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GM Tech (Cam)
GM Tech (Cam), Pontiac Technician
Category: Pontiac
Satisfied Customers: 2373
Experience:  GM Grand Master Technician 2007. 14 years experience.
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I'll try to make this simple. 1998 3800 v6 firebird. Cranks

Customer Question

I'll try to make this simple. 1998 3800 v6 firebird. Cranks but no run. Security light goes off after 5 seconds. No fuel pressure. When I turn key on, I get 12v out of the PCM to the fuel pump relay socket pin that operates the relay coil. When I put the relay in the socket it will not click. I have tried 3 relays. Same result each time. With the relay out of the socket, if I take the 12v from the pcm directly to pin 1 of the relay coil, and ground the other coil pin #2, to batt neg, the relay won't work, and the voltage goes to zero as long as it's connected, and back to 12v when I disconnect a jumper. I have checked all ground wires to the pcm and they are good. What do you think? I recently had the engine rebuilt. The very first time I cranked it , it ran fine, but now won't start.
Submitted: 2 months ago.
Category: Pontiac
Expert:  kalamykid replied 2 months ago.

Are you getting a positive power on the trigger wire to the relay from the pcm or a ground?

Customer: replied 2 months ago.
According to my wiring diagram there is only one wire, a drkgrn/wht , from the pcm to the fiel pump relay coil. It is labeled "fuel pump rly ctrl" and that's the wire from the pcm I am talking about that has 12v until the relay is plugged in. I also jumpered a small 12v lamp between this 12v and batt gnd and the light would not light and voltage went to Zero.There is no ground on it. It goes to 12v when I turn the key on. Is this the trigger wire you ask about?
Expert:  kalamykid replied 2 months ago.

Yes that is the trigger wire. The computer should turn it into a ground to activate the relay. I have to step out for just a few minutes. If you don't mind let me try to go run an errand real quick and get back to my desk computer. Then we can try and go over what's really going on and try and find your problem. That why I shouldn't have positive power on it ever.

Customer: replied 2 months ago.
OK By the way, If it never has power, how is the relay going to operate since this wire goees to the relay coil??
Expert:  kalamykid replied 2 months ago.

You have to fuse power source is going to the relay. Then the computer as a ground to complete the circuit to activate the relay and then the relations positive power out the fourth part To the fuel pump

Expert:  kalamykid replied 2 months ago.

I'm sorry that should be two fused porch I'm driving down the road please let me get back to my desk so I can help you.

Customer: replied 2 months ago.
Good idea, as I have no idea what your last message said. I think you were saying that the computer completes a ground via the drk grn/wht wire for the relay to operate. But the other side of the relay coil is solidly wired to ground already. The opposite coil pin goes directly to the pcm on this drk grn/wht wire. There is no ground on this wire.I have checked and confirmed that????What is a fused porch?
Expert:  kalamykid replied 2 months ago.

The pcm gives the relay its ground, there are two fused power sources to the relay, one is hot at all times, the other is hot when the ignition switch is turned on. The the 4th pin of the relay will be hot when the pcm gives the relay a ground signal. the 4th pin goes to the fuel pump.

Customer: replied 2 months ago.
That makes sense because normally you want to apply ground to close a circuit not 12v power. The wiring diagram I have differs from what you are saying in that It does not appear to apply a ground to the relay.. Pins 1 and 2 are the relay coil. Pin 2 is hard wired to ground. Pin 1 is connected to the pcm on the drk grn/wht wire that goes to 12v when the key is turned on ( its' labeled fuel pump relay control).. Pins 3 and 5 go to the relay contacts. Pin 5 goes to the fuel pump. When I put 12v from the batt on pin 5 the fuel pump runs. Pin 3 is hot at all times. I have traced each one of these wires out and they match the wiring diagram.. The relay only has 4 pins. Pin 1 voltage goes to 0 as soon as a relay in put in the socket.. I am confused?
Customer: replied 2 months ago.
I was gone these minutes to double check the voltages. And I haven't been drinking or watching football.
Customer: replied 2 months ago.
If it'll help, the relay is a small omron relay.
Expert:  kalamykid replied 2 months ago.

Let me pull up the wire diagram for this car. If you look on the relay it will have a 87a,87 and so on. With you saying pin 1, 2 and 3 I do not know what pin is what. There is no ground feed to the relay except for the ground from the pcm. You will have two pins with 12 volts with the ignition switch on, the wire to the fuel pump will have a ground on it, but that is from the fuel pump itself, and last you will have a dead lead from the pcm that will turn into a ground when the pcm comands the fuel pump on.

Customer: replied 2 months ago.
I am considering wiring the fuel pump relay coil to a 12v source that comes on when the key is turned on. If the car runs, I won't need that pcm function. Would that be a problem? Cars used to be that way..
Expert:  kalamykid replied 2 months ago.

