Bad ECU? (1 Viewer)

[gorbilax]

Hello all. I've been a long time reader here in 80-series tech, but this is officially my first cry for help.

After attempting to chase down a no idle problem (details below), my '94 LC (1FZ-FE) now cranks but won't fire at all. During the bulb check, my CEL doesn't illuminate. So far I've:
- tested my EFI main relay
- checked the 10a gauge fuse, 15a ECU-IG fuse, 7.5a IGN fuse and 10a ECU-B fuse with a multi-meter. All are good.
- Per the FSM, I have back-probed the following terminals at the ECU +B1, BATT, M-REL, STA, B+, and IGSW and all are giving ~12v with the key on.
- Per the FSM, I have followed the flow chart to test the MIL. Grounding pin W of the diagnostic connector DOES cause the MIL to come on, so I know the bulb is not bad.
- The next step in the flow chart was to check pin E1 for ground at the ECU, and there is a good ground at the pin. After this, the flow chart recommends I replace the ECU.

My ECU part number is 89661-60221

Does anyone have any thoughts on where to proceed from here? I don't want to drop the money on another ECU to a.) have it not be the problem or b.) have whatever is actually causing the problem fry the new ECU.

Some background: This all started after removing the TB and upper and lower intake manifolds to replace the EGR VSV, a blown PHH, some vac lines, and the fuel filter.

After reassembling everything, the cruiser wouldn't idle and would stall out when coasting to a stop once it warmed up. When cold, it would idle rough but wouldn't stall. Disconnecting the wiring harness to the IAC valve allowed me to limp around without stalling when the engine was warm. I borrowed a Snapon MODIS scanner from a friend and was about to start running through some diagnostics on the MAF and IAC when the scanner started randomly losing its connection to the ECM. I disconnected the scanner and returned it to my friend, and the truck ran (with IAC disconnected) for two days after using the scanner before I got in it the other day and it just cranked but wouldn't fire up (from cold start). That was when I first noticed the MIL wasn't coming on during the lamp check. I have not attempted to reconnect the scanner (I'd need to borrow it again), but I cannot blink any codes by jumping the terminals in the diag connector.

 

[TrickyT]

So from that ECU p/n, I'm guessing your vehicle is a '93 or '94, yes? Or possibly an earlier year and the ECU was previously replaced with the later model.

Check the connector to the MAF, and check that you are getting 5v to the MAF from the ECU.

 

[gorbilax]

Yep, it's a '94. TrickyT, which pin should I be measuring at the MAF? pin E2 (pin 5), pin VS (pin 6), or pin VC (pin 4)? E2 appears to be the sensor's ground from the ECM, and pin 7 (TCC) is the air temp sensor, so I'm assuming VS or VC?

 

[TrickyT]

Yep, it's a '94. TrickyT, which pin should I be measuring at the MAF? pin E2 (pin 5), pin VS (pin 6), or pin VC (pin 4)? E2 appears to be the sensor's ground from the ECM, and pin 7 (TCC) is the air temp sensor, so I'm assuming VS or VC?

I would measure at the ECU with the MAF connected. That's because there's a good possibility that something got messed up with the MAF or the MAF connector when you did your maintenance work. A bad ECU should be last on your list of suspect components, especially because the ECU you have is the newer model and your maintenance didn't involve messing with the ECU. Besides the MAF and connector, there could be a problem with the throttle position sensor and its connector, or possibly the idle air control and its connector, although I don't think a problem with the IAC would cause the no-MIL/no-start symptom you describe. My bet is something with the MAF or its connector - that's seems to come up fairly often.

Have you looked at this thread: https://forum.ih8mud.com/80-series-...es-faq-item-recommend-keeping-copy-truck.html ?

 

[gorbilax]

Yes, I went through that thread a few days ago. I jumped the diagnostic pins for the fuel pump and I can hear it running. I don't think I have a fuel issue. I haven't gotten a chance to check the spark yet, but I have tested the EFI main relay. The thing that's got me stuck on the ECU is that when I first started troubleshooting the idle issue, the CEL would always light during the bulb check when I turned the key on, but go out with the engine running. At that point, it would fire up and run, but eventually stall out. I knew I had some sort of MAF/TPS/IAC issue then.

