1989 MJ 4.0 No Spark

Hey there i have a 1989 Comanche base model MJ 4.0 5spd. I got the truck in poor condition and in pieces for free. I have recently rebuilt the motor (Rings and Bearings) and with all new paper gaskets except for head gasket, vlave cover and oil pan and of course main seal. I have duplicates of every engine sensor from junkyards. And replaced damn near everything since i only got half of it from the previous owner. New fuel pump, new 4 hole injectors, Rebuilt fuel pressure regulator and fuel rail, short ram intake. Stainless steel headers and exhaust. New cap and rotor on the cleanest distributor i could find. New spark plugs and wires and new battery. Soldered in a new fuse box with new fuses since the stock one had pins missing and corroded from the leaking master clutch cylinder. New clutch and throwout bearing/slave cylinder. I have never seen this engine run and hasnt run since i rebuilt it. Im pretty frustrated with it.

I have done everything on "Cruisers tips for renixs" that applied to MJs and of my year also, added all the grounds with both an additional battery ground and motor ground strap at 4gauge wire. All grounds have been cleaned, adding a strap from driver fender to radiator mount and instrument panel ground to steering column brace both added with 10 gauge wire. Cleaned the connecters on both ignition coils and slightly pushed out and cleaned the springy tabs on the control modules.

I have replaced the igntion coil and control module with a used one although im fairly certain the ones on the truck were fine. I have replaced the CPS with an autozone brand, oreilly brand, and installed currently a Napa brand all times pushing the sensor closer to the flywheel with no luck. I have pulled the CPU hoping to find it fried but it was fine and cleaner than one i pulled apart from a junkyard which happen to be fried. Still no spark and i believe no injector pulse either. I might try drilling out the holes for the CPS sensor next. I have read many threads before posting this and trying all suggestions on other threads with again no luck.

What are your thoughts on the cause of this no spark situation?

There are some plugs near the steering column unplugged one is 2 prong i think for speaker left of column another 2 prong that is in a 3 wire loom and the 3rd wire goes to the CPU loom right of the column and 3rd one close to 10 prong right under column. Id like to know what these are too.

I get into a situation like that and I usually check the larger yellow wire at the ignition coil (it is easy to get at) for power with the key in the run position. That yellow wire powers up a lot of stuff including the ECU. The yellow wire to the ECU powers up when the key is turned to run and switches the ECU on (like an on/off switch). The red wire to the ECU is the main power (constant power). You can check that same red wire at another point, the 30 pin for the fuel pump relay. The fusible link may be bad.

Does the fuel pump prime when you turn the key to run?

Next possibility, if the ECU is good and the CPS is good and power to this circuit is good, what about the wiring between the CPS and the ECU? Not a bad idea to check these wires anyway for resistance even if they aren't your main issue.

What are the wire colors to the connectors you are trying to identify?

Ill check the wiring out at the coil and fuel pump relay. Im looking for 12v at yellow at the coil with key on. and then 12v at the red 30 pin of the fuel pump relay too?

The fuel pump primes sometimes by sometimes i mean it will and others its like its still primed and doesn't try. I doubt the wiring is bad from the CPS to ecu there is only like 2foot of wire but ill check it too.

The wire connectors ill take a picture of or write down the colors.

I have a thread in the MJ section also with a reply saying it may be the ASD Relay what does that handle and what would i be checking for to make sure it works or problems it may have. I do have a very loud clicking in the relays i guess from this relay.

RedBeast said:

Ill check the wiring out at the coil and fuel pump relay. Im looking for 12v at yellow at the coil with key on. and then 12v at the red 30 pin of the fuel pump relay too?

The fuel pump primes sometimes by sometimes i mean it will and others its like its still primed and doesn't try. I doubt the wiring is bad from the CPS to ecu there is only like 2foot of wire but ill check it too.

The wire connectors ill take a picture of or write down the colors.

I have a thread in the MJ section also with a reply saying it may be the ASD Relay what does that handle and what would i be checking for to make sure it works or problems it may have. I do have a very loud clicking in the relays i guess from this relay.

