I did a search here and in the fuel section and didn't see anything similar.

I completed the stock LM7 swap into my Supra & have gotten almost all of the bugs worked out. The motor is a 2003 LM7 truck motor with 1998 wiring harness & PCM along with 2002 stock F-body injectors. The PCM related bugs I have not been able to overcome are:

1) The PCM will not control the fuel pump. I double checked the green/white wire that grounds the relay is going to the right pin on the PCM, and it has continuity from one terminal to the other. I have to ground the wire to the relay to get the car to start. Once it starts, it runs fine. I am running a VATS bypass box, and it has power and the blue wire to the PCM is intact and also has continuity. The PCM was reflashed by Keith at HP Tuners with a stock 98 F body tune. I have also tried a second 1998 PCM with the same result. I also disabled the VATS in HP Tuners with no improvement.

2) The tach does not work. I am running an adapter box for this which requires 4 wires: switched +12V, ground, tach in from PCM and tach out to vehicle. I have again tested the wires for continuity and they are OK. The box self-test feature (flashing LED sequence) works OK when I supply +12V and ground until I connect it to the harness. The box is calibrated for the LSx motor to a 6 cylinder tach, which is correct for my swap. It also shows OK with the box harness connected and the PCM 80 pin connectors off. I have tested power and ground for both boxes and all is OK. Based on all of this, it appears there is a problem with the tach output from the PCM.

I'm in the process of checking the wires to/from the PCM, but this will take a while. So far, the only question I have is the Evap stuff- there is a wire to the valve and another to the solenoid. I have the valve underhood and it is wired to the PCM OK (green/white to PCM, pink to +12V). There is a white wire that goes to the Evap solenoid, which of course my car does not have, so this wire was clipped outside of the PCM. I'm not sure if this would have any effect on the PCM turning on the fuel pump.

I was getting an EGR and a fuel tank code on the HP Tuners scanner.

Thanks for any help or advice.

EDIT: Short Attention Span Summary-
Thanks to Speartech and the others that posted here & PM'd me on this. It appears both problems were due to conflicts in the Toyota wiring that I did not anticipate. I bypassed the Toyota relays and used new wiring for a new fuel pump relay and I had to rewire two wires in the dash for the tach problem.

 

Updated info-

I had an oil leak that prevented the car from running for testing. Once corrected, I noticed the tach works fine with the transmission in reverse, but not any other gear.

In my HP Tuners setup, I have the PRN switch disabled. My harness is from a M6 car and the speed sensor from the Tremec trans is connected to the PCM where the T56 would normally be, both are 4K PPM output sensors. Any thoughts on why this is happening?

I have tested the adapter box and it is working as it should. Since the tach is operational at some point, I know the Toyota wiring to it is OK. The issue seems to be with the PCM somehow.

The fuel pump still has to be jumpered, it does not run off the PCM regardless of the trans position.

I should have a manual with wiring diagram soon so I can see what wire goes where.

 

The green/white wire from the PCM doesn't ground the relay, it provides 12 volts. So no wonder the FP relay isn't working.

Some of the other stuff doesn't make much sense to me. I see no relation between reverse gear and the tach working or not.

 

Thanks for the info. I must have carried that over from the LT1 wiring I did on the 280Z, I know the PCM FP wire grounds it there.

Yeah, the only in reverse is a challenge, but I've got plenty of time to overcome it.

Thanks again.

 

The PCM must put out miniscule current here. I tried a standard 30A relay to reverse the polarity & it doesn't have enough power to operate the relay. I did get +12V on my voltmeter, so the PCM is working correctly here.

I'll look for a lower capacity relay at Radio Shack & this should take care of the fuel pump issue.

 

You must have a weird relay there. The PCM has plenty of current capability to drive any normal relay.

BTW, ALL GM applications are 12 volts from the PCM to the FP relay, including LT1's.

 

I checked my diagram for the LT1 280Z & it shows plug 220-J being a green wire that grounds the fuel pump relay (it goes to terminal #85 in the shop manual if I recall correctly). That one had no fuel pump issues, so I know the wiring was not an issue.

