Headlights make engine run rough

[Gary Lewis]

I tested it with the voltmeter leads both pointing straight down on the top of the posts, with enough pressure to get a good connection. The posts are pretty clean and free of corrosion.

I'll try to check voltage from the rad support and the engine to the negative post next time I have some free time -- maybe that will show something.

I will check the voltage to the ignition box and the coil too. Does the coil have a connector I can unplug and test or do people usually just bury the needle lead in the insulation to get a reading? I am new to the art of electrical diagnosis!

As for vacuum leaks, I think I have an exhaust leak at the manifold, which means I might also have an intake leak at the manifold since they bolt together. Me and a friend tried to put in a gasket a month ago but it didn't make much of an improvement. I might have to have them planed flat.

Thanks all for the ideas! I'll get some results soon, let me know if you think of anything else to look at.

You can test the voltage at C321, as shown below. I don't suggest piercing the insulation as it can allow moisture in to degrade the wire itself.

And, you should check the voltage at the coil on the red/light green.

You should check the voltage from those points to the battery's negative terminal, with and without the headlights on.

[vjsimone]

Try unplugging your headlamps from the harness, that will eliminate the headlamp load.

Any change between H and L beams ?

 

[can0fspam]

You can test the voltage at C321, as shown below. I don't suggest piercing the insulation as it can allow moisture in to degrade the wire itself.

And, you should check the voltage at the coil on the red/light green.

You should check the voltage from those points to the battery's negative terminal, with and without the headlights on.

Just went and tested the voltage at the coil and solid state. With headlights off, both of them showed 12.2 volts. With parking lights on, 12.1 volts. With headlights on, they went down to 11.8 volts.

I replaced the solid state over the summer, so it should be good. The coil is of unknown age but the voltage seems alright.

Edit: Tested battery positive terminal to the valve cover. 14.2 volts running, no lights; 14.0 with lights, 14.0 with high beams.

[Gary Lewis]

Just went and tested the voltage at the coil and solid state. With headlights off, both of them showed 12.2 volts. With parking lights on, 12.1 volts. With headlights on, they went down to 11.8 volts.

I replaced the solid state over the summer, so it should be good. The coil is of unknown age but the voltage seems alright.

Edit: Tested battery positive terminal to the valve cover. 14.2 volts running, no lights; 14.0 with lights, 14.0 with high beams.

There's something wrong with regard to the voltage going to the ignition module. As you can see here, in Run you should have battery voltage to the module. So something is wrong if you have 14.2 volts at the battery and 12.2v at the module.

But, the voltage you are seeing at the coil is what I expected since there's a resistor in the circuit that drops the battery voltage to the coil.

I think the low voltage at the ignition module may well be what is causing the miss. I suspect that the module isn't always triggering the coil, either at the appropriate time or every time, and that causes the engine to miss. You could prove that by running a jumper, preferably fused, from the battery to the ignition module and then taking the truck for a drive and turning the lights on.

To fix it we need to know where the voltage loss is. As you can see in the page linked to above, it is a fairly simple circuit: A fuse link is attached to the solenoid, and that leads to the yellow wire that goes to the ignition switch, then from there it goes directly to the ignition module. I guess it is possible that the ignition switch is failing, but that's rare. And given the age of the wiring it is possible there's a bad connection.

But the first thing I'd check is the fuse link. A fuse link is a smaller sized wire with a heat-resistant insulation. I'd pull on that link to see if it appears to be compromised. If not, then I'd start checking voltages down the line from the solenoid, and you may have to use a pin to probe into the wire. While I don't like to do that, it is going to be difficult or impossible to determine where the problem is w/o doing that.

[can0fspam]

There's something wrong with regard to the voltage going to the ignition module. As you can see here, in Run you should have battery voltage to the module. So something is wrong if you have 14.2 volts at the battery and 12.2v at the module.

But, the voltage you are seeing at the coil is what I expected since there's a resistor in the circuit that drops the battery voltage to the coil.

I think the low voltage at the ignition module may well be what is causing the miss. I suspect that the module isn't always triggering the coil, either at the appropriate time or every time, and that causes the engine to miss. You could prove that by running a jumper, preferably fused, from the battery to the ignition module and then taking the truck for a drive and turning the lights on.

To fix it we need to know where the voltage loss is. As you can see in the page linked to above, it is a fairly simple circuit: A fuse link is attached to the solenoid, and that leads to the yellow wire that goes to the ignition switch, then from there it goes directly to the ignition module. I guess it is possible that the ignition switch is failing, but that's rare. And given the age of the wiring it is possible there's a bad connection.

But the first thing I'd check is the fuse link. A fuse link is a smaller sized wire with a heat-resistant insulation. I'd pull on that link to see if it appears to be compromised. If not, then I'd start checking voltages down the line from the solenoid, and you may have to use a pin to probe into the wire. While I don't like to do that, it is going to be difficult or impossible to determine where the problem is w/o doing that.

I do appreciate the thorough analysis, but before we get too far along the diagnosis, make sure you understand that I checked the ignition voltages with the engine off since I had to unplug things.

The battery shows something like 12.2-12.4 volts when the engine is off, and my 14.0 to 14.2 figure was tested with the engine running. Therefore I don't think there's a big jump in voltage from my test, unless the jump from 12.2 to 11.8 is enough to worry about; my apologies if my wording was unclear.

[vjsimone]

ChecK the 2 grounds for the Headlamps.

