Silver2000WS-6

iTrader: (1)

*Warning* I do not know the proper terminology for a lot of the things I am trying to explain so please forgive me.

I am trying to help my friend finish wiring a stock 1998 ls1 with stock computer from a Trans Am in his '81 Trans Am. He is also trying to use stock '98 f-body fans and has a stock fuel pump mounted in a stock tank with a Corvette filter/regulator combo. He has bought an aftermarket harness and had his computer tuned. I ASSUME they cut off the VATS, emisions, and etc. The aftermarket harness he bought already had the fuel pump relay wired in and the fuses for the necessary items. After getting power to the fuse blocks and buttoning up a few things the car crank and ran really well. The car had been cranked and ran probably 20 different times for three or four minutes at a time with no problems. The only thing I noticed was if you left the switch on, with out the car running, the fuel pump never shut off. I thought that was odd, but I hadn't really looked into that much because the car was running and all of that part was wired by a professional aftermarket harness company.

The problem began when I tried to help him wire in the factory '98 f-body fans into his fuse block and additional relays. I wired up the fan relays such that is was looking for a + voltage output from the low speed fan wire in the computer to close the relay and run one fan. The high speed wire was wired in the same way to another relay, but for the second fan. I know that this is not the way the factory swapped them from both running in series low then to parrallel for high speed, but I thought it would be simple and would work for him. However the fans never would come on so I started reading and I saw where the computer actually sends a - voltage out to the relay, so I swapped my ground to a positive 12v source. To test this, I turned the switch on, pulled the ECT sensor out and used a heat gun to heat the sensor until it was about 250*F. I put a multimeter on the low speed output wire on the PCM and never could get a change in voltage to command the relay circuit closed. Nor did the high speed output. I scratched my head all I could for the day and just left for the day. After I left my friend went to crank the car to move it and said it fired off, ran for a second, and died. He knew I had pulled the fuel pump fuse while playing with the fans because the switch was on and the pump kept running. So he started checking the fp fuse and relay. The fuse was good and he was getting power to the relay, but just not on the source out side of the relay. He swapped the relay and it cranked. He let it run for about five minutes with no more problems so he assumed the relay was just bad.

Now we went to crank it today and no fire on the back side of the fp relay again. So we put a new relay in and that did not fix it. I put a multimeter on the signal wire from the PCM to the relay and I could not get a voltage across it to ground and this did include within 2 seconds after turning the switch on. The pump would not kick on at all. After wiggling wires and checking atleast 20 times, I got a -6 volt differential between the fp relay signal wire and ground. I thought this was very strange since it is suppose to be +12 volts and the pump was still not coming on so I check it against the positive post and got another ~6 volt differential. So I feel pretty confident that it wasn't a bad reading. However the relay never triped to close the circuit and run the pump. I guess 6 volts just wasn't enough. After cycling the key switch I could never recreate these readings again.

I know it looks like I messed something up when I was working on the fans, but I did not touch anything on the fuel pump wiring except for pulling the fuse. I just can not understand why the computer is not commanding the fp signal to +12v. I have also checked numerous times to verify my wires are in the correct pins since the '98 apparantly has odd ball wiring. What all parameters have to be satisfied for the fuel pump to activate? Is it possible that it is a VATS problem even though I had ran it like 20 times before? Is there any other signal wires coming from the PCM that are suppose to be +12v that I can check to see if they are getting this? Thanks for any help and again I am sorry for the lack of proper terminology.

Chase

mantycarguy

I believe the fans in the 98 camaros are run in series for low speed and in parallel for hi speed. This is done with 3 relays with the ecm controlling the ground side of relay 1 for series/low speed operation and relays 2 and 3 for parallel/hi speed. If you applied 12volts to the ecm drivers directly it may have taken out those drivers and shorted out the board inside the ecm.

Silver2000WS-6

iTrader: (1)

I sure hope not, but it is possible. Is there any way to check it?

mantycarguy

Isolate the ecm control wires from each relay. Using a voltmeter, you can connect your + lead to bat+ and your other lead to each of the 2 fan control circuits one at a time while heating the coolant temp sensor. You get no voltage reading until the temp rises to approx 225 degrees for fan slow speed and 235 for fan hi speed. If you get less than 12+ volts on either circuit the ecm is cooked. Make sure to check your ecm grounds before chucking a good ecm.

Silver2000WS-6

iTrader: (1)

Thanks for the help, I will try to measure that again.

tommyboy

This diagram may help out too, its for 1998 specific models. Shows you how to wire up a standalone setup.

mantycarguy

After rereading your post its very well sounds as if you have a ground issue Check your ground wires from the pcm to the engine. When in doubt take the pcm ground wires and run a jumper directly to the battery negative post.

Silver2000WS-6

iTrader: (1)

Thanks for the input guys. I will start with chasing down all of the PCM grounds.

Silver2000WS-6

iTrader: (1)

Ok so all of the PCM grounds showed to be good however I noticed a few interesting things.

1) When the PCM grounds are disconnected and the key switch is on, both the high speed and low speed fan output signal wires go to +12v regardless of engine coolant temperature.. However when the ground is connected I get no output from these wires regardless of the engine coolant temperature.

2) With the PCM ground connected back and within the first 2 seconds of the key switch being turned, the PCM to fuel pump signal output wire would go to +4.5v then drop to +1.5v after that. This obviously was not enough voltge to activate the coil in the relay since the pump never came on. If I jump the fp relay and crank the car the signal wire voltage jumps up to 6.5 volts which still isn't enough voltage to energise the coil in the relay. So to me it sems like the PCM is sending output at the proper times, but just at half of the voltage. Does anyone have any ideas?

Thanks

deuceswildpacefl

try unplugging the temp sensor and see if the vehicle will run then if the sensor is bad it will not let the pcm put out a signal

lsxRanger94

A sensor shorted to ground will not let pcm control fuel and/or spark. If the temp sensor is ok. move to remaining sensors one by one until all have been checked before condemning the pcm. I have had the same experience and it turned out to be the egr shorted. I had no fuel or spark.

Silver2000WS-6

iTrader: (1)

I didn't really check the individual sensors. I only checked the PCM grounds so I will go back and check all the sensors. Thanks for everyones time.

71ANTICARB

iTrader: (23)

Did you figure this out? Got pretty much the same issue in my Chevelle.

jvert

iTrader: (1)

Did you figure out the solution to this problem? I'm dealing with the same thing right now. I believe my pcm is not sending signal to the fuel pump. Any help?

Thanks