I'll test the 1998 PCM for current output tomorrow & see what it is doing.

Thanks for the advice.

 

I don't understand why you are running a VATS disable box if the VATS is disabled in the PCM. This could be your problem, but maybe I'm missing something.

The fuel pump is run off the same 5.0v reference circuit as the TPS and AC pressure sensor. If you are getting code P01639 with fuel tank pressure code PO452, then the 5.0v circuit has a short. Usually (not always) a TPS code will be set as well. A bad AC Refrigerant Pressure sensor may also cause a short.

A quick test is to turn the ignition ON (engine off) and use a digital multimeter to probe the PCM side connectors of the TPS, AC sensor and fuel pump. They should all read 5.0v. For the fuel pump, it's important you test the harness before the relay. If both the TPS and AC sensor have voltage, but the fuel pump does not have voltage before the relay, the fuel pump wiring is bad. If none have voltage, there is shorted ground somewhere.

Worst case scenario would be a bad PCM, but if you swapped the PCM with known good one, with the same results, it's not the PCM.

 

Why don't you guys stop with the mis-information. The fuel pump does NOT run off of any 5 volt reference. Where do you come up with this stuff???

The fuel pump relay does NOT get grounded by the PCM on any GM applications!

The PCM puts out 12 volts to power the fuel pump relay; not 5, not ground; TWELVE VOLTS.

Here's the LT1 diagram show C220-J as the FEED to the relay, not the ground.


We ship hundreds of these harnesses out of here every year. I may have just a little more experience with these than the 1 or 2 harnesses you guys may have piddled with!

 

Speartech - I thought your were pretty cool, but there is no need to bash and demean me by trumping your expertise. I am not spreading misinformation. He mentioned a 98 PCM, so I posted based information based on my experience with the same year PCM and similar issue I had. I got it straight from the 1998 GM manual. Yes, the 98 PCM does have a 5.0v ref volt for the circuits I mentioned. Check your manuals. I'll leave now and let you figure out his problem. F-ing aye.

 

DrkPhx: Sorry, I'm not trying to come across as bashing or demeaning. I guess my real point is that basically ALL GM systems operate the fuel pump and fuel pump relay the same way.

It doesn't vary by year, model, or engine, they're all the same. He "knows" how LT1's did it and then you offered yet another alternative. I'm just trying to state the facts.

Didn't mean for it to come out as harsh as it sounds now that I re-read it.

 

No problem. I'm certainly no expert, I just know how aggravating it is to chase electrical problems related to fuel pumps on these cars.

 

FWIW, I don't know much, that is often why I post here. The reference to terminal #85 being a ground was due to my using Bosch aftermarket relays with diodes that showed terminal 86 as a +. Accordingly, I concluded 85 was a ground, but of course the factory diagram has shown that is not the case. That car is not in my garage right now, but I will look at the wiring when I see it next.

I appreciate all the help here, I'm always willing to listen to alternatives and sort things through. I don't have my 1998 full wiring diagram yet, so when this arrives it will be of great assistance.

I've already excised several electrical demons on this car such as the one tail light that didn't work (corroded socket) and the fuel gauge that didn't work (varnished contact on the sender). I picked up the solution to the second one on a Supra forum after tracing wires, taking the dash back out & checking the gauge.

This forum is 99.9% great to post in, especially considering others I belong to. I got decapitated the other day on an audio video forum when I asked a question about two different types of remote control setups. The "professional" that responded ASSumed several things about me, all of which were incorrect.

How about tach outputs while only in reverse?

 

Thanks for the reply. I think the fuel pump relay polarity issue has been resolved.

Part of my problem may be that since this is a transplant, there is no fuel tank pressure sensor or AC pressure sensor (using the Toyota system)- these wires are not connected to anything from the PCM.

I initially bought a VATS bypass box before I bought HP Tuners software so I could disable the VATS myself. I've tried it with the VATS enabled and disabled in HP Tuners with the same result.