The only problem with that is the fuel pump may keep the relay active and keep running.

Customer: replied 2 months ago.
The relay I have is an omron The four pins are numbered 1,2,3, and 5. I know it's confusing. If you draw a rectangle representing looking at the the relay bottom, with 4 pins, one in each corner, the top left pin is # 3, top right is # 1, bottom left is # 2 and bottom right is # 5. 1 and 2 are the coil. 3 and 5 are the contacts. There are no 87 or other 80 numbers. Like you I am used to relays with 80 numbers.The numbers are katty corner so you can't put the relay in wrong. Not sure if this will help. My real question is why does the 12v from the pcm go to 0 under load and not operate the relay? If this is the way the car was wired, it would seem there is something wrong with the pcm. But, I don't want to randomly change pcms without some reason..That's why I need help.
Expert:  kalamykid replied 2 months ago.

With the ignition switch off, how many pins in the fuse box with the relay out have 12 volts to them?

Customer: replied 2 months ago.
I will check..
Expert:  kalamykid replied 2 months ago.

also check it with the ignition switch in the run position please.

Customer: replied 2 months ago.
My apology. That is wrong.. There is 12v to only # 3 pin with key off. The other three are 0v.
Expert:  kalamykid replied 2 months ago.

Now check it again but this time turn the ignition switch to run please.

Customer: replied 2 months ago.
With key on there is 12v on pins 3 and 1.
Customer: replied 2 months ago.
Do you have any inkling that it might be the pcm?
Customer: replied 2 months ago.
I don't understand why it's wired this way.
Expert:  kalamykid replied 2 months ago.

Now with the ignition switch on, see what pins have a ground please. You will need a helper to see if you have a ground to two pins, onw will be constant but the other will only have a ground for 5 seconds or less.

Customer: replied 2 months ago.
Took a bit. # 2 pin has 0 ohms with key off, and goes to 30k ohms with key on. #5 pin has 0 ohms with key off and goes to 24k ohms with key on. Both hi ohm values stayed high as long as key was on.There was no 5 second or any variation of it of any kind. These ohm values are way to high for good compuer operation.
Customer: replied 2 months ago.
There was a braided ground cable that came off the car. I think it may have gone between the engine and car body. I know it was on the driver's side and I think went under the steering wheel. Can anyone tell me where to hook it?
Customer: replied 2 months ago.
That should be under the steering wheel shaft inside the engine compartment.
Expert:  kalamykid replied 2 months ago.

I did not ask for an ohms reading. I need to know what pins Have a ground on them. Hold your red test lead to the battery positive post, and check he pins with your black test lead with the relay out please. Make sure you have your meter set to dc volts.

Customer: replied 2 months ago.
Not a very respectful reply! Pin # 2, 12.15v: pin# ***** 12.15v: pin # 2, 0v: pin # 1< 11.59v.
Expert:  kalamykid replied 2 months ago.

I am sorry, but when you give me information that tells me nothing instead of what I asked for, you are wasting your time. Did pin have a ground for at least 4 seconds after the switch was turned to the run position?

Customer: replied 2 months ago.
All the readings I gave you went to that value and all stayed there as long as the switch was on.
Expert:  kalamykid replied 2 months ago.

You are not making any sense. You said that pin 3 had 12 volts on it with the ignition switch off, but in your last post there is no mention of pin 3????? You still have not told me what pins have a ground on them. If you do not want my help please tell me and I will see if another expert can help you.

Customer: replied 2 months ago.
First of all, you did not tell me which pin, so I stated what happened with all the pins. Secondly, I am a graduate electrical engineer with decades of experience in troubleshooting and working on cars, and we do not measure grounds by voltage. A good ground is a connection measured by having o ohms of resistance. If they read in the thousands of ohms, they are not good grounds, especially for computer work. You can still measure 12v on a bad ground. I have told you a lot with the resistance values I sent . If I said it was zero, that's a good ground. If 60k ohms that's a high resistance ground and not good. . It 's up to you to interpret them. Apparently we are speaking different languages, and I resent you implying I don't want your help, as I'm paying money for an answer!. For some reason you seem to think I am joking around. You couldn't be more wrong. You might want to relook at what I sent again. All I want is someone to tell me if it's the PCM. And, I think another tech would be a good idea.
Expert:  kalamykid replied 2 months ago.

I do my job and do it well. I have helped people that did not even know what a digital volt meter was fix there cars, men and women. I have even helped an older man rebuild a transmission over the internet. You want me to help you then please do as I ask and do not try to over think this problem. What I was looking for was a signal from the pcm to the relay. with all the information you have given me, the pcm trigger wire does not have positive power on it EVER. NJow if you would like to have me help you then just say the word, ME OR ANOTHER EXPERT.?

