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1992 Airstream Landyacht on P32 Chassis, 2001 GMC Savanna Van
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Discussion Starter · #1 ·
I've been having problems with getting my 1992 Airstream motorhome started if its sat for some time. The motorhome has a TBI 454 engine. I found that if I just try to start it, it will crank for a long time with no start but if I turn the key on and off and on a couple of times it will start right up.

What I found out was that my hot fuel module was damaged (fried transistor) and not providing a prime function to the fuel pump. The stock ECU only provided a 3 second prime function but the hot fuel module provided 20 seconds of prime in order to purge the fuel line of any vapor lock. Once started the oil pressure switch would turn the fuel pump relay on and would keep the pump running as long as I had oil pressure.

The problem I ran into was a lack of a replacement fuel modules. Seems that GM no longer stocks it and no aftermarket replacements were available. My module was repairable but I didn't like the design. My solution was to replace the board inside the housing with a board I created that performed the same function. This is what I ended up creating to solve my problem:



This ended up fixing my problem and provided me the ability to program just how long I wanted the pump to run. Right now it's set to prime for 12 seconds or 20 seconds depending on the status of the jumper at the top of the board. I also added an LED to indicate when the pump is being powered through this module.

Does 12 seconds sound like enough time or should I set it for longer? I know that the OEM module provided about 20 seconds of time but I just thought that was a little overkill. I can program it for just about any time interval since it has its own microcontroller instead of a fixed timer.

One side benefit of designing it the way I did was a higher voltage is now allowed to get to the fuel pump. The stock unit would have a significant voltage drop internally and only provided about 11 volts under ideal conditions. Low battery voltages would provide even less. Since this design uses an automotive relay to switch power, there's almost no voltage loss between ignition and pump.

What do you guys think?
 

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Hey, welcome.
Forgive me; but I have a very suspicious attitude when it comes to new guys joining the site and "showing off" a new magic bullet (especially when the board has revision 5 on it).

Anyway, I want you to know, I read your thread on the Airstream forum and you appear to be the genuine article.

At this point, without knowing you, I would endorse the product as a cure for what appears to be a problem on the heavy haulers with the 454.
 

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Welcome and that looks like a viable solution.

One thing though...
Once started the oil pressure switch would turn the fuel pump relay on and would keep the pump running as long as I had oil pressure.
As soon as the ECM gets DRP (Distributor Reference Pulses) from the ICM it turns on the fuel pump relay and keeps it on. The OPSU is just a backup.

Edit, it won't hurt anything to run for 20 seconds it'll just recirculate the fuel back to the tank. Plus it'll help to cool off the TB more and prevent vapor lock.
 

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1992 Airstream Landyacht on P32 Chassis, 2001 GMC Savanna Van
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Discussion Starter · #4 ·
Welcome and that looks like a viable solution.

One thing though...

As soon as the ECM gets DRP (Distributor Reference Pulses) from the ICM it turns on the fuel pump relay and keeps it on. The OPSU is just a backup.

Edit, it won't hurt anything to run for 20 seconds it'll just recirculate the fuel back to the tank. Plus it'll help to cool off the TB more and prevent vapor lock.
It's one of the reasons I decided to make it able to provide two different pulse widths. 20 seconds is the OEM default but the unit will allow for a shorter duration or longer pulse width if desired. All it takes is a little program change. Just wondering what others would like to see and why.

Yes, I'm the original designer. The reason for posting here is to gain additional information of what is needed and features desired. As for why revision 5, my protocol is to make incremental revisions on paper when refining the design even if the actual board design is never produced. That way I can always revert back to a previous level if I find I don't like the changes made. My original design (Revision 2) used a MOSFET to switch voltage, but I found that the relay method in this design produced more predictable results and was less expensive to produce. Revision 1 was the original GM design and board layout. I do have a schematic and board design that reproduces the OEM Module. The problem I've been dealing with is being able to retain the original housing and produce a replacement board that was more reliable with less voltage loss. I then discovered an automotive relay made available from Panasonic that would fit the housing and provide the needed current handling capacity. I think that by revision 5 I was able to do this. Revisions 3 and 4 were prototype versions for test purposes only.

