TR how to test differential pressure switch

As you may know I've been trying to figure out my TR's terrible cold-running condition. It seems that the main problem is the cold start injectors not running after the key goes from "cranking" to "engine running" position. Also after the car idles reasonably but still cold, I cannot rev the engine much...not that I'd want to anyway. But it is my understanding that the differential pressure switch (under right side fender) is supposed to cause the cold start injectors to spray extra fuel, in addition to their cold-running fuel, like an accelerator pump to allow the engine to rev up when still cold running. How would I test this switch? And if I have the function wrong....I hope somebody can clarify this for me. Thanks Guys!

The differential pressure switch has a diaphragm with a small fixed oriffice that goes through the diaphragm. When the engine is idling, manifold vacuum is high and is applied to both sides of the diaphragm. If the engine starts to stall, manifold vacuum drops. This causes the diaphragm to momentarily move toward the side with higher vacuum, closing the switch contact on the end of the unit. When the pressure equilizes on both sides of the diaphragm the switch contacts open. You can check the operation of the switch with a hand held vacuum pump (Mighty Vac) and a multimeter. While checking the resistance across the contacts, apply vacuum to the port on the switch. When you release the vacuum, you should see continuity across the contacts for half a second.

The easy way to check the whole system is to get a Noid-light for a Bosch connector. This is a small test light that plugs into the cold start valve connector. Plug it into one side with the engine running cold and using a vacuum pump, apply and release vacuum to the differential pressure switch. The light should come on for a second, I looked and the wiring diagram last night, and the system is pretty funky. There are two relays for the differential pressure switch. They are located in the relay box on top of the rr. fender. The first relay is supplied power by a coolant temperature switch, (Note: this is different from the thermo-time switch ), and appears to be connected to the throttle micro swich. It looks like the relay is only closed when the throttle is closed, so when the throttle is opened the system does not work. I will have to examine the tiny wiring diagram futher. Anyway, when the first relay is closed it supplys power to the second relay. The second relay is controlled by the pressure differential switch. When the differential pressure switch closes, the relay sends power to the cold start valves.

Hope this helps.
Brian

Brian....I can't Thank You enough for this information. Your explanation is very thorough and clear. I truly appreciate you taking the time to post. Thank You Again !! Jeff

Does anyone have a picture of what one looks like? I would like to see.

Sorry, no photos, It looks like a small flying saucer, about 3" in diameter with a male Bosch connector on one end and a vacuum port on the other.

Jeff,
I was wrong about the system working only with a closed throttle. Check your owner's manual. In the description of the fuel injection system there is a paragraph on the differential pressure switch:
Differential Pressure switch: Controlled by intake manifold vacuum and coolant thermoswitch, it activates the electromagnetic cold start injector for additional fuel under acceleration with cold engine.

There is also a partial wiring diagram of the system in the owner's manual in the emissions control section describing the air injection system.

Brian