a short to power or ground, an open, or high resistance in either the primary or secondary side of the relay, if that makes any sense.

Start by testing the affected relay. Check resistance between pins 85 and 86, this is the primary coil. Should be 80-100 ohms and there should be an open or infinite resistance between 87 and 30. Now apply 12v + and - to 85 and 86, there should be a click and continuity between 87 and 30, which is the high side of the relay allowing current to flow. Or just toss another relay in the slot. If all that checks out then you'll need to test the wiring.
 
awesome sounds good. what is the pcm? and just to ask ahead of time while im here, how would i test the wiring? just follow them around and test them for breaks?
 
PCM= powertrain control module in Ford-speak(my job), it's the main engine computer.
You're going to have to be able to look at a wiring diagram and understand what to look for on those wires that the relay plugs into if the relay checks out ok. I don't have a diagram in front of me but I'd imagine there should be two keyed hot leads and a computer switched ground, then another power lead that's only hot when the relay fires up.

What's the symptom, anyway, does this Jeep run?
 
yeah the jeep runs. it has been most irritating though. sometimes it starts perfectly and runs perfectly. then other times it starts hard, i have to hold the pedal to the flooor while i crank it then once it starts it smokes and runs rogh and the intake sounds like it is gulping air. after a minute it settles down but the idle is a little high. untill the engine is warmed up it will studder on acceleration but once its warm if i tap the gas for a moment the engine will rev far beyond what my foot tells it to. then other times it works beautifully.

ive already replaced

fuel pump
cleaned injectors
TPS
fuel pressure regulator
plugs and wires
cap and rotor
coil
O2 sensor

i think thats all that ive replaced since the problems started. and it runs better but not correctly yet. i just recently found out how to get the codes.
 
JJacobs said:
I don't have a diagram in front of me but I'd imagine there should be two keyed hot leads and a computer switched ground, then another power lead that's only hot when the relay fires up.
Click to expand...

Get out your diagrams.
 
The Ballast Resister is for older non-computer controlled ignition systems. I could be wrong, but I don't think newer vehicles have ballast resisters.

BUT, if you had a bad Ballast Resister, the car would ONLY have ignition while cranking the starter. As soon as you turn released the starter, the motor would die out.

OBD-1 and OBD-II are a Protocol or set of standards/rules mandated by the government for Engine Computers. So computers can be OBD-I or OBD-II COMPLIANT but that is NOT the entire workings of the Computer. The manufacturer usually has additional codes and ways of displaying the codes that are above and beyond what OBD calls for. So the Gov sets some min rules as too what the Engine Computer has to be able to test for and how it should read out the result (mostly just emissions stuff) the manufacturer figures out everything else and comes up with their own rules for the rest.

SO, don't get wrapped up about OBD-II or I, look up the specific codes for the make/model/year of your vehicle and go off the info for that, not some generic OBD-II or I info.

Testing Wires, I think there is a bit of a communications gap. You need a "Multi-Meter" and Wiring Diagram, you check the resistance and continuity (basically zero or near zero resistance, i.e. a complete circuit) at the different pins and contacts at the connectors, and with a wiring diagram to refer too and some basic understanding of electricity, you can deduce a lot from the results.

I found a short in the wiring to one of my fuel injectors this way (it was in my Neon R/T, NOT Jeep), One Pin in each connector had infinite resistance to ground, another pin had zero resistance (or continuity) with ground, obviously the ground wire. Except one contact for one of the connectors, the one that should have been an open circuit, its resistance was actually 130 ohms instead of the infinite ohms of an open circuit. Sure enough, after unwrapping the tape of the harness, I found a short in the wire and fixed it.
 
Last edited:
OBD-II codes can be read either as the SAE Standard directs (a letter and four numerical digits,) or with a two-digit manufacturer's code (like your code 76.) SAE allowed the manufacturers to use two-digit codes so that the flashin of the CEL/MIL could still be used to depict codes - the SAE standard codes require the use of a separate code reader. DTCs can also be portrayed using a hexadecimal numbering system (Base 16 - common in computers, uses 0-9 + A-F, or a digit span of 0-F) as well.

I've got a listing of OBD-II codes on my site (http://www.geocities.com/JeepI6Power/obd2.html for everyone to refer to, taken directly from Jeep FSMs (to reduce extraneous codes. I'm working in the SAE full listing, and it will be labelled as such.) There doesn't seem to be "DTC 76" until I refer to the hexadecimal numbers, when it becomes "Fuel System Rich" - meaning that the system keeps trying to correct "lean" and it doesn't get anywhere. This is a "hard" code, which will illuminate the CEL/MIL if set.

I hate to mention these people, but most of the "Chain" stores (like Autozone, or Advance Auto) have OBD-II readers they can use to retrieve the SAE DTCs gratis, and you can then have the code interpreted and get an idea to service your vehicle. Also, if you plan to keep the vehicle for a while, there are OBD-II readers well within reach of the typical hobbyist - down around $100-$150. Saves you a lot of running about... I'd get one. (I need to anyhow - we just got a 2005 Verona for her to drive about in, and it's OBD-II.