Expert:  kalamykid replied 2 months ago.

From what I have seen it sounds like you have had the relay in the fuse box but not all the way down and trying to test it that way. The only problem with that is the relay is giving you back feed and voltages from the fuses not the pcm.

Expert:  kalamykid replied 2 months ago.

Now I feel like a fool. I just looked up the wire diagrams for this car and it is totally different than most gm cars. the pcm does send a 12 volt power source to the relay. and this should activate the relay. we know that you have the 12 volt constant hot, so that means that the ground to the relay has to be bad, or the relay itself is. I am sorry if I made you feel bad, but that was not my intentions. I just wanted you to slow down and stop over thinking the problem. Can we keep working on this to find your problem? You do not have to pay me for the help.

Expert:  kalamykid replied 2 months ago.

Please understand that I am working with more then 10 people at a time and when half of them do not do the tests I ask for, it only takes more time to get to the root of the problem.

Customer: replied 2 months ago.
I appreciate that, and I understand, but I did what you asked. Also, I used a wire piercing tool for readings so I could keep the relay in the socket. In an effort to eliminate the socket as the problem, I took the relay out, jumpered it between the 12v from the pcm and directly to the battery neg. post. It did the same thing. The relay wouldn't work, and the 12v went to zero. I tried 3 different relays that work fine across the batt + and -. So, my question is have you seen this before, and do you think the pcm is the problem.
Expert:  kalamykid replied 2 months ago.

No the pcm is sending out the 12 volts to activate the relay. What we need to find out is why you do not have two grounds at the relyas pins. You should have the ground from the fuel pump And I am assuming that would be pin 1 according to the data you sent. Please look back over your responses where you said pin 3 had 12 volts?? But in the later data you omitted pin 3??? Please help me understand this.

Customer: replied 2 months ago.
Isn't the ground question secondary to finding out why the 12v from the pcm won't actuate the relay and goes to zero? I can fix no grounds by cleaning up the socket wire connections. I had a dead solid ground when I checked the relay to the batt neg post and it still wouldn't work. I have checked all ground wires to the pcm with the piercing tool and they are all under 7 ohms to the batt neg post.. Apparently you are under a lot of pressure. Don't let it get to you. Of course we can continue, and I certainly will pay you.
Customer: replied 2 months ago.
Pin 3 is one of the contacts that goes to the pump. It has 12 volts all the time with the key on. when the relay is activated and the contacts close, pin 3 is connected to pin 5 that is wired directly to the pump. By the way, when I put 12v on pin 5 from the batt, the pump runs.
Expert:  kalamykid replied 2 months ago.

If pin 5 goes to the fuel pump, why does t not have a ground? The ground should come through the fuel pump and back up to the relay. Do you have a test light? can you connect it to a good ground and probe pin 3 and see if the light comes on for 5 seconds and then goes out?

Customer: replied 2 months ago.
The fuel pump and the fuel pump motor are grounded to the body ground. I have already verified this back at the pump itself.
Expert:  kalamykid replied 2 months ago.

Ok but your readings say that there is no ground coming from the fuel pump to the relay correct?

Customer: replied 2 months ago.
That is correct. The only wire between the pump and relay is a gray one from pin 5.
Expert:  kalamykid replied 2 months ago.

And what about this question? Do you have a test light? can you connect it to a good ground and probe pin 3 and see if the light comes on for 5 seconds and then goes out?

Customer: replied 2 months ago.
I did as you suggested. I did not get any light at all.This is for the initial 4 to 5 seconds to build pressure when first starting? It goes off until you begin cranking?/?
Expert:  kalamykid replied 2 months ago.

Yes that is correct. There should have been 12 volts to the pin for at least 5 seconds. Can you do one more test for me please? Do the same test but this time along with th test light use your digital volt meter and see if you have any voltage with the test light connected and then with the test light disconnected form the battery.

Customer: replied 2 months ago.
with the light connected I got a voltage that varied between 7 and 8 v. It did not stay steady, but stayed betwween 7 and 8. Without the light I got 12 .5 volts steady.
Customer: replied 2 months ago.
I checked and 12.5 was the batt volts exactly.
Expert:  kalamykid replied 2 months ago.

I hate to say it but it sounds like something has either rubbed through the trigger wire, chewed through it, or the solder joints in the pcm have come loose and you are not getting the full amperage though the wire to the relay. The first thing I would do would be to check the pins in the pcm plug as well as run the wire back from the relay to the pcm and see if there are any wire rubbed almost through or chewed through. If you can not find any problem with the wire or the connector at the pcm, then the problem will have to be in the pcm. I am 90% sure that you will find a bad place in the wire from the relay to the pcm.

Customer: replied 2 months ago.
Thanks for your help.
Expert:  kalamykid replied 2 months ago.

You are very welcome.

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