I tested the output on the PCM fuel pump wire, and it is 1.92A for a couple of seconds then it drops to a trickle, so it looks like the relay I was using is bad. I'll switch it out with a new one and see if this does the trick.

 

Put a new 30A 12 volt relay in tonight & got the same result . I think it is time to work on something else for a while.

 

Check the resistance of the relay your are using. Either it requires too much current to be used in this application or something else is wrong. Also, check the resistance both ways; from 85 to 86, and from 86 to 85. This will let you see if it uses a transient diode or not. If it does then the the polarity matters when wiring it up; if not, it doesn't matter which is positive or negative.

We use these high current relays all of the time with no problems at all:

 

Thanks for the additional info Speartech. I suspect it is in the "something else is wrong" category. I found the Toyota uses a "circuit opening" relay in series with the fuel pump relay. I am going to bypass both of these and run a relay for the pump with new wiring.

When I laid this setup out, I was trying to mirror the functions and wiring of the Toyota ECU (engine control computer, their version of the PCM/VCM) with that of the LSX one. I guess it was not too bad for a first attempt as everything except the tach and fuel pump worked. The SES lamp, oil pressure, volts, and coolant temp are all 100%.

It looks like the tach issue is a Toyota harness issue. As best I can tell at this point, there is something different about how the M5 and A4 Supras are wired in this area (my car was an A4 converted to a M5). I took the power for the B/U lamp switch from the stock location and wired the ouput to the plug at the rear lamp harness and the tach still only works in reverse!!! I have a lot of spaghetti to sort through.

Thanks again for all the responses and help, I think I will have these hurdles overcome by the end of the weekend, or will be doing a lot of driving in reverse .

 

Sure enough, when I looked at both dashes the problem became apparent:

On the '88 AT dash, two other wires going to the tach are on different harness pins as compared to the '91 MT dash. I am using the '91 tach and gauges in the '88 panel since the '88 tach had AT lights on it: O/D off and an LED for the trans setting (Normal/Power).

It looks like the tach was only getting proper power while the backup lights were on.

What a mess , but this should do the trick.

 

I've been busy with my RX7 swap, so I need to bring this one back for an update since I'll be working on the Supra again in the near future.

The tach still doesn't work after the above rewiring. I noted that it does come on when the car is in reverse as mentioned above OR if there is a heavy electrical load such as rolling down the power window.

To recap the wiring:
Toyota tach has three connections:
1) Power, shared with several other items on the dash
2) Signal
3) Earth (ground)

My PCM tach wire goes to a signal converter box from Baker Electronix. It gives a normal LED test sequence when it is powered up and not connected to the PCM. It fails the power on test when connected.

From this box, the tach signal goes to the stock Toyota tach feed up into the cabin and to the dash.

I don't have any shorts and have tested the integrity of these wires with no issues.

When I have HP Tuners connected, I am able to get a tach reading with no problem, so the PCM is outputting this data at least to this program.

The only possible thing I can think of at this point is that the GM alternator may be putting out more voltage than the Toyota one did, so a potentiometer connected to the just the tach's power feed may solve this problem.

I'll update if this works, it is one of the oddest electrical problems I've seen.

 

You ever solve the fuel pump relay/VATS problem? A bunch of FFR GTM builders are experiencing the same type of challenges. I know that John from Speartech has been helping a few of them with new ECU's and harnesses.

Interesting thing... it appears that a few guys have found relief from the problem by disconnecting the alternator. Replacing the Alt solved the problem... but the original Alt came out of a working car. Some solve the problem by a re-program, and then after leaving the battery connected for a period of time, the problem comes back. Some feel it may have something to do with adapting the stock C5 harness and that we may have possibly cut out a resistor or have an open circuit that is triggering the VATS issue, or creating a problem that acts like it.

Others see resistance between ground and the serial wire on the ODB connector, and others dont...

If your interested in seeing what the FFR guys are running into go here:
http://www.ffcobra.com/ubb/ultimateb.../t/001753.html

If anyone has any tips at all... please do post. If we find this problem, its gonna make a bunch of folks really happy!

David