Problem with Starter Circuit

I suck at electrical stuff. Double the problem when it is a rat's nest that someone else has already hacked up and so stock guides aren't good any more. I need someone who can give me advice on how to track down where the problem is in an electrical circuit.

The car in question is my 944Spec race car. It is an '88 924s with an 85 944 motor in it. The normal key start was bypassed for an ignition switch and a starter button. Pressing the starter button will not engage the starter, but I can hot wire it with a screwdriver across the solenoid. Pushing the starter button does engage the fuel pump to prime though.

A while back, it kinda had this issue, and it seemed like there was some leads or something under the torn out dash that I could wiggle and make the button work again. This does not seem to be the case anymore.

I do have a cheap multimeter. I only kinda know what I am doing with it. Any guidance on where I might look to identify the problem?

Do you get voltage at the starter solenoid when you push that button? It sounds like that button is sending juice, if that's what's priming the pump. Unless the button has two separate parts (i.e. contains two separate switching mechanisms), the issue would seem to be either between the button and the solenoid, or it is the solenoid itself.

If you can hotwire it, the big wires going to and from the solenoid are fine. It's just either not receiving triggering voltage, or it's not doing anything when it gets that voltage.

BTW, I'm not familiar with this prime-on-starter-engagement arrangement... just in case that's something that's uncertain.

Priming is probably the wrong word. Maybe being sure there's pressure in the fuel rail. When I push the starter button, it makes a whirring sound that sounds like the fuel pump. Could very well be something wired up wrong though.

The switch has only two wires going to/from it. One on either pole. One comes from the ignition switch (so I presume that's my power source), the other goes out to elsewhere.

As I said, I have a multimeter but don't really know how to use it. What leads should I check to be sure the solenoid is getting voltage (I will have to do this when I have a spare set of hands who can sit in the car and push the button).

I think maybe I don't really understand how a starter solenoid works. So... the solenoid is basically a switch attached to the starter motor. The ignition in the car does not close the circuit to engage the starter. The ignition in the car sends a current to one lead on the solenoid that triggers it to close the circuit on the starter.

Correct?

So... if the solenoid is bad, even if it gets current it won't do what it needs to do to close the circuit to engage the starter?

Working from last post first: Correct! The solenoid is, mostly, a beefy switch designed to deal with starter-motor loads which would melt the little switch (or button) you're manipulating to fire it up. It uses an electromagnet powered up by the starter switch/button to throw that beefier switch.

When you hit the button, you're basically telling the car to engage the "real" start button... If the solenoid's bad, it won't do that, and the starter won't fire. If you're just bridging the heavy starter wires with the screwdriver, you're taking the solenoid out of the picture.

If I didn't know that you have plenty of automotive experience, I'd be less hesitant about suggesting that you're not hearing a fuel pump, but instead an attempted start (buzzing from a failed solenoid or relay)... I've never seen a pump wired into the starter button, but I guess I could see doing that if you had an ECU that didn't have prime-on-ignition-on functionality and you just wanted to fire it that way on cranking and let the ECU keep it up after the engine catches (I think most OEMs, and I know Megasquirt, will fire the fuel pump for a couple of seconds when you turn the key on, then won't run it again 'til there's an ignition signal indicating the engine's cranking or running; the starter isn't usually involved in firing the fuel pump in my limited experience)

Here's where I start getting concerned that there's a bunch of 944-specific stuff I'm not thinking of... Does it actually use the ignition switch to fire a smaller relay which in turn fires the starter solenoid? What about the clutch safety switch?

In any case, after a quick google-peek, I'm guessing your starter/solenoid has a big ground wire, a big wire to battery +, and a smaller wire. That smaller wire is where you want to see 12V + when you hit the starter button. Er, between there and ground. Depending on what sort of failure mode the solenoid is having, you may need to disconnect that wire from the solenoid to see voltage on it (it could be dragged down by a short, which is also no good for upstream components).

Having someone else to push buttons may also allow you to determine where that fuel pump sound is coming from, and whether that's what it actually is...

Think of the solenoid as a big relay. It gets a 12v low amp signal and activates to pass a 12v high amp current from the battery to the starter.

Step 1: cut a hole in a box.

No wait, that's not it.

Step 1: find the large cable in to the solenoid from the positive battery terminal.

Step 2: Use a scrap of wire to connect this terminal to the small trigger terminal on the solenoid. If the engine does not crank you have a bad starter solenoid.

Step 3: if the starter does crank you have a problem with your trigger wire. Trace it from the solenoid to the starter button. You should have 12v to the trigger wire when you press the button.

Oo, I like his way ^^^^^

I should've asked which terminals you were bridging with the screwdriver...

bgkast is money.

i just did this troubleshoot on a '94 ranger that clicked but didn't crank. the little wire to the "start" pole on the solenoid was OK. it was the big cable from B+ to starter that had a crappy ring terminal (poorly crimped and not taped) that was badly corroded.

In reply to bgkast:

Great little test!

Result is, the solenoid appears to be good. It just isn't getting power from the starter button. Now I get to figure out where the break in the circuit is. -grumble-grumble-

Try directly touching the wires which go to the button? That'll test whether it's the button itself... That allows you to at least rule a component out without a bunch of wire-tracing.

In reply to Beer Baron:

Pull the button and make sure you have 12v in to it with the ignition on, and 12v out when you push it. That should isolate where the issue is.

I checked yesterday for resistance across the button. It went from open circuit to showing some resistance when pushed. So the switch apparently works. I will have to check for actual voltage though.

Electrical gremlins are gremlin-y. After months of being a bitch, I just pushed the starter button and it cranked the car starter.

Apparently I wiggled exactly the right wire, or sufficiently flagellated myself to appease the demons that possess the car.

How much resistance when it's pushed? Maybe your button is giving up the ghost.

bgkast wrote: How much resistance when it's pushed? Maybe your button is giving up the ghost.

This. A switch or button in the "on" position should be damn near zero ohms.

Sure it's not a Honda? I heard somewhere that they sometimes have trouble with ground wires.

Resistance was like .015 ohms.