OBD-II was used in the XJ beginning in the 1996MY. Should you have a 1991-1995XJ, you actually have OBD-I - which is a different coding system (since that was set by the manufacturers, not SAE.) In OBD-I, DTC 76 denoted trouble with the Fuel Pump Ballast Resistor Circuit - and it was a "soft" code (did not illuminate the CEL/MIL.) That's an easy fix, and most ballast resistors run about five bucks. OBD-I readers tend to be a little cheaper as well - ~$100 - but you have to make sure to get a "Chrysler" code reader (most OBD-I readers come with adapters for various plugs, and you just tell them what you're working on.)



CEL - Check Engine Lamp
MIL - Malfunction Indicator Lamp
OBD - OnBoard Diagnostics
MY - Model Year
SAE - Society of Automotive Engineers
DTC - Diagnostic Trouble Code
 
5-90 said:
OBD-II codes can be read either as the SAE Standard directs (a letter and four numerical digits,) or with a two-digit manufacturer's code (like your code 76.) SAE allowed the manufacturers to use two-digit codes so that the flashin of the CEL/MIL could still be used to depict codes - the SAE standard codes require the use of a separate code reader. DTCs can also be portrayed using a hexadecimal numbering system (Base 16 - common in computers, uses 0-9 + A-F, or a digit span of 0-F) as well.

I've got a listing of OBD-II codes on my site (http://www.geocities.com/JeepI6Power/obd2.html for everyone to refer to, taken directly from Jeep FSMs (to reduce extraneous codes. I'm working in the SAE full listing, and it will be labelled as such.) There doesn't seem to be "DTC 76" until I refer to the hexadecimal numbers, when it becomes "Fuel System Rich" - meaning that the system keeps trying to correct "lean" and it doesn't get anywhere. This is a "hard" code, which will illuminate the CEL/MIL if set.

I hate to mention these people, but most of the "Chain" stores (like Autozone, or Advance Auto) have OBD-II readers they can use to retrieve the SAE DTCs gratis, and you can then have the code interpreted and get an idea to service your vehicle. Also, if you plan to keep the vehicle for a while, there are OBD-II readers well within reach of the typical hobbyist - down around $100-$150. Saves you a lot of running about... I'd get one. (I need to anyhow - we just got a 2005 Verona for her to drive about in, and it's OBD-II.

OBD-II was used in the XJ beginning in the 1996MY. Should you have a 1991-1995XJ, you actually have OBD-I - which is a different coding system (since that was set by the manufacturers, not SAE.) In OBD-I, DTC 76 denoted trouble with the Fuel Pump Ballast Resistor Circuit - and it was a "soft" code (did not illuminate the CEL/MIL.) That's an easy fix, and most ballast resistors run about five bucks. OBD-I readers tend to be a little cheaper as well - ~$100 - but you have to make sure to get a "Chrysler" code reader (most OBD-I readers come with adapters for various plugs, and you just tell them what you're working on.)



CEL - Check Engine Lamp
MIL - Malfunction Indicator Lamp
OBD - OnBoard Diagnostics
MY - Model Year
SAE - Society of Automotive Engineers
DTC - Diagnostic Trouble Code
Click to expand...


Haynes lists 76 as "fuel pump resistor bypass relay circuit," or words to that effect. Of course you'd presumably have to have a fuel pump bypass resistor and relay for that code to be correct. There's no such code in the 95 FSM list, for example, as one would expect since there's no resistor. I don't know why such a code would appear on a 96. I'd suggest getting it read by an OBDII reader for confirmation.
 
Matthew Currie said:
Haynes lists 76 as "fuel pump resistor bypass relay circuit," or words to that effect. Of course you'd presumably have to have a fuel pump bypass resistor and relay for that code to be correct. There's no such code in the 95 FSM list, for example, as one would expect since there's no resistor. I don't know why such a code would appear on a 96. I'd suggest getting it read by an OBDII reader for confirmation.
Click to expand...

I'd suggest similar.

However, the fuel pump ballast resistor was added in 1988 for reasons of NVH (Noise, Vibration, Harshess) due to customer complaints. It was a voluntary recall for 1987 and early 1988 vehicles which were not so equipped.

Typically, if there is a ballast resistor, it is for normal operation, and is bypassed when starting (similar to the ignition coil ballast many of us here are familiar with.) The fuel pump ballast may also be bypassed safely - at least on RENIX, in my own experience (my 1987 never had one, and I removed them on my 1988 and one of my 1989's.)

I don't know how far up the fuel pump ballast resistor was used, but it's reasonable to suspect it may be present. It would take the form of a small ceramic block, about 3/4" square by 3" long or so, and it should be forward on the driver's side fender liner underhood.

You say you have a 1991, yes? Confirm, confirm the code (1991 is not RPT not OBD-II, and you probably have the ballast) and I can dig up my 1991 electrical FSM to see what I can find out...
 
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