Hi Jon,

If you get a chance, can you share your latest circuit and the relay PNs you are using? You can PM me and I will give you my email, if you don't wish to post it. Would be helpful to include any wire lengths and gauge sizes as well.

Without seeing the latest design, a couple of thoughts come to mind:

Are you experimenting on a bench with a power supply, or on an actual vehicle with a battery? Relay chatter can sometimes be caused by voltage fluctuations due to the high inrush current of a DC motor. (Relay energizes, motor activates, current spikes, voltage drops, relay deactivates, repeat...) Inrush can be much higher than running current, so a supply that would seem to provide enough current normally, may be temporarily dipping low enough to affect electronic circuits.

Also, and you probably checked this, but are you sure that the transistor that switches the relay is fully into saturation? Without going downstairs, and blowing the dust and spider webs off my circuit design books, I seem to recall that when used as a switch, the rule of thumb for bipolar transistors was to run base current 10 times normal saturation to ensure that it stays out of the linear region, and does not fall back into cutoff. Perhaps you could try reducing the value of the 3.3K resistor that feeds the 4401 base? Maybe clip like a 500 ohm or 1 K across it ?

I remember building basic automotive relay drive circuits in the past for quick tests. In order to drive the relay coil I sometimes used a TIP 121/122. It is a Darlington design, so it has tons of beta, and can drive up to 5 amps.

TIP

I used to also sometimes drive relays using the parallel port from a PC. (This was wayyyyyy before USB) To electrically isolate the PC from whatever it was we were testing, I used optically coupled relay drivers something like these.

Opto Isolator

They also have really good beta (transfer ratio, technically), and can easily drive an automotive type relay.

Lastly, do you have access to a scope? It would prove very helpful in debugging transient conditions like this.

Best Regards....