1990 will not start - Chevy chassis

[Contributor]

Here is a YouTube troubleshooting your same issue on a 1992 RV with the 454. Pretty good guide.

 

[tilliepeople]

Your pretty close to right
I got a meter,testlight and a couple of jumpers.
Can I put 12 volts on the injectors or will that just fry them.
If I pull the ECM out where I can get at it i may be able to figure out which pin to check or jump.
The schematic shows the injectors go from the ECM to the injectors through a brake switch and back to the ECM which does not make sense to me

 

[Old_Crow]

Disconnect the plug at the injector, put a standard 12v test light across the terminals in the plug and crank the motor. Test light should flash. If it doesn't flash, no injector pulse.

 

[Mark_K5LXP]

Careful, be sure there isn't some other input missing like maybe a crank sensor that's inhibiting the injectors. You can throw parts at it and you can get lucky sometimes, but if you're not sure it's the ECM all you're doing is saving the troubleshooting time for later when the new one has the same symptoms as the old.

Mark B.
Albuquerque, NM

 

[Old_Crow]

Careful, be sure there isn't some other input missing like maybe a crank sensor that's inhibiting the injectors. You can throw parts at it and you can get lucky sometimes, but if you're not sure it's the ECM all you're doing is saving the troubleshooting time for later when the new one has the same symptoms as the old.

Mark B.
Albuquerque, NM

The more I think back on the reverse polarity situation that started this whole thing, the more I tend to agree with him about the ECM.

I'd still check injector pulse before I threw one in there. Crank sensor would be another thing to check, because with no crank signal, the ECM wouldn't know to fire the injectors.

A '90 ought to have an old style 12 pin ALDL connector under the dash somewhere. On that older model you could short across a couple of pins and get flash codes off the SES light. I think he stated that he also doesn't have an SES light on the dash, that might be another clue pointing at the computer.

 

[Ex-Calif]

Do you have the chassis manual?

Here is a description of the sensors involved.



The TBI unit itself incorporates the fuel injectors, pressure regulator and Idle Air Control valve.

Because you are not getting fuel I would pull the plug off the pump and turn on the key. If you get no power then move on.

I am a bit unorthodox in my troubleshooting and the next thing I would do is jump 12V to the pump to see if it pumps.

I don't know how sophisticated the OBD is for these old rigs but I would also check to see if any codes are being thrown, like crank sensor which IMO would be high on the list but also not something I would think would fail due to reverse polarity.

Note that the text says the first thing that happens is fuel pump on for 2 seconds - I interpret that to mean, "regardless of the sensors" - they come into play only after fuel pressure is detected by the ECM is what I interpret.

 

[Mark_K5LXP]

The "flash codes" you can get by jumpering the test mode pins on the ECM I'm not sure would reveal much since with all the electrical mayhem going on here I'd say any codes stored would be arbitrary. But, if it flashes anything at all it's an indicator it's "alive" and unless it does otherwise I'd still be checking inputs and sensors. My last throttle body car "died" and it turned out to be several sensors that went bad and I didn't know it because the car ran fine, until it didn't. Turns out that the ECM can disregard wildly erroneous values of a few failed sensors until enough data goes bad it can no longer "guess" what it should be doing. The only errors or failures that are required to trip the CEL are emission related so I discovered you can have enough wrong with the engine it won't run, but still not set the CEL. Fortunately even though it was a pretty intelligence limited ECM of the day it blinked out enough codes I was able to chase everything down.

Mark B.
Albuquerque, NM

 

[Old_Crow]

The "flash codes" you can get by jumpering the test mode pins on the ECM I'm not sure would reveal much since with all the electrical mayhem going on here I'd say any codes stored would be arbitrary. But, if it flashes anything at all it's an indicator it's "alive" and unless it does otherwise I'd still be checking inputs and sensors. My last throttle body car "died" and it turned out to be several sensors that went bad and I didn't know it because the car ran fine, until it didn't. Turns out that the ECM can disregard wildly erroneous values of a few failed sensors until enough data goes bad it can no longer "guess" what it should be doing. The only errors or failures that are required to trip the CEL are emission related so I discovered you can have enough wrong with the engine it won't run, but still not set the CEL. Fortunately even though it was a pretty intelligence limited ECM of the day it blinked out enough codes I was able to chase everything down.

Mark B.
Albuquerque, NM

The flash code I was really looking for a code 12, which would indicate that the ECM is alive.
I know that the ECM can ignore the MAP and BARO and maybe the TPS sensor, but without a crank sensor reading it won't fire the injectors. The crank sensor should also throw a code.
The spark is fired by the ignition module, so it could still have spark with a dead ECM. Running off starting fluid indicates that it does have spark and compression.

As to what Ex-Cali said, yes, the relay runs the pump for a few seconds, but without injector pulse all it's doing is pressurizing the lines. This is why I suggested checking for injector pulse first. If you have injector pulse, and no fuel spray, then you can worry about crawling under the coach to bang on the tank with a rubber mallet to see if you can shock the pump awake. If it starts at that point, the deal is to drive it to a place where you can drop the tank out of it before you shut it off.

