Auto Electrical Forum

Automotive Electrical Tech Tips and Questions Answered

Jim Asks:
I have a modified 1987 Fiero GT.
It had a 1995 Cadillac 4.6 Northstar in it.
Question:

Fuel connector C502 in the center rear engine bay, connects to fuel tank.
Has a pink wire, a tan/white, and a black wire.
I have added a external fuel pump.
I’m ready to start motor up.
The C502 connector, shows volts when key is on.
(Fear blowing up car.)
The tan/wht wire is for the fuel pump,
which I have elimamated.
Should show voltage?
Question: should that C502 connector show voltage?
Thanks Jim

Answer:
I think this is something that others may have an interest in, so I am going to frame my answer in a generic form.
When doing an engine swap there are many things that must be accounted for in regards to fuel injection. You are most likely going to use the ECU or a modified ECU for the engine you are swapping into the vehicle. Now lets look at the obvious, the fuel lines should be the same size, keep in mind we need to move the same volume of fuel not just the same fuel pressure.
Next does the new engine have a fuel pump control module or is it a simple relay control. If it is a relay design then the wiring is easy, just replicate the wiring and you are ready to do. If the new ECU has a fuel pump control circuit, internal or external, it must be inspected carefully. You can just wire it in because it may have “G” sensor circuitry, this will adjust pressure based on what the vehicle is doing, keep in mind a Cadillac will not corner the same as a Fiero. A mistake here can cause over rich or lean out during cornering or under accretion or deceleration. You may be better eliminating the controller and this will require duplicating the ECU inputs so the ECU will not trip a code or go into “limp home”.
This is how I normally wire it in, step one check the wiring to determine if the ECU controls ground or voltage. Most ECU’s supply the ground for the fuel pump and the voltage is supplied when the key is turned on. To play it safe I will run a relay, the ECU controls the low current side and the battery supplies voltage to the fuel pump and ground is run through the relay.
Finally make sure your fuel pump can make the specs of the engine you are swapping in. Now a hot tip, if this is a daily driver with some part time racing, check out our adjustable external regulator. With this you can run 14 volts when not racing but turn the voltage up to 16 – 17 volts when racing, the fuel pump will supply all the fuel you will ever need at 16.5 volts. We have seen .5 sec to a full second gain in the quarter mile.

Jeffro Asks:
Can I run some radiator fans from a Nissan Primera on AC power?
I want to use them in a heat transfer system in my house.

Answer:
The simple and easy answer is no, however you can purchase a 12 volt power supply to convert ac power to dc and that will power the fan, however I think it would be more cost effective to purchase a ac fan.

Jake asked:
hi guys im from Australia and need to rewire my roof for my s13 convertible due to a fire.
Basically it has a motor in the rear boot with hydraulic oil which drives the hydraulic arms to open or close roof.
there’s a +/- wire on motor, at the moment I just use car battery terminals +/- to open roof. how can I wire a switch in cabin?_any assistance will be great, I have a 3 way switch, will I need a relay? fuse? run power from where?
thanks Jake

Answer,
most all motor circuits work the same regardless of if it is a window motor, door lock motor, or a top. Of course newer vehicles are more complicated, many are controlled by the body control module and cant be easily bypass.

By way of review the system works like this; push the switch one way and the top goes up, push it the other way it goes down. The big question is amperage, something like a hydraulic motor will require around 30 amps, compared to a door lock motor that will only be around 10 amps. So for this problem you are going to need a switch that can handle 30 amps, however that may not be the most attractive switch in your interior.

The answer here is a low current switch and 2 relays. Now the easiest way to do this is to order a switch relay kit, it gives you everything you need to make the system work and allows you to run your own switch. If you would like to order one contact our sales department here sales@rushps.com.

If you want to wire it your self here is the layout of the two relays. tape 2 relays together, on the pin side wire the connections as follows:

pin # 85 from one relay, pin #86 on the other relay, and pins #87 on both relays will connect to a 12 volt source using 10 or 12 gauge cable, be sure to use a 30 amp inline fuse and attach the power directly to the battery.

pins # 87a on both relays connect to ground,

the switch should be wired with grounds, the center pin will go to constant ground and the up and down leads will go the relays

ground input from the switch will go the pin #86 of one relay and pin #85 on the other relay, this is low current os you can wire this with 16 gauge wire.

Pin #30 of one relay will go the black wire of the motor and pin #30 of the other relay will go to the red wire of the motor.

Now when you push the switch up or down you should hear a click, this is normal. You may notice the switch is working backwards, that is you push up and it goes down. Just switch the leads at the switch or pin #30 on the relays.

Robert asks:

having a problem with my gauges the fuel gauge pegs all the way past full when key is on. and I have no voltage. fuel has 2 tanks and wont switch over . this is a 89 ford f250 7.3 can some one tell me were I might be able to start to look for the problem

Answer:

First lets cover the basic operation of the fuel gauge, this is not all of them since newer vehicles are handled by the Body Control Module (BCM), however we need a background for the operation.

In the gas tank there is a float that is attached to a rod, which moves a pointer along a resistor, a ground signal is sent across the resistor and then to the gauge, the resistor limits the ground that goes to the gauge, more resistance = less gauge movement, or less gas in the tank.

Now hopefully we are talking about a factory installed tank switch, if this is the case you will be able to find wiring diagrams to trace the wiring. If the switch is a aftermarket switch then you will need to contact the manufacture for wiring information.

If it seems I am concentrating on the switch it is because I am, with wiring always go with the most logical problem. In your question you mention that the tanks will not switch over, in most designs the switch also controls which fuel gauge is sending a signal to the gauge. So I would start with testing the inputs and outputs of the switch wiring and replacing the switch if it is bad. But again you will need to know what type of system you have.

You may want to stat at the dealer parts house, give them your vin number and see if you truck had factory installed dual tanks or aftermarket.

If it turns out that the switch is not the problem I would next check the GEM module, it is bit like a early BCM. The GEM is a fairly common item to fail on Ford products and finally the gauge could be bad, check all fuses and follow the wiring diagram.

This is a good time to announce a new service we are offering, finding wiring diagrams is difficult enough but then how do you read them? We now offer personalized wiring diagrams, pricing ranges from $39.95 to $99.95 depending on the problem and vehicle. You would receive a diagram of the circuit, what each wire color voltage should be, and how you can find the wire on your vehicle. So you need a volt meter but hey for less than an hour at the dealer you could fix your electrical problems. To get yours just send a note to sales@rushps.com

Now one final note, when working on wiring on newer vehicles never use a clamp on test light, you must use a volt/ohm meter. Test lights send too much voltage through the wiring and can damage electrical components.