Getting through the emissions test

1994 Jeep Cherokee OBD port should be located under the hood on the passenger?s side next to the powertrain control module (PCM).

Never look down on anyone unless you are helping them up - Jesse Jackson
1994 Jeep Cherokee OBD port should be located under the hood on the passenger?s side next to the powertrain control module (PCM).

Never look down on anyone unless you are helping them up - Jesse Jackson

I have never seen an OBD plug on my Jeep but I'll have a look tomorrow. I thought it was too early (1994) for that. I have always used the flashing check engine light to get the codes.

I have been working my way through the sensors trying to find something dodgy, but no luck so far. The problem is to get a connection for the meter on those tiny pins. I am making up all sorts of bent wire creations to go over the pins. Paper clips are quite useful.

In the event of having to replace one or two where can I find a list giving the genuine jeep part numbers? Everything I look up has multiple offerings that all look the same.
It takes ages to keep putting the vehicle info in for every supplier and having done that the lists contradict each other!!!!!!
This is a real pain since everything has to come via air mail. It also amazes me how the same part can be priced from say, $5 to $95 for the same thing.
It's not easy living in Spain. Nothing is ever easy and straightforward, you always seem to be on an uphill struggle.

i have heard that, hard to get stuff deal
I have been looking into all the other sensors. I have got a new water temp one on order as the car boiled up just after the test so I'm assuming the one on the thermostat housing is the culprit.

I have also been looking at the temperature sensor on the cat that could be an oxygen sensor but isn't.
It is actually a thermistor probe, see attached pics. No one can tell me why it's there or what it does. EGTs were not fitted to Jeeps at this time.
The braided lead (2 core ) runs up to behind the dash near the steering column to a small 'control' box. Being a thermistor it will change resistance with temperature like most sensors. Thereafter I have no idea.

The only clue is that this Jeep was exported to Japan when new, then imported to the UK where I bought it, so it might be for some strange Japanese requirement. We bought two of these Jeeps about a year apart and they both have it.
Not knowing what it does makes it impossible to check so I really need help here. Can anyone offer any suggestions or recommend somewhere I could ask some questions? Is there any way to contact Jeep themselves? Thanks......Mike


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if you unhook it, does it cause any changes in the jeeps behavior? i wonder if they have some weird check in japan that is required and that is how the testers access the results?

Got a problem using quick reply. It won't go, so it's on here as well.

I did try to unscrew it but gave up as it just would not move even after soaking with diesel for a couple of days and applying heat. If I destroyed it I doubt I could get another. So I chickened out. I might have a closer look at the top end where to two wires connect to heavens knows where, but where it's situated makes it incredibly difficult to get at and I'm not that flexible any more.
Anno Dominae is not kind.
I'll be checking the MAP sensor today.
totally understand. wrist arthritus from when i went in the army after high school. started having problems about two years in so it was in my medical record. way worse now, to the point that driving can be painful and if i have to work on something, i have to take medication to sleep.
Hi all,
I have been progressing through all the sensors. I have ordered a new MAP sensor as it did not seem to work when I put vacuum on it. Unfortunately that is now lost between here and Germany.

I now have yet another problem. The (brand new) battery was totally flat yesterday. Checking today, having recharged it, I have got a 0.25Amp continuous discharge via the auxiliary fan circuit. It is indicated on the check engine light as well.
I have pulled the small fuse for it in the centre which stopped it, but pulling the relay makes no difference. So the problem seems to be on the control circuit.
I have not got a circuit diagram for this. I assume it is controlled via the computer but have no idea what or how.

Has anyone got a diagram for this circuit or can tell me where to find one on the net? My manual does not go this far. Thanks for the help. Mike

Hopefully someone has that diagram. I know i dont
Hopefully someone has that diagram. I know i dont

I have actually found the complete (I think) set of genuine Jeep wiring diagrams and the workshop manual. Each section is a separate pdf, 41 sections in all. It's a big document, 92 MB.

I have worked my way through some of the individual pages of the wiring diagrams but haven't found out if the individual pages can be built up into one big diagram if I were to print them. All the pages overlap each other.
Jeep diagrams are quite difficult to follow as you only get a little bit on any one page.

I have a question re the fuse and relay panel in the engine compartment to which I cannot find an answer in the manual. There is a 20A fuse labelled I.O.D. on the underside of the lid. What is it for? I can't think up anything that might fit the letters.

The more I look at the pages the more I seem to think I might have a problem in the computer section that controls the fan. All the 'exterior' bits like fuses, sensors, relays and fan motors are OK in their own right.

Are the ECUs in the 4.0lit XJ known for giving problems. What's it like trying to find another if needs be?

The basic principle is simple. The temp sensor is fed with a 5v supply from the computer (along with the other sensors) it then feeds a varying voltage back to the computer in proportion to the temperature (it's a thermistor). It then decides when to bring on the aux. fan at the pre-determined temp. by activating the fan relay and hey presto you get cooled.