 

[tilliepeople]

The good news is the injectors work fine. Jumped then, filled up the bowls and started it up. Ran for a whole 5 seconds.
started to follow pink/blk wire led me straight to EGR.OHM check gave no reading but that could be right as it does not turn on until running (vacuum). OR it could mean it's dead.
Tried sucking on the vacuum hose and i could hear the diaphragm moving but still no 12 volts but I was actually running the vacuum backwards.
At that point i gave up.
This thing has no check engine light, by that I mean there is no light to turn on.
I tried jumping the data link but the only thing it did was move the alt gage a little (not enough to read)

 

[tilliepeople]

I got a really strange question
I have a fuse marked (crank) but according to the books I have a 1990 does not have a crank sensor (96 0r newer only).
What am I missing

 

[Ray-IN]

I got a really strange question
I have a fuse marked (crank) but according to the books I have a 1990 does not have a crank sensor (96 0r newer only).
What am I missing

If you said I forgot. What year is the chassis/engine. Many MH's are built on a chassis the MH mfgr. purchases the prior year, I've read of a few MH's on a 2 yr old chassis.

 

[Old_Crow]

Assuming a Chevy chassis, I think (emphasis on "think") that if you remove the "crank" fuse that the starter is disabled. Easy enough to check.

 

[Contributor]

I think that crank position sensors are used in the newer fuel injected engines with individual injectors per cylinder. TBI is a glorified carburator that gets rid of the injector pump and still uses engine vacuum for the ecm to control fuel delivery based on throttle position. I could be wrong, but that's the way I remember it.

 

[tilliepeople]

According to the Haynes book I have the the crankshaft and camshaft sensors were only used in 96-2000 chevys.
I also note on pre 88 models the crank signal come from the starter solenoid which makes "old crows comment seem quite reasonable.
Also having a tough time finding a pinout for a 90 7.4(454)

 

[John Canfield]

I've been letting this thread run in the Winnebago board since I wasn't sure if there was something other than the chassis involved. Pretty sure this is strictly a Chevy chassis problem so I'm moving it to the Motorhome board.

Fingers crossed you can find/fix the problem.

 

[Contributor]

I just remembered helping a neighbor troubleshoot a chevy truck that wouldn't start. he had it towed to a shop and it turned out to be the ignition module. You may want to try that

 

[dktool]

Create an account and post questions here;

GM EFI Systems

TBI - TPI - MPFI - CFI - SFI

www.gearhead-efi.com

This is the single best source of help for gm obd1 systems.

 

[tilliepeople]

Can anyone help on this one?
I have a 1989 wiring diagrams
My question is that it makes reference to what I believe is different engines
I think I should be looking at (L05 or L19) but there are others that it could be.
It's a good read but I want to make sure that I am looking at the correct information


1989 WIRING DIAGRAMS R-V, P TRUCK MODELS

START (DIESEL) LL4, LH5 21
IGNITION (6 CYL.) L25 22-23
IGNITION (8 CYL.) LE8, LT9 24
IGNITION (8 CYL.) LO5, LT9 25
THROTTLE BODY INJECTION 26
FUEL CONTROL & IDLE AIR CONTROL (8 CYL„) LO5, L19

 

[Frizlefrak]

The news today is not too good.
I disconnected the fuel line at the carb and sure enough when i turned the key I did have fuel.
That means the injectors are not firing. I really don't know the procedure for testing them.
I do not have a Noid light and I tried the 9-volt battery trick with no results. That means either the injectors are bad, or the battery was to week to work.
I wrote down the numbers on the ECM, but they must be wrong because nothing comes up when I search for those numbers.
As far as I can tell there are a couple of switches left to check but I am not sure which way to turn.
ECM is around $100, and the injectors are at least $150 for a pair, and I really don't want to just throw parts at it.
Thank you for your patience with me

You noted that you "disconnected the fuel line at the carb and sure enough when i turned the key I did have fuel. That means the injectors are not firing." No, it doesn't.

Do you have a carburetor, or is this engine fuel injected? How many lbs of fuel pressure? Fuel just dribbling out of the line, especially on an EFI engine, isn't going to cut it. Fuel pressure needs to be measured with a gauge. Throttle body injection usually requires 9-13 psi of fuel pressure. Port systems typically require 40-45 psi. Even an old school carburetor needs about 4-5 psi. If fuel pressure is deficient, the engine isn't going to run, and then you need to find out why. Test for voltage at the fuel pump relay, then test fuel pressure regulator before replacing the fuel pump by pinching off the return line to see if pressure comes up. On GM OBD1 engines, you can supply 12V directly to the diagnostic port terminal G to cause the fuel pump to run full time, bypassing the ECM directives.

Noid light setup is cheap at Harbor Freight. If the noid light test determines your injectors aren't firing, before condemning the ECM, measure voltage at the Throttle Position Sensor. It's possible that the sensor is stuck wide open, which the ECM interprets as "clear flood mode", and then disables fuel trim.

That should keep you busy for a while. Troubleshoot, don't guess.

 

[tilliepeople]

After checking and rechecking pretty much everything I broke down and ordered an ECM.
I watched a millionish videos and read a dozen books and did all the tests that I could do with the equipment I have.
Some checks indicated a short or open circuits while others were inconclusive or drifted toward an ECM
I decided it was time to try an ECM and if i was wrong I could always return it.
as soon as I turned the key to on I knew I had made a good choice. I heard the injector fire once, so I turned it off and back on just to make sure and sure enough they both fired again.
It cranked over a few times but sure enough it started.
Now all I have to do is put everything back together.
Thanks for all the suggestions and guidance without which I would still be at it