Truck is burning out solenoid relays at an alarming rate. Please help troubleshoot.

Factual Questions
1

We have a 2001 f150 v8. Recently the solenoid relay went bad and we replaced it. The part autozone gave us turned out to be the wrong part, though, and while it worked briefly, it quickly also burned out. So we replaced that one, and the next one lasted a few days longer, but soon died again. It has also been replaced. Clearly we have a problem.

We put a multimeter on it, and with the truck off, we’re getting 12v to the solenoid. With the truck on, it’s 14v. Our friend has the same kind of truck, and when we tested his, there is no power to the solenoid when it is off or when it’s running, only at startup, which is what you would expect.

When we disconnect the ignition wire on ours, we still get the 12v. What might be the problem here? I’ve googled and I’m not finding much.

2

Feedback from the solenoid itself?

Probably time to replace it.

Edit: Are you speaking of a separate solenoid and relay?

3

I’m suspicious of your diagnosis.

If you had 12v to the starter relay, the truck would be cranking all the time. You would probably notice this.

4

It is not continuiously cranking. I’m referring to the solenoid relay that is attached to the firewall up by the battery. When we test with the multimeter, we get 12 v (actually 12.4, same as the battery). This exact test on our friends truck (same year and model) yields different results. this truck apparently also has another solenoid on the actual starter, but we seem to be having problems with the relay.

Obnoxious hood ornament, this new part was installed yesterday.

5

This might be too simple but let me work it through for you. I am a bit more familiar with older Ford equipment and the 2001 might be a little fancier, so adapt as necessary.

The starter relay (aka solenoid, very old-fashioned term akin to “condenser”) has a heavy-duty input terminal that should go basically straight to the battery, a heavy-duty output terminal that should go straight to the starter, and one, two or more small terminals. One of those small terminals goes to the ignition switch. Ignore the other ones for now.

When you close the circuit by turning the key, 12V goes to the small terminal and through it, the relay coil and the frame ground, closes the extremely heavy-duty (600-1000 amp) relay terminals. This lets the current from the battery terminal cross to the starter terminal and fire the starter. (Whose circuit is completed by the grounded engine block.)

I’ve never seen a starter relay burn out in a short time. It sounds like it might be being triggered by a short or faulty connection on the ignition line, maybe while driving, and continually opening and closing and thus burning the contacts.

You need to define the failure here. With the truck off, there should be 12V only at the battery terminal. There should be 12V (more likely 9-10V because of the cranking draw) at the starter terminal while the truck is cranking, but 0V otherwise. You should able to detect 12V at the ignition terminal whenever the key is turned to the start position, but 0V otherwise.

If you detect 12V at the starter or ignition terminal any other time, something is wrong with either your starter control wiring (from the key to the relay) or internally to the relay.

If the relay is “burned out” at the ignition terminal/coil level, nothing will happen when you turn the key; you will detect 12V at the ignition terminal but the relay will not click shut (can feel it) and 12V won’t appear at the starter terminal.

If the relay is “burned out” at the relay contact level, it may operate (click, and close) but the bad contacts will only allow a little current through. The starter might hum, or clack from the (actual) solenoid, but not turn, and you will measure a very small voltage at the relay’s starter terminal (2-6V, maybe).

That make sense? Any of that help resolve the problem?

6

Thanks, nitropress. I forgot to mention that the solenoids are scorched on the backs when we remove them, if that’s a clue.

Ill have to wait until my husband gets home to do more testing, but I do know when I disconnect the ignition line, I’m still getting 12v at the big terminals on the relay. So doesn’t the problem have to be somewhere in between the battery and the relay? But it’s just direct cables, as you say, so I don’t get it.

7

Nitropress’s entire post was outstanding.

When we have relays quickly fail in the equipment we produce, it’s always due to a faulty input causing the relay to chatter, which fubar’s the contacts.

Sounds like that relay shouldn’t have power on it unless the key is turned, so perhaps the output from the ignition leading to the relay has fused shut.

8

Just spoke to my husband. They re-tested his friend’s truck with a different multimeter (they’re at work and used one there, since my husband forgot to bring ours with him this morning.) retest showed the same amps as our truck is giving off, so apparently that’s normal and not the problem.

Still no idea what the actual problem is, of course, but apparently the constant power to the solenoid isn’t it.

9

The coil is burning out from continual power, or the relay is being activated frequently, burning up the contacts. There’s a metric buttload of power going through that unit durin cranking (about 500 amps or 6000 watts, typically) and any extended contact or switching will damage things in a hurry.

Just to make it clear: measure for 12 volts (not amps - you want to measure voltage, not current) between a good chassis ground and each of these terminals. The battery-side terminal is the only one that should show 12V or so with the key off. With the key turned to start, the starter terminal will show about 8-9V and the (little) ignition terminal about 12V. Those should show 0V when not cranking - again, between them and ground, not between the terminals. You’ll always have 12V between the battery and starter terminals because the latter is essentially a ground terminal. Measure each terminal to the metal bracket of the starter relay or another good piece of bare chassis metal.

