Imperial Home Page -> Repair -> Fuel -> 1981-1983 Tips -> Fuel Pressure Switch
Tip from Dick:
I have recently been on the track of an identical erratic starting situation, and I think I have tracked it down to a faulty Fuel Pressure Switch. Just to confirm it is easy, finding a new one will be a little tougher, although the function is very similar to that of an idiot light sender for a car with an oil pressure warning light, in that it closes on low pressure and opens on high pressure.
To confirm a bad FPS, take off the lid of your air cleaner, find the FPS, it is identical in appearance to any other pressure switch, like a brake light (hydraulic type) or an oil pressure idiot light sender. It hangs upside down, like a sleeping bat, from the fuel lines connected to the control fuel pump, and has one wire clipped to its lowest part. Pull this wire off (sideways in the direction of the wire travel) and take your VOM, measure the resistance from the FPS terminal to the fuel line. It should be zero ohms, or close to it. If it is higher than about 50 ohms, it is probably what is causing your problem. I found one of them that is very high, like an open circuit, in a car that was acting the way you describe. Dick Benjamin
In response to a hot-restart problem, where the car will not restart when it is warm and has sat for awhile:
This is a common problem with this car, even when everything is to spec. This problem was addressed with a TSB in mid-'81 in which it was stated that changing the Fuel Pressure Switch to PN 4091901 was the factory cure. It was not a complete fix. (By the way, this part is not available from MoPar.) The purpose of the new Pressure Switch was to allow more pressure to charge the pump before it opened thereby assuring a better start.
There is a fuel pressure regulator on the end casting of the Control Pump that was revised to change the spring pressure on the relief valve. The new spring is identified by a dab of yellow paint on the top of the cap over the valve housing. Also, you can fashion a tool with a small piece of brass tubing, by cutting four, equally spaced "Vee" notches in one end of the tube. Insert it into the well, push down gently and turn and remove the valve in its housing. You can inspect this for dirt and contamination.
This being said, hot starts remain a problem. The fuel literally boils off in the heat soak period after a prolonged shutdown. I took some aluminum-faced, foam backed insulating material and cut it to fit in the various places on the underside of the HSP to reflect the heat away. I also wrapped the fuel lines, (after removing all of the supporting brackets from the restraints on the rocker cover), with a thick wall rubber hose that will fit over the piping outside diameter. The fuel line coming up the face of the bulkhead is also insulated this way. All of this to retard heat soak and the resulting loss of fuel. Unless this area can be ventilated to remove the heat, we'll have this problem. Bob Harris
A few years ago, I tried to find a suitable substitute for the FPS in an oil pressure "idiot light" type sending unit. The electrical function of this switch is identical, that is: to short the connection to ground until the pressure comes up to 20 PSI (the original pressure spec, right Bob H.?). Most oil pressure idiot lights are set to come on at a much lower pressure, typically 6 PSI or so, but I did find one which had a much higher threshold - as I recall it had an application in some Ford truck engine. It had the wrong threads, so it would have to be adapted to fit, and since the space available is so cramped, it would be a bit of a kludge to fit in, but it would be possible. The thresholds for the switches are shown in the NAPA buyer's guide. The one which comes closest to the correct electrical parameter is OP6604. Its pressure threshold is specified as 11 to 17 PSI - if you sorted through these things to find a high one (17 PSI), it would probably work OK in most conditions. If one could be found with a higher and better controlled threshold, it would solve our problems. So far, I have not found one. -Dick B.
In Reference to adding a �Start Dammit� Switch, a switch that could be used to replace the FPS grounding circuit. This switch could be placed inconspicuously under the dash, and wired to they wire normally attached to the FPS. When starting the car, the switch could be pressed, grounding the circuit and driving the In-Tank and Control Pumps at full speed, then released when the engine started. Dick B. explains why the switch would need to be pressed BEFORE turning the key on:
With the key off, you will have to push the button (grounding the FPS contact) BEFORE you turn the key on (order of events is important here) and immediately crank the engine; it should start as soon as it sees the gas, then you can release the button. Pushing it again will not do anything, it only works when it is pushed BEFORE the key is on. If the car doesn't start, turn the key all the way off, and do it all over again.
Question from Ed:
Dick, why is it that you must ground the switch BEFORE turning the ignition on?
Reply from Dick:
The power module circuit which controls the purge cycle is designed to purge the vapors only once per start sequence (to avoid pumping too much gas into the throttle bores). The way they do that is to trigger an SCR (silicon controlled rectifier) when the pressure comes up to 20 PSI, but they didn't want it to quench and begin pumping again if the fuel pressure should drop below 20 PSI (which it could do under some conditions, like a very long downhill coast at low speeds.) So the circuit is designed to latch into the non-pump mode any time that the 23 volt power supply is on, when, IF AND ONLY IF, the FPS opens. If the FPS is working right, this will never happen if the car has been sitting; the switch will always be grounded before the power is turned on, but with our auxiliary "start dammit" switch, and assuming the FPS is bad, the circuit would think the FPS is open if you haven't pushed the button before the power is first applied, and latch into the "don't-pump" mode.
I guess I should point out that an SCR is a device that is an open circuit until triggered, then becomes a diode in conduction when triggered until the current through it drops below some very low threshold, or reverses, at which point it reverts to an open circuit. If you go on the internet and look up a Motorola 2N5062 (thanks again, Bob Harris!), you'll see what I mean. That is the famous Q52 in the power module circuit.
Tip from Mike:
I have been reading your posts on the fuel pressure switches and wanted to inform you of a possible solution. In the Summit racing catalog I Saw a oil pressure switch that was rated at 15psi and one rated 30psi--maybe one of these would work.
Follow-Up:
Thanks to Mike's tip, I have contacted Summit and bought one of the 30 PSI sending units and tested it on my 82. They do indeed seem to work fine, the closed contact resistance measured less than 1 Ohm, which is great! The unit is adjustable from 20 to 40 PSI; I left it at 30 PSI, which is a tad higher than the original Chrysler spec, but this seems to me to be a good compromise, given that our normal problem with these cars is not enough fuel for warm startup, rather than too much.
The bad news is that the sender is larger than the original FPS, so you have to be a little creative to install it, but it can be done, with a little careful positioning of the wire connector, and by rotating the brass block a little off vertical. The other bad news is that I was only able to purchase it by buying a whole whiz-bang kit deal with fitting, wires, and a fancy light that is supposed to warn the drag racer that his oil pressure is dangerously low (one would think the pistons flying through the hood would serve the same purpose).
I have reverse-engineered the sender and contacted the vendor of same, and am awaiting a quote on supplying just the sender, in some quantity ( I suggested lots of 20), intending that if I am able to obtain these directly, I will just make them available at my cost to anyone who needs an FPS, or would like to keep one in reserve for the proverbial rainy day.
Not to be secretive, the maker of the sender is Faria Corp, in Uncasville, Conn.
Another caveat is that I had some considerable difficulty getting all the fittings to stop leaking after disturbing the fuel lines, these run up to 80 PSI, after all.
Anyone with a fuel pressure switch problem, I have found a cure at http://www.jeqs.com
Jeg's sells a pressure switch made by Auto Meter. It is designed for drag racers to operate a warning light to be used with a gauge as well.
The part number is # 105-3242 and it runs at 30 psi.
I purchased one of these and put it on my '82, and so far the car starts right up every time.
It was a tight fit getting the switch to fit. It has two connectors that hang straight down. One I ran to the ground on the support plate. ( I had to make a wire to do this). I took the plastic insulator off the wire that plugs into the switch, and just slid it on the other prong. I had to connect the wires before putting switch in. And then screw it in.
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