Jump to content

1990 Jeep YJ Wrangler 2.5L TBI Has No Spark, No Gas: Need ECU info on board drivers for the Ignition Module and Injector

Recommended Posts

Hi Guys! I've scoured these pages the last couple days, but can't find anything that matches my specific dilemma.

I have a 1990 Wrangler w/ 150k miles on it, recent acquisition by my future son-in-law. It was running relatively fine, but it quit suddenly on a recent trip. It started cutting out when hitting bumps, then finally quit altogether.

After dragging it home, hitting the forums, swapping a few parts, following all the sensor tests and wrapping my head around this TBI setup, I am afraid I have a partially dead ECU.

-All ignition parts replaced- Ignition Module, Coil, Wires, Cap and Rotor, Plugs.

-The truck will crank the engine just fine, but has NO spark at coil or plugs, and NO gas at the injector. The fuel pump comes on with ignition switch for 1-2 sec, there is fuel in the line at the throttle body, I have not checked fuel pressure yet.

-A new CPS was installed, I checked the CPS, 217 Ohms (slightly low), 0.54 VAC while cranking the engine. 

- All other sensors checked out nominal. There is 5 VDC to the TPS and MAP sensors, so the ECU is not entirely fried.

-While tracing the sensor wiring, I found a wire broken internally between the CPS sensor and the ECU (red-white wire tied to pin 28), I'm guessing that was the root cause of the failure. I'm afraid the ECU board may have been damaged in the process of the wire failing while driving.

-I am looking for information about what I should be seeing for the output signals for the Ignition Module and Fuel Injector from the ECU. I expect they will be low voltage AC pulses, grounding the DC power supplied to the Ignition Module and the Injector. I have not found any specifications for the AC voltage  or pulse duration for either the Ignition at pin 27 or Injector at pin 21 of the ECU. I measured both of these signals while cranking the engine, I got ~0.1 VAC at the injector, and basically zero (0.004 VAC) at the ignition module output. These numbers lead me to believe that the output drivers for these functions are damaged or dead, although I can see no signs of physical damage, no burnt smell, melted traces, cracks, or corrosion anywhere in the ecu components. 

Is there any function programmed in the ECU which would completely shut down the spark- and injector-outputs based on some combination of other inputs for some safety reason ? 

Any info on the specific layout of components relating to the Ignition and Injector Drivers in the 1987-1990 ECU would be greatly appreciated! And Thank You to all who have gone before me here, the info trail has been invaluable!!


Link to post
Share on other sites
  • Administrators

Hi, Rusty!  Generally, the jostling/stall symptom begs a look for issues like a bad ground(s), defective ignition switch or an alternator short, loose connections, etc.

Regarding output voltages at the ECU and related circuits, the ECU will either provide voltage and ground signals to the injector and ignition circuits or not.  Seldom is there a low voltage output signal, more likely a resistance source will drop voltage in the individual wiring circuits.  Low enough available battery voltage and the injectors or ignition module will cut out.  Load test the battery to see if there is a dead cell or an open under load.  12.5 or less volts is usually the cut-out point where the engine will misfire and lose the injector pulses.

Here is a valuable illustration of the ECU and its related circuits and devices.  The schematic applies to 1987-90 Jeep YJ Wrangler/Model 81 2.5L TBI vehicles.  Zoom into the PDF for details:

1987-90 Jeep YJ 2.5L ECU Schematic.pdf

Think in terms of circuits.  See the relationship of grounds that can create voltage drops and poor or low available voltage to the ECU.  Trace out the grounds and wiring connectors involved with the ECU supply voltage or ground circuits for the injectors and ignition module.  On 12VDC negative ground systems, grounds are just as important as hot/positive leads.

I would check for poor grounds like the infamous engine to body ground near the dipstick to firewall.  Check the voltage  and ground continuity to the ECU before and after these ground checks.  If you have access to a Fluke 1587 or similar insulation resistance tester, that would be very useful when testing for opens or shorts to ground or shorts between wires under load.  (This tool is cost prohibitive unless you have one or can borrow one.)  Here are details:  http://en-us.fluke.com/products/insulation-testers/fluke-1587-insulation-tester.html.  You can somewhat simulate these tests by beginning with a voltage drop test between the battery's ground post and the ECU ground point.  Also compare ECU input voltage reading to the actual battery voltage reading.  

Caution:  To avoid creating a spark at the battery, I test battery voltage away from the battery, usually at the starter motor end of the positive battery cable.  If the battery is defective, a spark near the battery can ignite battery gases and cause an explosion.

