1990 F150 won't crank unless I jump across the relay posts
 

I have a 1990 F150 with a 5.0 engine and an automatic trans. I went out to move the truck the other day and it wouldn't crank. All the dash lights, fuel pump, etc work fine. But, it wouldn't turn over. The linkage on the column appeared to be engaging the ignition switch as it should, so I swapped the switch(happened to have a new one here) but that made no difference. I opened the hood and used a screwdriver to jump across the terminals of the starter solenoid relay and the truck fired right up like it always has. I tried to check to see if I was getting voltage from the switch wire at the top of the relay, and I don't think I'm getting anything there.
What now?

Neutral safety switch?
Issue in the wiring?

Welcome to the forums.
Try in neutral? See if that position works. Or try wiggling the shifter a bit and see. Might be NSS?

I bet if you try to loosen one of those bolts on the solenoid, the whole thing crumbles apart. was a $15 part last I replaced one. That's where my money is at, since you were able to start it by jumping the posts. grounds are another thing to address. Make sure all your cables are in good shape, clean the grounds on the frame and block. The one between intake and firewall, and the one on the rear since you're in ground cleaning mode. And all the terminals on all the posts of battery/solenoid, plus the fender ground over there. Can't have too much ground.

I almost said welcome. 14 years and one post? Can we get a slow clap, everyone?

LOL! I am guessing I was searching for info when I was working on my '76 F100 since it was that long ago.

I had tried that, but no such luck.

https://cimg3.ibsrv.net/gimg/www.for...231065298d.jpg
Find the pos output wire on the connector for the ignition switch. Make sure it is hot with the key in the start position. If it is then I'd just tap into it there and run a fresh wire to your solenoid. I'm assuming the solenoid is good.

As you've figured and the others have said, this problem can have multiple possible culprits.
Checking the output of the ignition switch at the ignition switch is the first step and will tell you if it's there, or farther down the line.

It can be a bad neutral safety switch to the point that simply jiggling it or readjusting it won't help. But testing with an ohm-meter or volt-meter will let you know that as well.
On the '90, is the switch on the steering column or the side of the transmission? Probably by then it was a whole "range switch" with all sorts of wires going into and out of it. Not sure what colors to look for in that case, but it used to be a simple check of the Red w/blue wire pair on the old transmissions and NSS setups. Sorry I don't have a diagram of the '90, but I bet it's been posted up here before.

It it's not the ignition or the neutral safety switches, it's the wiring in-between there and the starter relay. Sometimes as simple as the little push-on connector at the "S" post getting loose. Again though, easy to test.

Your relay is the vertical style (I think called the "pancake" style) of the later models, correct? That's where the two large terminals stick out the top, rather than the sides of the relay. Assuming yes unless it's been changed by a PO, and if it's that type it likely has the big factory molded connector that holds the starter and battery cables in a big bungle of a "strain relief" connector? It can go bad too, but it's rare and you've already verified it's good by the jumper test. And when it fails it's usually an old cable anyway, rather than the connector at the relay.

Back to testing the relay, you verified it's working when you jumpered the terminals. Just to be sure though, you did jump the small "S" terminal to 12v correct? Or did you jumper the two large terminals across the top?
If you did the latter, then you actually bypassed the relay and did not actually test it in a way that would tell you if it's still good. That is simply jumpering the battery directly to the starter. But if you used the smaller terminal post, you verified that the relay itself is still good.

With any luck it's a connector or a switch, rather than the wires themselves. That would be a real pain to fix, vs replacing a relatively easy part. Unless you just run a new wire like was suggested. That's pretty easy...

Good luck.

Paul

my guess is a broken ignition actuator in the column. common problem with these old trucks.
easiest way to test it is remove the ignition switch and use a small screwdriver to slide the switch and see if it will start.
if it does, the actuator is bad. if it does not start, it is either a bad NSS, bad solenoid, or broken wire between the switch and solenoid.

Thanks for the help everyone! It turned out to be just the solenoid relay.

Good video on replacing this here...
This applies to 1980 - 1991 trucks