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Discussion Starter #1
Working on an older Cadillac, 83 Deville, 4.1L.

Had an issue with no 12V to the injectors and have already solved that. We are currently not getting an a Negative pulse. While I haven't dug in to this issue much yet but wanted to maybe check to see if there are some known items to focus on.

Right now:

Fuel Pump is working, proper pressure
12V to both injectors with IGN On
Oil Pressure light works and goes off when running
Fuel injectors are now working but only when a negative trigger is jumped to the injector

I see in the schematic that the oil pressure sending must work to get a pulse and appears to be working. I have not traced the wires back to the ECU yet but that will be the next step.

Does anyone have a flow chart or list of sensors required to obtain the negative pulse aside from the Oil Pressure sending unit?

We just got a fresh battery in the car today so I am not sure if any codes are present yet but hope someone may have some information for me.

Appreciate any feedback you can provide
 

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95 FWB 81SDV 96 FWB 94 Fleetwood
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The only time I had this happen to me it was a power transistor in the ECM. But it was only one transistor, so one injector was still working.It would just about run and stall if I touched the gas.
 

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The injectors have 12v to them all the time and the ECM gives them a ground to open them. Thats the best way I understand it.
 

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Discussion Starter #5
The injectors have 12v to them all the time and the ECM gives them a ground to open them. Thats the best way I understand it.
Exactly, most injectors have a constant 12V + and they are energized by a negative from the ECM. I am just wondering what else besides the ECM is responsible for sending the negative trigger. I know the Oil Pressure sensor is one thing that is required for the ECM to provide the - but not surer what else on something this old. I have the schematic but not a great copy and is a little blurry.

Any help is appreciated.
 

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Discussion Starter #6
This is what I found so far, but have nothing about the circuit before the ECM. The newer vehicles will use a cam or crank sensor as the input to trigger the injectors but this vehicle is to old to have those.

Does anyone have a more detailed schematic for this vehicle or can point me to one?


Screen Shot 2020-02-09 at 5.59.51 PM.png
 

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Older cars used the distributor pickup coil to send a signal to the ignition module then to the ECM.
No ignition pulse to the ECM then no pulse.

For cranks no start it's important to check for spark also.
If it has no spark then work on that first as it may be the cause of no pulse.

Also they had a clear flood mode. Press the gas pedal to the floor & the ECM would cut out the fuel pulse.
Designed to correct a flooded car. Trouble is when the TPS sensor went bad it could give the ECM a false clear flood reading. Besides checking for codes with a paperclip and or scanning the computer codes / sensor readings there was another way to eliminate this possibility. Unplug the TPS sensor to see if it gets fuel pulse or starts.

Another cause for the injector pulse to cutout is the resistance on the injectors got out of spec shorting out the ECM. Or something else shorting out the circuit. The circuits are ECM internally protected so that when the problem is fixed it should work. But there is always a chance the ECM got smoked. Normally it happens to just one of the circuits (injectors) at a time. They did have a problem with the injector resistance going out of spec. Check with an ohm meter. I can't remember what they should be. But they need to have some resistance & should be about the same.

ECM's also just crap out. Sometimes it's from something it was connected to caused a problem. Such as the OP orignal issue or the dianosis related to it. It's kinda rare that several things that cause a no start all go bad at the same moment.
 

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Discussion Starter #8 (Edited)
Just verified the
Older cars used the distributor pickup coil to send a signal to the ignition module then to the ECM.
No ignition pulse to the ECM then no pulse.

For cranks no start it's important to check for spark also.
If it has no spark then work on that first as it may be the cause of no pulse.

Also they had a clear flood mode. Press the gas pedal to the floor & the ECM would cut out the fuel pulse.
Designed to correct a flooded car. Trouble is when the TPS sensor went bad it could give the ECM a false clear flood reading. Besides checking for codes with a paperclip and or scanning the computer codes / sensor readings there was another way to eliminate this possibility. Unplug the TPS sensor to see if it gets fuel pulse or starts.

Another cause for the injector pulse to cutout is the resistance on the injectors got out of spec shorting out the ECM. Or something else shorting out the circuit. The circuits are ECM internally protected so that when the problem is fixed it should work. But there is always a chance the ECM got smoked. Normally it happens to just one of the circuits (injectors) at a time. They did have a problem with the injector resistance going out of spec. Check with an ohm meter. I can't remember what they should be. But they need to have some resistance & should be about the same.

ECM's also just crap out. Sometimes it's from something it was connected to caused a problem. Such as the OP orignal issue or the dianosis related to it. It's kinda rare that several things that cause a no start all go bad at the same moment.


Thank you, appreciate that. I verified the Negative wires from the injectors to the ECU and they are good. I do see on the schematic that each injector has it's own circuit so it is hard to believe both went at the same time but someone looked at this vehicle before me so who know what they could have caused.

I have the book ands schematic on the way, but my guess was the distributor as there are no cam crank sensors on the older models.

I will check out the TPS, I certainly didn't even consider that so it is new to me. Aside from that I am either missing the input to the ECM or the ECM is bad. The injectors do work fine and test fine otherwise.

My schematic is missing the pin outs of the ECM so without them I am just guessing.

Thanks again, appreciate your feedback.

Edit: It has spark and will start and run when a - is added to the side of the injector. We did have a missing + which has since been fixed. We found a broken connection in the aux relay panel to power the injectors and also corrosion in the ignition switch connection.
 

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1980 FBC
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You are saying the car runs when you ground one side of an injector?

Somewhere in the past I remember one of these where it has spark and no injector pulses from the ECM.

Can't remember what made the problem go away.

