1970's Ford Truck 302 v8: Carb auto-choke question

The auto-choke on the carb has an electrical connection. When I remove the lead, and ohm the auto-choke connector out with the engine off and cold, it appears to be open. High ohms. But if I start the engine and let it run a few minutes, then turn the engine off and check it again, it shows low ohms. Is this normal?

Don’t know what the ohm readings should be. The choke should be closed when the motor is cold. When you turn the key the “heater” in the choke is energized and in about a minute the choke butterfly should start to open. There is an adjustment on the housing of the choke. Basically if it opens up in about a minute it is working properly. Adjust by trial and error until you achieve the cold start and warm up results you want.

The choke has a heating element (the wire) to make it open quickly. The ohms measured should be pretty constant and fairly low…under 10 ohms…An “Automatic Choke Heater” was once a common replacement part. Today, maybe not so common.

The thermal choke has a bi-metal spring under the plastic cover. When current is applied to the thermal choke that spring heats up and starts to twist which comes in contact with the choke plate shaft lever. This then causes the choke to open up.

It could be that when you check the resistance of the thermal choke the bi-metal spring isn’t in contact with the choke lever. Thus the high resistance reading. But when you start the engine and current is applied to the thermal choke, this allows the bi-metal spring to twist enough where it comes in contact with the choke lever and you get a lower resistance reading.


Testers point is good…In order for the choke to work properly, it first must be adjusted properly , be in the correct position, so it has a ground path…Remove the black plastic cover and investigate how the tang on the coiled spring is positioned…When cold, the spring tension should hold the choke plate closed. The moment the engine starts, a vacuum choke pull-off should crack the choke open slightly…Then as that heating element gets hot, the spring tension is reversed and the choke opens fully…

They make manual choke kits to replace all that…

Even though there is an electric heat element in the choke housing, is there also a small pipe exiting the rear of the choke housing, connected to the exhaust manifold? If so, the electric choke heater is quite redundant.

Thanks for the comments. As mentioned in your comments, I checked and the choke plate is fulled closed when the engine is cold, and it cracks open a bit after starting. After about 5 minutes the chock plate is fully open. All these tests done when ambient temp is 65 degrees.

I looked in a carb repair manual for the same carb I’m testing, and it indicated on some electric chokes there are something called a “silver contacts” in the choke heating circuit which appear to open and close the heating circuit, but it doesn’t say how they work or under what conditions they are supposed to open and close.

I can imagine you’d want the choke heating element to heat up only unitl the engine is fully warmed up. Then you’d want to disconnect the heating element. Otherwise, it would just waste power, heat the engine compartment more, and probably more importantly, reduce the life of the heating element. I’m guessing the silver contacts are designed to open the heating circuit and stop the heating once the choke temp reaches engine operating temperature. But that wouldn’t explain why my electric choke heating element would be open when the engine is cold.

I’m guessing there is some problem yet to be discovered. I’ll take it apart per the recommendation above and look to see if the spring is bent or broke.

Thanks all.

If you remove the electrical connector from the thermal choke, connect a meter to the electrical connector and set it to read 12 VDC, and then start the engine, you’ll see a constant reading of about 8 VDC at the electrical connector. So all the while the engine is running, the thermal choke is getting that 8 volts to keep it heated up.


It seems your choke is working properly. You are trying to fix a problem that isn’t a problem. Your assumption that there should be no current is not valid.

automatic chokes on those carbs are horrible! it is probably functioning…(they just dont function well as a state of normal). i have a separate post regarding a 68 mustang but it actually is a 1971 302 with holley 650. the first thing i did was rip the electric choke off of it. I think they are somewhat reliable in normal temperatures. but they are not vital to startup if you remove it and install a manual choke. If temps are not very cold you might be able to get away with not having one at all. carb tuning will help with this too.