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Computer questions

M

Matt&Mandy

NAXJA Forum User
Location
Greencastle PA
I have a 91 XJ that I've been told has a 95 motor. The computer that is in the Jeep is from 91 and I suspect that it's fried. Are these computers interchangable between years? Do I need to get one for a 95? Everything is hooked up and I've replaced alot of parts since buying the Jeep including fuel pump, O2 sensor, distributor, Crankshaft position sensor, etc. The motor is a 4.0 and the previous owner said it was out of a 95. It has never run perfectly since I bought it and now it won't even start. Any help would be appreciated. Thanks in advance!
 
91-95 were OBDI HO's so it should be fine. maybe go to junkyard or borrow someone else's computer.
Does it crank/no start? If so, try checking voltage/connections at your CKP (Crank Position sensor), Sometimes if you unplug the connector and plug it back in it will start (If that is the problem). The connector is near the rear of the engine on the driver's side is a 3 wire connector. Sensor itself is on the housing of the trans between the trans and the block.
Check all of your connections, fuses, relays, fuel pump, etc.
 
Here's a lil more info. I put a new fuel pump in already, I put a new crank position sensor in as well. I have checked the fuses and relays and still no power to fuel pump or spark at distributor. The Jeep also has a new exhaust manifold and new O2 sensor. When the Jeep did start and run it was hit or miss, sometimes it would run fine and I could drive around for an hour or two with no troubles, other times it would backfire thru the manifold and sputter and often times stall and not restart for a while. Any suggestions or is my theory about the computer more or less adding up?
 
Is the pump priming? If not, try swapping the ASD relay with one of the others. If the ASD relay is bad the coil won't get power or the fuel pump.
 
Which one is the ASD relay? If it is in the main fuse box with the other relays it has been switched and pulled several times. If it is somewhere else how do I locate it?
 
Should be on the passenger's side fenderwell, three or four relays in a row. Try testing the relay sockets to see if you are getting power/grounds where they should be.

I don't know which is which on the HO (Renix RULES!).
 
That has all been checked then. That was one of the very first things I did. Thanks for the advice though. And Renix does rule. My 89 was great until it caught on fire. Long story. Any other tips?
 
Ok, but is the fuel pump priming for 3 seconds when you turn the key to the ON position?
 
Matt&Mandy said:
No. I hear NOTHING from the fuel pump.
Click to expand...

Ok, that 3 second running of the pump when the key is turned to the ON position goes through the ASD (or latch) relay. I don't have the FSM for the 91, can you get access to one?

And Semper Fi, Mac! 1969-1979.
 
I'm trying to right now. I have a close friend working on this issue with me too and the Jeep is actually in his garage right now. I plan to call him in the morning and let him in on this information and we'll go from there. I'll report back tomorrow evening. Thanks!
 
I stand corrected. With the new computer in the fuel pump does prime for three seconds however there is no fuel pressure at the rail and there is still no power at the distributor or coil.
 
I didn't read the whole thread but to answer the original question, there are multiple generations for the XJ electronics, and the components are only going to be directly interchangeable within their generation. For example, the fuel injectors are different between 1991 and 1995, O2 and TPS sensors are listed in the parts catalogs as 91-93, the fuel rail plumbing was changed around 1995, and so forth. Maybe the 91 parts on your engine will work with a 1995 computer but I would be surprised if it was not problematic. You really need to coordinate the computer and the electronics. The vintage of the engine block is not significant.
 
Matt&Mandy said:
I stand corrected. With the new computer in the fuel pump does prime for three seconds however there is no fuel pressure at the rail and there is still no power at the distributor or coil.
Click to expand...

If the fuel pump runs for three seconds but you don't have fuel pressure at the rail, clearly you have a bad pump or blockage. Still no pressure after priming 3 or 4 times?

Some people have claimed to have pumps that were wired backwards???????
 
Last edited:
The Jeep ran with the pump that's in it before. Not well. But it ran. So the pump should be good. I will check for blockages again tonight. Weather permitting.
 
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