A test meter like the Fluke 1587 can test for shorts, opens and issues with actual voltage available, quickly covering entire point-to-point wiring circuits like the battery or relay box to fuel pump and other 'end to end' chassis wiring.  The meter's built-in, non-destructive high voltage/low amperage insulation resistance tests can be very helpful when there are suspected opens or damaged insulation/shorts buried within a harness.  If you can't access or afford this type of meter, a quality digital volt-ohmmeter can simulate the other tests.  

Let us know what you find...We can go from there.


Link to post
Share on other sites

Moses, thanks for your suggestions and for all of the invaluable info on your site. That Fluke 1587 meter is interesting, but costs almost more than than the Jeep was purchased for. 

Honestly, this is a very black and white problem. The Jeep was running and being driven daily, then suddenly there is no spark or fuel signals. The owner and I spent several days cleaning grounds and sensor plugs, and tracing circuits with both resistance readings and audible signal. From all I've read so far (Please correct me!), only the WOT switch has the ability to completely stop the gas injection signal. I've verified that the WOT switch is not closed while we are cranking the starter. I have seen no mention of any condition other than failure of the Crank Sensor (CPS)  which would prevent the spark signal while cranking the starter, we have cleaned and verified the sensor leads and tried a replacement part, no luck. The Jeeps owner hauled it to a local professional diagnostic shop but they were also unable to find any other faults.

I have done lots of electronic equipment diagnosis from vacuum tubes to modern surface-mount fabrication. I'm confident I can replace any of the components on the ecu board except the microprocessor, but I was hoping someone might have specific information about which components (the large, heat-sinked transistors) on the board are the source of the spark and injection signals  and likely to fix the failure if replaced. I guess I'll just wing it, and get a backup replacement from ebay just in case.

Link to post
Share on other sites
  • Administrators

Rusty...I have seen CPS sensors fail and also develop resistance in the wiring...something to consider.  A seeping rear main seal or exhaust manifold heat near the wires can impact the sensor performance , too.  An aftermarket header on our XJ's 4.0L six scorched the insulation of the CPS wires, created a short to ground and gave the same symptoms you describe.  

Note:  A defective battery (one dead cell "open", failed a load test) produced the same trouble symptom on another occasion.  The engine could start but would cut out under electrical load and while driving down the road.  It stalled at low engine speeds as if the ignition and fuel had been switched off.

If you can, with a degree of certainty, narrow this down to the ECU, there are aftermarket rebuilt boards available.  You might, as you hint, be better off addressing the board yourself.  You're very well qualified.

Nobody at the forums has "rebuilt" his/her own ECU board, but several members have replaced the ECU on 1987-90 YJ 2.5L models.  There are sensor ohms-resistance readings given throughout the factory workshop manual for the 2.5L TBI models; however, I've not seen any specifications for ECU troubleshooting use.  Clearly, the OE expects owners and shops to replace the ECU when any kind of trouble is suspected.

I suggest that you contact ModuleMaster at Idaho.  They rebuild customers' ECU and ECM units and make upgrades to offset factory weak spots.  My son had a Kelsey-Hayes ABS module fail on an S/T truck (very common) and G.M. wanted $800 for a new module.  We sent the OE module to ModuleMaster:  $130 later, the unit worked better than new.  They were friendly and helpful.  Try these folks for tech insights or an actual ECU rebuild:  https://modulemaster.com/rebuilds/.

Let us know if this helps...


Link to post
Share on other sites
  • 3 weeks later...

Just wanted to close this thread out with the resolution so others can hopefully benefit.

The owner had purchased a new but cheap Crank Position Sensor as the symptoms pointed to the CPS. That sensor was checked and was just slightly low on the resistance check, and seemed to be putting out ~0.5 VAC when cranking. 

After miles of wire tracing, and epic growling and gnashing of teeth, another, higher quality CPS was installed, and the Wrangler started up and ran. 

Moral of the story- don't trust the cheapest part in the list...

Link to post
Share on other sites
  • Administrators

Thanks for updating and sharing, Rusty...I learned this lesson with a Brand-X TPS and oxygen sensor on my 4.0L XJ engine.  Each did not perform well, the off-shore TPS from a popular high-volume auto parts chain store actually failed, the O2 sensor caused a mysterious engine idle issue.

I'm once again a staunch advocate of OEM spec parts.  Avoid the generic pieces that fit a variety of applications and may be adequate if your application happens to be the benchmark for the part.  You're an electronics pro and can appreciate this.

One solution is to buy OEM supplier parts.  An example is that NTK supplied Chrysler with the OEM O2 sensor on my '99 XJ Cherokee 4.0L.  I took the NTK number from the OE sensor and simply replaced Brand-X with an NTK unit of the right part number.  I sourced the best price on the NTK sensor strictly by its part number.  Win, win.


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...