Possible replacement for Hot Fuel Module

[92Landyacht]

One of my recent projects started as a fix for a problem I was having with my Airstream. If the MH sat for a while without being started I found that it would crank FOREVER before it would finally start. I did find that it would start quickly if I turned the ignition ON/OFF/ON a few times. After that it would start right up. Long story short I found that the "Hot Fuel Module" was the culprit. When I tried to find a replacement, I found that none were available OEM or aftermarket. Opening up the plastic box that housed the board inside I found that it was easily removed. Further inspection identified the damaged component. The output transistor was burnt up. I did fix the module by replacing the transistor but also found that the output voltage from the module to the fuel pump was much lower than ignition voltage. Looking at the design I realized that it had an inherent voltage loss by design. Decided to design a new replacement board with more up to date components and a high power relay to replace the output transistor. The end result was a programmable module that I could set to any prime timing I wanted no significant voltage loss. I was a direct plug and play solution to my problem. Installed it into my MH and everything is working great! Here's a pic of the new module:

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Here's a rendition of the original module that GM created:

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It was the Q3 transistor that was damaged in mine. Seen several others since.

While I was doing the design I decided to add a couple of "features". You'll notice an LED at the top. The LED shows me when the module is switching power over to the fuel pump. It can't be seen when the housing is closed but can be seen when the lid is open. The other feature is a pair of jumper pins. This allowed me to program it for two different timing pulses. With no jumper the timing is currently set to 12 seconds. Providing the jumper changes the duration to the OEM 20 seconds. I also took advantage of a double sided board and the two edge connector that gave me twice the contact area.

Right now I'm going to just gauge NEED. If there is a need in the community I'll be looking at producing more to fill that need. Not trying to make big bucks. Just trying to see if I need to have more boards made up and how many. It was very frustrating to me that this part was not available anywhere and was a bit of a nuisance when not working. In one case, the OEM module was shorted out and kept the fuel pump running for as long as the ignition was turned on. Normally this would not cause a problem but in his case he left the key on to troubleshoot a "no start" problem. The low voltage output from the module ended up frying his fuel pump before he found the problem.

I've got some additional information on the OEM module if anyone is interested. Here is a schematic of the module:

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One related question... What timing duration do you guys feel is needed? I can program these for any duration from milliseconds to days with only a couple lines of code to change.

I could add any other features that you folks can think of. Was thinking of a troubleshooting mode that forced the fuel pump to be always on as long as the ignition is kept on. Maybe a button press to force it into this mode. Would exit this mode on the next ignition cycle. Just thinking.

 

[Dale Stevens]

OK; Explain " hot fuel module " . (Nice project) . Dale

 

[92Landyacht]

The "Hot Fuel Module" was added to late 80's and early 90's big block TBI vehicles to avoid vapor lock. It was deemed necessary to avoid vapor lock on higher temperature producing engines. It was added to the wiring harness near the ECU to those vehicles instead of making changes to the ECU. Basically it causes the fuel pump to run for about 20 seconds (as opposed to the ECU default of 3 seconds) with the ignition on even if the engine isn't started. This allowed the vapor lock to be purged and enhanced engine start.

Here's a schematic showing where it's located. It's called simply "Fuel Module":

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As you can see it simply supplies voltage to the fuel pump in parallel with the ECU, Fuel pump Relay and Oil Pressure Switch.

 

[Schurkey]

God bless you. I get all tender when folks find a need, and fill it even though the Big Bad OEM and the normal aftermarket sources have abandoned us.

I don't own a vehicle that needs this module; and I don't know anyone who does. I am almost certainly not a potential customer. Even so, thanks for doing this work.





There's no aftermarket or OEM source for "wiper modules", the circuit board that controls the windshield wipers on 1st Generation GM W-bodies--I eventually found a Treasure Yard module to make my wipers/washer work on a '92 Lumina.

 

[L31MaxExpress]

I had one of those on my G20 van try to catch fire and smelled the inside of the van up with electrical smoke. Unplugged it and never had an issue without it. That being said that is a cool engineering project. If someone is tuning the TBI system anyway the fuel pump prime pulse is adjustable up to 60 or 90 seconds. When I had TPI on the G20, I set the prime pulse for 10 seconds. Even with empty lines and an empty fuel rail, 10 seconds fully primed the fuel system and had the rail holding 40 psi.

 

[PlayingWithTBI]

If someone is tuning the TBI system anyway the fuel pump prime pulse is adjustable up to 60 or 90 seconds

I didn't know that, where in a 7747 .bin is it?

 

[92Landyacht]

I had one of those on my G20 van try to catch fire and smelled the inside of the van up with electrical smoke. Unplugged it and never had an issue without it. That being said that is a cool engineering project. If someone is tuning the TBI system anyway the fuel pump prime pulse is adjustable up to 60 or 90 seconds. When I had TPI on the G20, I set the prime pulse for 10 seconds. Even with empty lines and an empty fuel rail, 10 seconds fully primed the fuel system and had the rail holding 40 psi.

That's the reason I decided to default to 12 seconds of prime with the option to select 20 if needed. 20 seconds just seemed like a little overkill.

The burning up transistor is the other reason I decided to redesign the unit. With the availability of a high current (20A) low profile 12V relay I decided to that route instead of a semiconductor switch. Some have simply removed the module with no issues, while others have had starting problem. In my case it did help.

 

[92Landyacht]

There's no aftermarket or OEM source for "wiper modules", the circuit board that controls the windshield wipers on 1st Generation GM W-bodies--I eventually found a Treasure Yard module to make my wipers/washer work on a '92 Lumina.

Sounds like a neat new project. I just need a sample of one that's damaged and in need of repair. From that I can usually determine how it works and how to reproduce or improve it.

 

[Schurkey]

I probably tossed my non-functional wiper module; or it went back to the Treasure Yard as a core. If I find it (unlikely) I'll get ahold of you.

 

[Insert Quarter]

I'm interested, I was just trying to figure out how to fix this and came across this thread!!!