Click to expand...

The larger yellow wire is hot only with the ignition switch in the run or start position. And supplies switched power to many sub systems including the ignition module. The smaller yellow wire at the ignition module is the trigger wire for the module and goes to the ECU. The ECU decides whether it wants to trigger the module and coil with inputs from various sensors, like the CPS.

The red wire to the ECU and the fuel pump relay is constant power, hooked directly to the battery, through an in line fuse (fusible link).

The connectors in the Renix are prone to corrosion, this corrosion can cause issues. I spent hours one day trying to find the resistance in my CPS to ECU wires, I finally gave up and ran new wires. The CPS only puts out around half a volt, it doesn't take much resistance to degrade the signal. Even a little oil in a connector can do it. The connectors can also degrade the voltage for the various sub systems, some won't work reliably if the voltage gets too low.

The early XJ (Renix) don't have an ASD relay, the ECU controls basically the same functions in other ways.

You might want to pull up the relay block and look underneath it, it can get really nasty under there. And maybe look very closely at the relay female connections in the relay block, for discoloration, corrosion or whatever, Those same female connections can get loose if they overheat.

This is the second relay 30 pin the Fuel Pump Relay With only 3.71V



I cleaned the relays when i was finding the fuel pump issues and they are pretty clean

I wasnt sure of the yellow wire you talked about so here is the Single Small Wire in the Dual plug at 9.23V



Here is the Big Yellow Wire in the Triple Plug at 12.12V

The 1st 30 relay pin i think for the heater has no voltage the second 30 relay pin for fuel pump is 3.7v and the last 2 30 relay pins for oxygen sensor and something else are at 12.6v. What would cause the red fuel pump wire to be at only 3.7v is it a bad ground or is it something to do with the wiring to the relay? You said it was a straight shot from the battery so could it be a connection at the big mess at the starter/ignition module that the 8ga battery wire goes to?

There is a clump of inline fuses, fusible links, on the starter relay. One of these is for the fuel pump relay and the ECU. Fusible link bad, wire bad, connection under the relay block bad, bad splice or the worst is a broken wire under the insulation.

Sometimes there is no easy way, first ohm test the fusible links, if they all ohm test OK, then you'll have to dismount the relay block and trace the red wire back to the fusible link. Too many red wires running down there to try and guess which one (IMO) all you can do is grab a wire end and follow it. Be careful. you don't want to do more damage than good, I try to avoid yanking and pulling and/or disturbing anything I don't have to.

And another note, don't assume the relays are in the same order as they appear in the book. My 87 had two reversed (which drove me nuts for awhile). The fuel pump relay should have an orange wire on the 87 pin. Orange wires are often something to do with the fuel system, yellow wires are switched at the ignition switch and associated with most of the major components.

So i have to undue that bunch of wires on the starter relay and find what is causing me to have only 3v instead of 12v right? What wires am i looking for thick gauge wire running in the loom to the ecu or to the fuse block?

What do you mean by fusible links? The fuse block or a plug wire connection? Or like an inline fuse or something soldered together? Im confused when you say fusible link.

Ill pull the relay block and check it out and maybe even go grab one from the junkyard? Would it be worth it to solder in a new one? By ohms test you mean im gonna check from one end of the wire to the other for the least amount of ohms for a good connection and high ohms is a bad connection or 0 is broken/no connection right?

And are you saying that the 9v on my yellow coil wire is also a bad connection or ground problem too? If so what do i need to check for that problem?

RedBeast said:

So i have to undue that bunch of wires on the starter relay and find what is causing me to have only 3v instead of 12v right? What wires am i looking for thick gauge wire running in the loom to the ecu or to the fuse block?

What do you mean by fusible links? The fuse block or a plug wire connection? Or like an inline fuse or something soldered together? Im confused when you say fusible link.

Ill pull the relay block and check it out and maybe even go grab one from the junkyard? Would it be worth it to solder in a new one? By ohms test you mean im gonna check from one end of the wire to the other for the least amount of ohms for a good connection and high ohms is a bad connection or 0 is broken/no connection right?