Now, the symptoms have changed. I have no CEL at all during the bulb test with the key on AND the engine won't fire at all. Before I had the CEL and a firing engine. The FSM's flow chart led me to notice the lack of CEL during lamp check, and that chart led me to the ECU being suspect.

I'll back-probe the ECU harness with the ECU and MAF connected and see what I find, but something somewhere has lost power that had power before.

 

[gorbilax]

Also, which terminals at the ECM should I be testing for voltage in regards to the MAF? Again, I'm not sure of the purpose of the various MAF leads, I just know it has a ground and three outputs, two for the air flow and one for the air temp. The FSM only indicates resistance (ohms) values to test for in the MAF, nothing about where to test for voltage (the 5v you mentioned earlier.)

 

[TrickyT]

Sorry, but I only have a 1997 EWD and I know there are at least some differences from your vehicle (i.e., there's no OBD-II on a '94 and I'm not sure if the transmission control for the A442F is the same as my A343F). So if you have the EWD (electrical wiring diagram) for your '94 you should definitely go by that. Look at the Engine Control section, then at the Service Hints page (just past the wiring schematics). There it will call out a bunch of voltages to test. For the MAF you want to look at THA and VG. For the TPS look at IDL, VTA, and VCC.

Edit: The +5v I mentioned is labeled VCC in the schematics. For my 1997 vehicle, it's found on ECU connector E5, pin 1 and should be 4.5v - 5.5v with the ignition in the on position.

 

[gorbilax]

My new (to me at least) ECU from arcteryx arrived today. I replaced the MAF and IAC, and then replaced the ECU. I turned the key, and I got a check engine light on the lamp check, cranked the starter, and it fired right up.... for half a block... and then promptly died in the middle of the road.

I'm back to having no check engine light on bulb check, and it would appear that I've killed a second ECU. What in the hell randomly kills an ECU as I'm driving down the road?

Any thoughts at all on what to test now would be greatly appreciated... I can't afford to keep toasting ECUs....

 

[Kernal]

Could a wire in the engine harness have gotten pinched/burned/melted/shorted when you put it back together? Fusible links good? IDK.

 

[TrickyT]

Check the wiring harness behind the EGR valve for burned insulation and shorted wires - a well known problem area.

(And I still don't believe you really killed your original ECU.)

 

[gorbilax]

The fusible links are fine... or at least they were before I replaced the ECU. I'll test them again, but I have a hard time believing it's a coincidence.... I'm leaning more toward the pinched harness theory. I'm going to go over the whole thing end to end on Sunday if I have time...

TrickyT... When I did the valve cover gasket, I had a pretty clear view of the EGR area and didn't notice anything, but I will check it again. Maybe I missed something. I hope I didn't kill the first ECU... but it's the only thing that makes sense.... I removed and reinstalled the first ECU twice when I was testing the harness and still no start. I think it's too much of a coincidence that I pop in a new ECU and it fires right up.... and then does the same thing ten minutes later and dies in the road...

 

[Brian894X4]

Just to eliminate it, are you sure there are no cracks in the intake hose in between the AFM and the throttle body? Since this part was removed and messed with. That would at least account for a rough running engine and no run, if the crack allowed enough air to enter.

As far as the wiring, the main wiring harness near the ERG valve tends to get toasted and bare wires are exposed. If the harness is not messed with at all, it could last for years, but once the harness is moved, bare wires start kissing each other and bad things happen. The most often result is throwing codes, but maybe the other wires are toast and its frying the ECU past any fuses. You may want to tear into this harness right where it almost comes into contact with the EGR and see what it looks like.

 

[davegonz]

Bad voltage regulator?

 

[davegonz]

You might also want to check the wiring near the firewall by the EGR crap. That's an area where the wiring insulation often fails.

Edit: Just read through the thread and it looks like you checked that.