10

Hi, is this the part you are burning out? HERE
These are basic high amp switches that are very reliable but can fail through long operation as in cranking for extended time due to failure of engine to start, but I would submit that it is more probable that a starter may be drawing high amps and causing SOLINOID failure. I use an inductive amp meter to test starter amps like this one, HERE
That link in picture 2 showers the small gauge I am referring to.
The test you are explaining is incomplete because you would always have battery voltage on the battery side of the large terminal and have that voltage on the starter side only when the solenoid is closed electrically. Or are you referring to the ignition switch terminal?
If you are having power on the ignition terminal, and the other is a ground for the switching coil, you could have a B.O. Ignition switch or wiring problem.
However if your solenoid is energizing when it shouldn’t there should be a sound that will give this away, and if you have a amp gauge in the dash there will be indications there and also when running headlamps you would see a dimming that would also be evident in a dome light if you were to watch one even in day light. Unusual Sound will be the best indicator.

11

If the solenoid in question is the one on the firewall and not the one on the starter you may also have a bad ground issue with it. It may be seeking a ground from the back of the solenoid causing the scorched look. These solenoids have a surge of amperage as they pull down but don’t really require much amperage to stay down. The surge on pulldown culd be causing some arching from a bad ground. Tell hubby to check the small ground terminal for a good ground, he will need to have at least some load on it to check it properly.

12

Thanks, everyone. We’re going to mess with it this weekend and see what we can find. I will report back.

13

Here’s how it should be:

The starter relay has two large (battery cable size) wires attached with nuts to the two large posts, one coming from the battery, the other going to the starter. The one will always be at battery voltage, the other should read zero volts except when the relay is energized (during cranking/starting), at which time the two will be connected and it will also be at battery voltage.

The third connection at the relay is a thin tan w/ red stripe wire. This is the “trigger” circuit from the ignition switch that operates the relay; it should read zero volts except during cranking/starting, when it will read ~12 volts. Note that this is part of the starting system, not part of the ignition system – it is not properly called an “ignition” wire.

Test the voltage at the tan/red wire, with the wire attached to the relay, with igniton off, ignition on (but engine not running), during cranking, and with engine running. Also test its voltage with the wire disconnected from the relay with ignition off, ignition on (not running), and during cranking; then reconnect it, start the engine, disconnect and test with engine running. It should only have voltage (~12 volts) during cranking. If it has significant voltage under any of the other conditions I’ve mentioned here, there is a power bleed-through (short circuit) to this circuit, and it’s a pretty good bet that this is what’s taking out the relays. The short circuit is likely in the ignition switch or, if it has an automatic transmission, the tranny range switch (which includes the neutral safety switch). It’s also possible, though uncommon, for it to have a short somewhere in a wiring harness.

Report the observed voltages from the above tests and I’ll see what I can make of it.

If the wiring at the relay is different from what I’ve described, I’ll need the 10th character of the VIN (not just the engine size) to positively identify which engine it has so I can check further.

14

It’s possible that the problem lies with the starter - if it were faulty, it would draw too much current and/or cause chattering of the relay, both of which would lead to premature failure. Starters are tough and can keep working with major faults.

OP: Is there ANYTHING unusual about the way the truck starts - hard starting, faulty starter engagement, too much noise, weird noises?

15

I talked with my boss yesterday about this very kind of problem he had with a 1979 Ford F150 with a 350. He had done a lot of restoring on said pickup and starter solinoid would not drop out after the truck was started. He had several other mechanics help him and although some thought there must be a diode in the electrical system that failed or was not connected up right none of them could find a diode in any wiring schematic. But to remedy the problem, he did put a diode into the starter soliniod system and I cannot tell you exactly where but will see him again on Tuesday and if your problem still exists I will get more info from him.
Good luck

16

We aren’t completely clear on how to conduct this test. Can anyone point me to a tutorial or something?

Ok. With the tan wire connected to the relay, we only get voltage during cranking (12 v). Good.

With the wire disconnected, we are getting 4.2v at the wire with the ignition off, and 2.9 v with the ignition on. So that’s bad, right? What next?

17

This problem started when we were having extreme cold weather, -14 and windy. I tried to turn it on and got one click, and then nothing. The battery was fully charged. Other than that, it generally starts up normally, no odd noises and quickly. It runs fine, but the check engine light is on-again-off-again. Right now the check engine light is off.

18

This is getting confusing and I’m not sure how to straighten it out. It’s possible to get slight “phantom” voltages on car wiring; you can only tell if they’re meaningful by leaving the wire connected and testing at the terminal - typically, these wires push on to a screw stud and have a nut around the bottom where you can touch the voltage probe. I’d guess that if things are normal, this connection would pull those two slight voltages to near-zero.

The really, really short answer is that if you’ve genuinely burned out a couple of starter relays, something is either keeping the relay engaged or overloading the relay capacity. Only current flow through the starter could be doing this, without obvious burned wires or arcing elsewhere. At this point, I’d look at the starter and its engagement solenoid to make sure there are no problems there. You may need a professional to test these properly without dropping the starter to look at it. (The good news is that Ford starters are easy to drop on most trucks, and relatively cheap to replace.)

19

Yes.

Not sure yet. What voltage at the tan wire with the engine running and the wire disconnected?

20

Also, is the tranny an automatic?