And are you saying that the 9v on my yellow coil wire is also a bad connection or ground problem too? If so what do i need to check for that problem?

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The wire lug on the starter relay has one big wire coming from the battery and four (or five) screwed to the same connection. Each of the red wires coming out has a fat spot in the wire shaped like a little can, these are fuses. Which of the fusible links goes to what is anybodies guess, you have to check them all. The simple way is to disconnect them and make a tiny cut into the insulation the same direction as the wire near the end of the fusible link and check for continuity, 0 ohms is good or you can check for voltage at the cut with them connected (which may or may not be accurate). Any one of the fusible links supplies power to multiple sub systems, there are usually splices in the wire harness where the power branches off to various points.

Just check out the bottom of the relay block for corrosion and broken wires, clean the crap out. It would be much easier to fix the relay block you have if needed, than to try and swap it out.

Two yellow wires going to the ignition module one thick and one thin. The thick one is power in, usually a volt less than battery voltage with the key on. The thin yellow wire is the trigger wire and goes to the ECU, there is little to nothing you can measure on this wire with a volt meter, it is a kind of AC (alternating current) most meters can't measure or if the meter does get a reading it lies to you.

Here is a picture of a fusible link (in line fuse), which one goes to where is anybodies guess. I sure don't know. Disregard the "green" in the picture, as it doesn't apply to this situation.


kostenlose bilder

Sorry if I'm unclear in some of my explanations, first off I'm bi-lingual and sometimes my English suffers some in translation. Secondly, I'm generally a piss poor teacher.

Well i cleaned the bottom of the relays cleaned out all the grease which i was assuming was di-electric used brake cleaner which made it allow easier to clean no broken wires. So everything should be good there.

I was double checking grounds and the starter relay wires and when i was checking the loom on the firewall i found 2 ground wires for the ecu that the previous owner had tucked in the loom. So once i cleaned them up and attached them to the dipstick stud i had fuel injector pulse i can smell the gas in the exhaust too since its not hooked up past the cross pipe.

I had to run to oreilly and finally got around to having my ignition control modules (original and junkyard) checked and they both came up needing to be replaced. So ill be making another run to the junkyard and hopefully the grounds and bad control modules were the cause.

Once i get a working ignition control module and if it still doesnt start ill work on checking the rest of the starter relay wires.

Also i have a red power wire in a female spade connector coming off a loom heading to the heater fan motor that has 2 wires orange and green in a 2 prong plug. Where is this red wire supposed to go it has about 15inchs of wire longer than where the orange and green go into the heater plug connection. I ask because i have no heater fan. it doesnt blow air on any setting.

Alright got it running my ignition control module was bad then those grounds i found in the loom on the fire wall were for the ecu so i cleaned them up and attached them to the oil dipstick stud then i got spark. still woundnt run after some checking and double checking i found i had the distributor 180 spun it around and now it runs.

New problem tho It runs and idles at 600 rpms fine no stumble or miss but from 1800-3000 I have severe backfiring struggleing to gain rpms then it races up from 3000-5000. So it runs fine from 600-1800 and 3000-5000 rpms. But horrible backfiring from 1800-3000 rpms what would cause this? I recently cleaned the throttle body tried a different MAP sensor checked the vacuum lines off the intake for leaks with no change and same results. My main guess is that the ignition timing off or is not working properly. Possibly a weak coil? Ive heard of erratic injector timing working then not working. Its fuel injected but im wondering if this is right at 600 rpms timing off cylinder #1 im getting roughly 14 degrees of timing. Is this right or is it advanced or what? It does advance timing when the throttle is opened so it is advancing fine.

Have you set your TPS yet? There are other inputs, like vacuum, but the TPS has a lot to do with the timing. Is it possible your distributor is in there one tooth off, though it doesn't sound like it with only the mid RPM range screwing up.