 

[elhombre]

You didn't specifically mentioned this but did you peal back the insulation that is wrapped around the wiring harness by the EGR tubes? You need to unwrap it to get a good look at the wires. Don't try to seperate them and check individual wires because that WILL break off insulation and cause a new problem.

You also said you replaced 3 things at once, then it started working. Try reading the voltage at the MAF and IAC connections of the new parts, then swap back the old parts and see if there is a difference. I would also replace the ECU back to the old one and only replace each part one at a time to see if I get a change.

Stick with it!!!! Once you get this figured out, you'll be drinking a beer and laughing at this whole thing.

 

[gorbilax]

I briefly spoke with an electrical engineer this morning who told me that the voltage regulators in the ECU would protect it from over-voltage on the +12v input side. That leads me to rule out the voltage regulator on the alternator (which I can't really check anyway since I can't get the truck to run). He told me that the easiest/most common way to fry the ECU was +12v battery voltage shorting into a sensor lead (most of which use a regulated 5v reference voltage and have no protection against anything higher), so now I'm definitely leaning toward a shorted/cracked harness somewhere. I never inspected the EGR harness that closely, so I'll start there. I'm also wondering if coolant leaking from the old PHH may have ran down in something and shorted it out.

elhombre, the reasoning behind replacing all of those parts at once was that they were all pulled from the same truck, so I wanted to keep them together in case something of mine was killing the ECU. I already had trouble with those parts in the past, so I replaced them all to be safe. I'm definitely going to test the voltage to all of harnesses to the sensors in the area I worked.

The engineer I talked to told me that there wasn't anything the sensors could do to kill the ECU, as a direct short in them would only feed +5v back to the computer, since they are only ever fed +5v to begin with, which I suppose makes sense. I'm not sure what would happen if the 5v were grounded to the body, though.

In any case, looks like I have a lot of harness to inspect this weekend. I'll post my findings. Thank you all for the suggestions.

Does anyone have the factory wiring diagrams? Are they worth picking up? I only have the FSM.

 

[arcteryx]

gorb- PM me your email, I can send them to you.

 

[gorbilax]

Ok, TrickyT.... you were right. The ECU wasn't dead. After reading this I decided to test the reference voltage on the VCC pin (16-pin connector). It was reading at about half a volt when it should be reading 5v. According to Toyota, I have an intermittent short to ground on something using the 5v ECU ref voltage. I disconnected only the 16-pin connector to probe the pin itself and my check engine light came on. So I guess my engineer was technically right... a short to ground on a sensor can't fry the ECU, because my ECU hasn't been fried. It can, however, prevent the ECU from operating.

Now here's the frustrating part: after I reconnected the 16 pin connector, the check engine light stayed on. I back-probed the VCC connector, and it's still reading +5v. I turned the key, and sure enough, she fired right up. SO... what on earth shorts to ground until it's disconnected, and then stops shorting to ground? I suppose something on the 16 pin connector makes the most sense... back to the wiring diagrams....

 

[Brian894X4]

Have you looked at your wiring harness by the EGR yet?

 

[TrickyT]

...Now here's the frustrating part: after I reconnected the 16 pin connector, the check engine light stayed on. I back-probed the VCC connector, and it's still reading +5v. I turned the key, and sure enough, she fired right up. SO... what on earth shorts to ground until it's disconnected, and then stops shorting to ground? I suppose something on the 16 pin connector makes the most sense... back to the wiring diagrams....

Intermittent electrical shorts are like this. Simple things like change in ambient temperature causing metal to expand and contract a tiny bit and one moment you have a short and the next you don't. Or pulling on a cable as you try to gain access to connector pins to measure voltages causes a wire to flex a little and suddenly the problem goes away. But all of these point to frayed or burned insulation somewhere in your wiring harness, and the only way to find that is through careful inspection. Since this all started after you serviced the EGR VSV and R/R'ed the manifolds, begin looking there. In addition to the area around the EGR valve we've been telling you about, look for places that rodents might have munched on. (Someone here or on the Yahoo 3FE list had a horror story about going on a camping trip and having their wiring harness eaten during the night!)

Let us know when you've tracked it down.