Do you have access to the factory trouble trees?
 

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Back in the day we used a "noid" light to check for injector pulses. One really needs to put some load on the circuit to test it.

Most VOMs will show voltage even when there's almost no available current. That fools a lot of people.
 

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Yes Apeman When my 81 still had a 368 with that 8-6-4 . I had one of the power transistors burn out and only injector was working. The thing started and idled , not smooth, and I drove it for 2 miles at 2 mph and got it home. Changed the bad transistor and it was fine.
 

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A family member had an '81 back in 1984. He bought it for a song because of all the hoopla about how they were no good.

It blew one of the 3 amp injector fuses and would run very poorly.


Yes Apeman When my 81 still had a 368 with that 8-6-4 . I had one of the power transistors burn out and only injector was working. The thing started and idled , not smooth, and I drove it for 2 miles at 2 mph and got it home. Changed the bad transistor and it was fine.
 

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Never tried to ground the negtive side of the injectors to get an engine to run.
It's meant to be a pulse, not just grounded constantly.
At an idle the noid light would be dim & brighten with higher RPM.

IDK what happens to the injector & ECM when the circuit is shorted to ground.
Perhaps backfeeding the ground to the ECM could have caused damage to the transistors.
Or mayby the injectors overloaded / overheated from the constant 12v+ & 12v-.

As the ECMs evolved they seemed to be more robust against circuit shorts & overloads.
In 83' that was not the case. They were a fragile sensitive piece of electronics probably developed in the late 70's then value engineered.

Wiring could cause this problem if there is a fault between the ECM & injectors or ignition module.
But I would say it's highly unlikley unless there is clear damage to the wire harness.

Again 2 injector circuits with the same problem is also highly unlikely.
But to have the 12v+ problem first, then this problem with both injector circuits failing, is strange.

Don't know the cause & solution for the orignal problem.
Sorry to say but I would guess the ECM may be smoked.
Possibly damaged injectors also.

Simplest way to test with an injector noid light & both injectors unhooked.
They are cheap to buy or can even get them on loan from some parts stores.
And test the resistance across both injectors.
 

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Is the ECM generating any diagostic codes or activating the Service Engine light? The ECM needs a signal from some type of cam sensor to activate the injectors. I'm not sure how primitive these early 4.1's really are. I only worked on 4.9's.
 

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Discussion Starter #15
This vehicle doesn't have cam sensors, it takes the input from the distributor. While I am waiting for the book, I grabbed some schematics from ALL DATA and have now found the following:

No 12V at the computer, I am reading 7V, went to the fuse directly and only have 7V there too. There is another power line is a 3amp fuse which I believe is for the memory with 0 Volts, so that appears to either be the problem or just the next problem to solve. I don't have those schematics at this point but should be here any day now.

I measured the voltage at the fuse even with the computer being disconnected with both the key off/on/start positions and have the same thing.

I also noticed and maybe I should have thought previously that I also do not see any 'Service Engine' light on at all, I remember noticing that before but blew it off as I didn't have 12V at the injectors so I solved that issue first. I do get the Oil light, Seat Belt light etc but see no Service Engine Soon light.

While the voltage drop is my next step, am I correct that there should be a Service Engine Soon or Check Engine light of some sort on this model? I would expect it to be on when the vehicle ignition is in the RUN position and not started.

Can someone confirm that?
 

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Discussion Starter #17
The light should illuminate when cranking. There is an input to the ECM of 7 volts.

That is good to know. I am waiting for the book still but will see what else ALLDATA has.

Thanks for your input, I was 'NOT' expecting to see 7V but the other is still an issue as there is 0 Volts on the other fuse.

I did not see any sign of the Check Engine light with the ignition in the run or start position. I do see the oil light come on and disappear once cranking but no check engine light at all. I will check again but did not see it which I thought was strange.

Do you know where the CEL is located? I assume in one of the places next to the other lights so maybe it is even a bad bulb.
 

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Discussion Starter #18
The light should illuminate when cranking. There is an input to the ECM of 7 volts.

According to the schematic, it should be 12V on that line. PIN 15 & 16 on the P2 Connector shows that it should be 12V Batt and 12V Ignition. I have 7 on one, 0 on another so that will explain the CEL light not coming on.

Appreciate your input though, greatly appreciated.
 

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Discussion Starter #19 (Edited)
So I found a broken wire at the fusible link on the power distributor by the battery, I repaired that and now I have 12V at the AUX Fuse Block for the ECM, I checked and also have power at the ECM along with solid grounds.

Now however, I still don't have a Service Engine Light nor can I read the codes from the computer. So at this point I think the computer is bad.

IMG_2619.jpg IMG_2616.jpg

Those are the only lights and no response for the codes.

The one question I have is this plug by the fuse block, I don't see it connected nor can I see anywhere to connect it. Does anyone have an idea as to what it is used for? It may be for options this vehicle doesn't have but figured I'd ask. It is located right by the main fuse block. There are some plugs like it but they are plugged in above the fuse block.

IMG_2621.jpg IMG_2622.jpg

Hope this helps the next person.
 

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I recall there being a 7 volt input into the computer in addition to two 12 volt sources. The two service lights on a RWD are amber should stare you right in the face with the engine cranking. Possible two bad bulbs, but the other cause is no communication from the ECM, which sounds like you have traced to a power problem. The lack of diagnostics confirms a lack of communication. Check all pins of each of the three connectors and try to pull the tang that engages the end of the board down a little bit. Clean contacts with an eraser, etc.

That is a connector used in production, likely to prime the fuel system. There are a couple of them throughout the harness.
 
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