I'm slowly working the bugs out of my '87 NA after a cold 5 year sleep. I noticed the ThermoWax plunger was stuck in the extended position and managed to get that unfrozen.

Now when starting cold, the car starts and quickly rises to 3000 rpm, then settles down to about 1500. But then as I let the clutch out the rpm's drop to about 400 and the engine quits after a few seconds of struggle. With my foot on the clutch I can cause the problem by simply pushing in the interlock switch with my finger so it's not a mechanical thing. the car never had the Cruise Control option. Frankly, I can't see from the FSM how the Interlock affects anything but the starting solenoid. Anybody seen this one or have a suggestion where to start?

 

When fluids are cold viscosity becomes very low.

When the clutch engages it spins the transmissions input shaft and the trans fluid is very cold.

The engine not expecting this it can't keep up to the increased load and dies.

 

...and BTW, gear oil viscosity is higher when cold.

 

So idle speed drops with the interlock switch pressed/clutch pedal on the floor, or when the switch is released/clutch engaged? If it's the second, bypass the interlock switch. A small piece of wire connecting the harness-side plug contacts will do it.

From the wiring diagrams, the starter signal wire (which receives power via the interlock switch, from the ignition switch, etc...) splits and runs to the ECU. But as far as I know, this is just there to send the start signal to the ECU. When the key is not to 'START', there is no reason this wire should be powered, and for the same reason the interlock switch should not have power running through it either.

And yeah, for Newtonian liquids, viscosity increases when temperature decreases.

 

It's the weirdest damn thing. I can have the clutch pedal on the floor and reach down and press in the interlock switch and the idle suddenly stumbles when cold. Once the engine warms up it idles at 750 with the clutch pedal fully released. So, is there something electrical in the warmup that could be disengaged when the interlock switch is opened, so the car thinks the engine is warm? If it'll ever stop raining I'll put a meter on it and find out whats going on inside the switch. The ECU seems to function normally in all other respects. Same for the ignition switch.

 

Do you remember the color of the wires to the Interlock switch?

 

It's all stock Satch; BR/BG. I just shined a flashlight on the switch to confirm that and noticed the plunger looks a bit angled and there's a philips screw on the pedal hitting the metal case of the switch. Is that normal? I'll mess with it some more tomorrow when I have better light. Bear in mind this car has 200K miles and the pedal pad is worn clean through so I wouldn't be surprised by anything at this point.

 

Does this car have the factory alarm and therefore the Starter Cut relay or just the ignition circuit related blue plug located near the trailing coil?

 

Sorry about the delay Satch. I had a problem with my daily driver that soaked up my attention the last couple of days.

The car is a base model with very few options. No factory alarm and no cruise control. I finally had a chance to crawl under the dash and do some detective work. I figured I'd remove and test the switch. Thats when I discovered the thing wasn't wired to the starting circuit and wasn't the interlock switch at all! In my defense, the BR/BG wires were taped in such a way that with casual inspection they looked like they led straight to the clutch switch wires.

The "clutch switch" is for a cruise control option my car doesn't even have. THAT is the switch causing my stumbling problem which is probably what you suspected when asking me to confirm the wire colors. The clutch switch harness is BY/YL, and the BY is switched 12V. The YL seems to go to the ECU where it does who knows what. Anyway, switch works fine so I removed it from the pedal, plugged it back into the harness. Car runs fine through the warm-up and even seems to run better when warm. I haven't had time to drive it. BTW, clutch switch is normally closed, open when clutch pedal pushed in which disengages cruise. OTOH, interlock switch is normally open, closed when clutch pedal extends nearly all the way to the floor.

Any idea why this switch would start causing problems so late in life and more important, anything wrong with just leaving it unattached to the pedal.

 

I'd leave it unplugged or remove it. If you don't even have cruise control, there's no reason to have it there at all. There isn't much in the FSM to say what it's there for; but evidently the ECU is playing with the mixture as if cruise control is being engaged/disengaged. There's one on the brake pedal that serves the same purpose too.