[Gary Lewis]

I do appreciate the thorough analysis, but before we get too far along the diagnosis, make sure you understand that I checked the ignition voltages with the engine off since I had to unplug things.

The battery shows something like 12.2-12.4 volts when the engine is off, and my 14.0 to 14.2 figure was tested with the engine running. Therefore I don't think there's a big jump in voltage from my test, unless the jump from 12.2 to 11.8 is enough to worry about; my apologies if my wording was unclear.

Sorry, I misunderstood. At this point I like Vinny's idea - check the headlight grounds.

[can0fspam]

Sorry, I misunderstood. At this point I like Vinny's idea - check the headlight grounds.

Alrighty, just went and gave it another look!

The headlights' positive contacts to ground both returned 10.8 volts with the lights on -- not tooo bad. Same voltage from the high beam contact too.

The headlights' negative contacts to battery ground both showed a resistance of 0.01 ohms -- seems like the ground connection is fine.

In the meantime, I thought I would investigate the rest of my wiring -- my alternator idiot light works once in a blue moon and my brake idiot light never works. Could be bad bulbs; how hard is it to get to the bulbs in the cluster?

[Gary Lewis]

Alrighty, just went and gave it another look!

The headlights' positive contacts to ground both returned 10.8 volts with the lights on -- not tooo bad. Same voltage from the high beam contact too.

The headlights' negative contacts to battery ground both showed a resistance of 0.01 ohms -- seems like the ground connection is fine.

In the meantime, I thought I would investigate the rest of my wiring -- my alternator idiot light works once in a blue moon and my brake idiot light never works. Could be bad bulbs; how hard is it to get to the bulbs in the cluster?

You should install a headlight relay harness, as discussed here: Electrical/Lighting/Headlight Wiring Harness. You'll get essentially battery voltage to the headlights and, therefore, have much brighter lights. Plus, your headlight switch and wiring will only be carrying a very small load - just the relay coil's pull-in current of less than an amp.

And, as I think about it, this might fix the problem of the engine missing when you pull the headlights on. I don't like fixing problems when I don't know exactly how it fixed it, but there's something bizarre going on here that I don't understand. So, fixing the headlights and the miss in one go wouldn't be a bad deal. A relay harness will for sure help the headlights and may fix the miss as well.

Oh, as for the ground, you really can't measure connection resistances with a digital volt meter - at least not the kind we mortals possess. The current our DVM's use to assess resistance is far, FAR too small to find a bad wire or connection as the electrical properties change with increasing current. For instance, I've seen battery connections that will run the headlights just fine but won't crank the engine. However, you can measure the voltage drop across a wire or connection, as explained here: Electrical/Voltage Drop Testing. In this case you'd put your DVM on the 20v scale and put one lead on the headlight's negative terminal and the other on the battery's ground post. With the headlights on you should see a very small voltage, but if the connection is bad you may see several volts of drop.

On the gauge bulbs, you have to pull the cluster. Start by pulling the wiper and headlight knobs, then the shroud around the steering column, then the instrument bezel and then the cluster. To do that you'll have to reach behind and disconnect the speedometer cable and the wiring connector to the cluster.

[can0fspam]

You should install a headlight relay harness, as discussed here: Electrical/Lighting/Headlight Wiring Harness. You'll get essentially battery voltage to the headlights and, therefore, have much brighter lights. Plus, your headlight switch and wiring will only be carrying a very small load - just the relay coil's pull-in current of less than an amp.

And, as I think about it, this might fix the problem of the engine missing when you pull the headlights on. I don't like fixing problems when I don't know exactly how it fixed it, but there's something bizarre going on here that I don't understand. So, fixing the headlights and the miss in one go wouldn't be a bad deal. A relay harness will for sure help the headlights and may fix the miss as well.

Oh, as for the ground, you really can't measure connection resistances with a digital volt meter - at least not the kind we mortals possess. The current our DVM's use to assess resistance is far, FAR too small to find a bad wire or connection as the electrical properties change with increasing current. For instance, I've seen battery connections that will run the headlights just fine but won't crank the engine. However, you can measure the voltage drop across a wire or connection, as explained here: Electrical/Voltage Drop Testing. In this case you'd put your DVM on the 20v scale and put one lead on the headlight's negative terminal and the other on the battery's ground post. With the headlights on you should see a very small voltage, but if the connection is bad you may see several volts of drop.

On the gauge bulbs, you have to pull the cluster. Start by pulling the wiper and headlight knobs, then the shroud around the steering column, then the instrument bezel and then the cluster. To do that you'll have to reach behind and disconnect the speedometer cable and the wiring connector to the cluster.

That sounds like fun. I definitely could use brighter headlights! Sounds like redoing the electrical system may be in order. How difficult and time consuming is it to put in relays? I want to do it but I don't want to tear it apart and get lost or stuck somewhere with a truck w/o headlights.

Also I'll read up on voltage drop testing and try that out too!

--

As for electrical I just remembered I wanted to ask you about this:

I was looking at the voltage regulator trying to troubleshoot the charge light the other night.

There are four "ports" on the voltage regulator, but there are five wires coming out -- two from the "A" port -- and one of those (orange wire) ends with what looks like a rubber heat shrink connector and nothing on the end of it. Is this supposed to connect to something?

The rest of the wires go into a wrapped harness towards the alternator. I think the "I" wire is the idiot light. It showed 12 volts with the key in run but I couldn't make the light turn on by grounding it.