If you have air con the fan should run all the time the air con is running. Sadly my air con doesn't work, but the fan has always worked as it should for the engine cooling side, until recently that is.

I'm back to it this afternoon. Hopefully I can find something. This all started when it failed the emissions test, which still needs sorting. The fan used to work before I took it to the test centre. I'm not sure if any of this is connected.
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No, the ecu are usually fine on an xj. The zj had ecu trouble

This is beginning to get expensive. I have a new battery (probably not connected to this issue) and numerous new engine sensors and I have just fitted a new MAP Sensor. Now the battery is flat again.
There is something draining the battery while the car is stood. Oddly enough I can't pick anything up by putting a meter in the battery lead, which does not make sense. What could possibly draw current with everything turned off?

I have one fuse in the relay box under the bonnet (hood) called I.O.D. 20 amps and I have no idea what it is for and I can't find it in any book I have got. It's in between the the Aux. fan relay and the small fuse labelled Can anyone help with this?

Once I can get this sorted I can get back to the emissions problem.
Does your DMM measure current?
If so place it between the negative battery post and cable to read current. With the key off you should read less than 50 mA. If higher start pulling fuses until there is a large jump down. That will show the draining circuit.
Then inspect what is on thst leg of the circuit.
If draw is less than 50mA have your battety tested and clean the outside.
Ive had the original battery for 10 years. 2x i brought it back. The last time it was leaking so I replaced it.
One time the cells wrre low on fluid. The other the battery looked wet and dirty. I pulled it cleaned the outside with mild soap and some baking soda to neutralize if it was an acid. You don?t soak the outside but wipe with a near dry cloth then dry behind as you clean.
Last time i also found my multi position switch was worn allowing my fog lights to intermittently turn on with ignition off.

I.O.D. Is ignition Off Draw.
This goes to dome lights, possibly radio clock, lights if you turn them on or flick them. Pcm memory?

Never look down on anyone unless you are helping them up - Jesse Jackson
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Or pull each fuse and listen carefully for the popping sound of electricity arching. Make sure doors and closed and the jeep is off so no lights are drawing power. That should help to find what circuit is pulling power

Many thanks for the IOD definition. I blew the fuses in my meter when I put it in line with the neg terminal so I put a small bulb in series with the lead and sure enough it glowed. I opened the door and it immediately went bright. Next I started pulling the fuses to see what turned it off and funnily enough it was the IOD fuse that did the trick. As it only seems to be for the radio memory and I have no idea what the problem might be, I'll just leave it out. The radio still works but won't remember any presets or where it was last tuned.

This brings me to my next problem The electric cooling fan has stopped turning on. The fan motor and relay are OK and the sensor has been changed. The temperature gauge is working and today I have had it way over in the very hot area and still no fan. The relay gets its signal from the ECU so obviously something is wrong there. How can just this one function not work when everything else is OK?
If it turns out to be a computer fault, I'll remove the relay and just wire in a manual switch.

As usual suggestions are welcome. I can't believe how many problems I have had recently since it failed the test, it seems to go on and on.
the gage works off a different sensor, if i remember right. i think its sensor is at the back of the head, top on the usa driver side. the sensor for the fan is located in the radiator or thermostat housing, i just cannot remember which. edit: google seems to say the fan sensor is the one in the thermostat housing

here is a link from others who have the same issue you have. some had to change the relay, some had to change the fan, some had another issue
94 xj 4.0 i6 Auxiliary Fan Relay / Overheating - Jeep Cherokee Forum
Put the fuse back. Ha Ha ha.

Idk but you might verify it. Look for other broken fuses.
Im surprised you blew the fuse in the meter it is usually good for 1-10A

Never look down on anyone unless you are helping them up - Jesse Jackson
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Until the fuses went in the meter I had no idea there were any inside. I have got some new ones now. Re the sensors, I have renewed the one in the thermostat housing (see pic) and have looked all over the radiator and head and can't see anything else that looks like a thermostat/sensor.
Mine is RHD so things will be in different places to what you are used to, but I think everything is there somewhere. I've attached some pics.
In the circuit diagram I have it shows a wire from the ECU to the coil of the fan relay. If I can find it again I'll attach it.

In case it comes to this and the ECU is the problem, the part number of the current one is 56028004, from the label on the side. Can some kind person please see if this is the same for all '94 XJs 4.0 and if a replacement can be found. Working second hand would do. Thanks.


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The sender should be just past the intake, near the firewall, if i remember right. But you said the gage works so no reason to mess with it.

The sensor you replaced on the thermostat is the fan controller.

My wranger is rhd also and other than the rhd particular parts, its the same as the other wranglers i have owned, engine, transmission, and ecu wise. I would geuss your cherokee will be also but i no longer have any cherokees so i cannot verify it physically
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Are you supposed to use tape on the temp sending unit? Should it be grounded???

Check the relays by swapping the fuel pump with the fan.

Never look down on anyone unless you are helping them up - Jesse Jackson
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