How do you simulate a light bulb?

I am supposed to have a light bulb for my alternator regulator wiring (actually this is not needed, since it also gets voltage from ignition, not like Fiero alts), and for the coolant temp light.

I have a Quad 4 swap, and when I dont have a light bulb for the coolan temp light I get a code 26, and the ECM goes into limp mode, but my Sunbird instrument cluster does not have a socket for that or the charge light. So I need to add resistors. I looked up the charge circuit for an 84, since they have no light, and according to my 84 service manual, it uses a 10 ohm resistance wire, so I got 2 10 ohm resistors, as soon as I turn the ignition (when the lights come on), the resistors get very hot and start smoking within a second.

What do I need?

The one thing you forgot in your bulb substitute was the wattage rating (voltage * current)

You failed to mention the wattage of the 10 ohm resistors. Also the resistance should be 10 so why did you use 2 10 ohm resistors? If it was to increase the current capability that will do it but it will change the resistance also.

To determine the value of two resistors in series:

Resistor1 + Resistor2 = Total Resistance
10 ohms +10 ohms = 20 ohms
so that 2 10 ohm resistors in series will be 20 ohms

To determine the value of two resistors in parallel :

R = (R1*R2)/(R1+R2)

For example, if a 10 ohm resistor is connected in parallel with a 10 ohm resistor, the total resistance is equal to (10*10)/(10+10) = 5 ohms.

I would guess that you used 1/8 or 1/4 watt resistors the type Radio Shack carries as standard.

To simulate a bulb you will have to have the same resistance and the same current carrying capability.

Sample list of bulbs:

Bulb Voltage Current Draw Wattage
#161 14 .19 amps 2.66

#168 14 .35 amps 4.90

#192 13 .33 amps 4.29

As you can see from the bulb samples it would take up to a 5 watt resistor to hold the current.

You can use a 10 ohm resistor rated at 5 or 10 watts. The resistor will however get hot just like the bulb does at the same current draw as the bulb.

10 Ohms is way too low. Typically a 470 Ohm 1 Watt resistor is used to simulate a bulb in the alternator charge circuit.

The 10 Ohm resistance wire is used to simulate a lamp. It's a low resistance when cold but much higher when it warms up with current flowing through.

Ok, I will try a 470 ohm resistor. All it does is go to the ECM, if I leave it disconnected, then I get a check engine light. both of the lights are only on during bulb check though.

The actual current draw for the charging circuit is very forgiving. Just be sure to get a higher wattage resistor.

One source says

quote Use a resistor of any value between 35 ohm, 5 watt, and 500 ohm. 1/2 watt between battery and "L" terminal

The lower the resistance, the higher the wattage (P=EČ/R). An old rule of thumb is to always use a resistor with a power rating two times greater than the expected requirement.

In the case of the coolant temp switch sensor you could probably use something like a 2,000 Ohm, 1/4 watt resistor. The ECM is only looking for the open circuit voltage or a ground indication.

[This message has been edited by spark1 (edited 04-14-2005).]