Try unplugging the O2 sensor and see what happens. Especially if your backfiring or bucking is happening in the mid range. Severe bucking usually happens on a warm motor, with a bad O2 sensor or the O2 sensor, knock sensor, engine temp. sensor wires are cooking on the exhaust manifold behind the power steering pump bracket. It may have nothing to do with the O2 sensor though, when the sensor goes bad top end also suffers, especially above 3500 RPM. The bucking tends to be the worst around 2200-2400 RPM and then the motor kind of craps out again above 3500 when the O2 sensor or the wiring is toast.

When you test the TPS, check it with an ohm meter as you slowly work the throttle and check for a dead spot. I had that on my 87. Then check the voltage with the key in the run position, should start out around 0.8 volts (ideal) and end up around 4.6-4.8 volts (normal but it does vary some). Check the TPS ground for resistance, 0 ohms is good, test between the ground wire connected on the TPS side and battery negative pole.

The easy way is to test on the TPS side of the connector, just to make sure the issue isn't in the connector itself. I had a TPS connector somebody had sprayed (inside) with WD40, my Ohm readings were never the same twice, drove me nuts.

You seem to be picking this up pretty quickly. Sorry I haven't been more help, all I have to offer is 27 years of fiddling with XJ's and I'm sure I haven't seen everything yet.

I usually test sensors and most all voltages to chassis ground or battery negative, That way you test the sensor or 12 volt power itself and then I test the sensor ground with an ohm meter separately. You never really know if it is the sensor or the ground, unless you test them both.

One quick test is to test the ground wire near the sensor for voltage to the battery negative terminal. Any significant resistance in the ground circuit between the sensor and the battery negative terminal usually show up as voltage in the ground circuit, I call it standing voltage.

Sorry double post.

My TPS was set to .60v and about 4v at full throttle testing the green and yellow wire.

I have since set it to about .78v at closed and 4.4v at full throttle. I checked the TPS slowly opening it and there was no dead spots in the readings tho it would jump about .1v every time it did show a change. This hasnt made a change except to verify the TPS is working also when i said it could hit 3000-5000 rpms thats only every now and then. Most of the time it will run fine from 600-1800 and backfire like crazy from 1800-3000 and not let the engine go above that it only revved above 3000 maybe 4-5 times since ive got it running. but in theta 3000-5000 range it runs fine no hiccups or backfires.

So maybe a better question is what will make it backfire so crazy it cant get above 3000 rpms?

The ground wire has 15-20ohms of resistance to the negative battery post. And the connector connector was pretty clean for the ost part i did try and scrape it a little cleaner.





I did unplug the O2 sensor but with no change. And when i installed the engine was connecting plugs i made sure to ziptie the loom away from the exhaust so its not melting and shorting on the exhaust same with the new CPS sensor.

Clean all your ground connections and c101 connector as per cruisers renix tips. 15 ohm is pretty high. Some sandpaper to all surfaces and mine measures .04 ohms~

Nm, missed that line where you did that.

Although, 15 ohms is still on high side.

Find out where the resistance is, various ways to do this. Start on one end and measure the resistance between the block and the battery negative. Or the other end and measure the resistance for the ground at the O2 sensor. Or measure the resistance in the MAP ground, which is pretty near the middle. The ECU is the actual middle and you want to test the D 3 pin. This ground circuit goes through numerous splices and connectors.

The sensor ground may be your issue, it may not, but you are chasing your tail if the grounds aren't right.

It doesn't have to be just one thing, it can be a combination of things that add up.

I dont think that the sensors slightly being off from a slightly high resistance ground is the source of my problem it may add to it but it isnt my main cause. Ill check my ohm readings at the various sensors but this isnt what i think is causeing it. Does anyone have a timing light that they can check the timing at their dampener? At 600rpms off cylinder #1 i have roughly 15 degrees of timing. I want to know if this is right. Im going to try retarding the distributor a tooth too see if it runs any better if not then try advancing it a tooth. I think the major cause of my backfire is ignition timing and valve timing issues (like its firing when a valve is open) not sensor inputs although they may be slightly adding to the problem.