I've been using my own RV as a guinea pig and am pleased with the results thus far. There is a revision 6 on paper but I'm still collecting data on rev 5. Revision 6 will include some changes to make it more reverse voltage tolerant. It will also have more rounded corners for easier insertion. One of the features I added on previous revisions over the OEM board is to take advantage of the two sided board design and provide contacts on both fingers of the connector. This reduces any contact resistance and provided me with a larger ground plain.

Look, I know that this is my first posting on this forum. I understand the skepticism of a first time poster. I just wanted more feedback before I commit to additional boards being produced. The Airstream forum is where I started but this group contains a larger base of big block GM TBI users.

I still can't believe that nobody has attempted reproduction of such a simple but critical item.

If this type of posting is out of line or inappropriate, then I understand and will adhere to whatever is decided.
 

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1992 Airstream Landyacht on P32 Chassis, 2001 GMC Savanna Van
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Discussion Starter · #5 ·
Welcome and that looks like a viable solution.

One thing though...

As soon as the ECM gets DRP (Distributor Reference Pulses) from the ICM it turns on the fuel pump relay and keeps it on. The OPSU is just a backup.

Edit, it won't hurt anything to run for 20 seconds it'll just recirculate the fuel back to the tank. Plus it'll help to cool off the TB more and prevent vapor lock.
As I understand the original design, the module only receives power when the ignition is on. Once turned off the module no longer receives power and the fuel pump stops running. No fuel will recirculate from that point on. The purpose of the Hot Fuel Module is to provide fuel circulation and eliminate vapor lock before the engine starts. High under hood temps can cause vapor lock and this was GM's method of "fixing" the problem without having to alter the ECU timing just for certain big blocks.
 

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Look, I know that this is my first posting on this forum. I understand the skepticism of a first time poster. I just wanted more feedback before I commit to additional boards being produced. The Airstream forum is where I started but this group contains a larger base of big block GM TBI users.

I still can't believe that nobody has attempted reproduction of such a simple but critical item.

If this type of posting is out of line or inappropriate, then I understand and will adhere to whatever is decided.
Not to worry, just making sure we had a real member and not a bot.

What's your background?
 

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1992 Airstream Landyacht on P32 Chassis, 2001 GMC Savanna Van
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Discussion Starter · #7 ·
Not to worry, just making sure we had a real member and not a bot.

What's your background?
I've had a wonderful 23 year carrier as an electronics test engineer with several well known US and Japanese companies. Now my wife and I own a franchise business together.

I've designed hundreds of electronic automated test systems verifying manufacturing and in design development of cellular phone products. Not saying I'm God's gift to electronics but I do know my way around circuit design and prototyping at various levels. I also am a proficient software programmer (C++ and Visual Basic). Now it's just a hobby. I have a small lab setup in one of my spare bedrooms. Nothing elaborate... Just an O-scope, logic analyzer, power supply, DVM and a couple of solder stations including a hot air solder station for surface mount devices. Any board production is done at a small fab prototype shop (OSH Park) in batches of 3 at reasonable prices. Mouser Electronics is a great source for parts as well.

I like to find things that I'm in need of and design something to meet my needs. My wife and I like to hike trails so I designed a GPS tracking system that allowed me to see my current GPS position and record data on an SD card every 5 seconds. Module can run for 11 hours on a single portable battery charge. Allows me to verify my current position and later to plot my path using Google Map.

I also developed an Aquarium controller that maintains water level and temperature. It also controls the lighting and feeds the fish. Every month it cycles the tank by draining it down 1/3 and then refilling it. Will text message me if there are any alarms.

I make my own beer and wine so I've started to automate the process. I made a stir plate system that maintained stirring action on a yeast culture. It gives me control over stir speed and even gives me a visual indicator of actual speed using hall effect transistors to measure speed.

Always looking for new things to keep me entertained.;)
 

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As I understand the original design, the module only receives power when the ignition is on. Once turned off the module no longer receives power and the fuel pump stops running.
Not exactly, when you turn on the key, engine off, the ECM turns on the fuel pump relay. If it doesn't see any DRPs within 2 seconds, it'll shut off the relay. Then, when you start cranking AND it sees DRPs, it turns the relay back on until it doesn't see any more (once the engine is shut off). If the engine relied strictly on the OPSU you'd experience longer cranking times.
 

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1992 Airstream Landyacht on P32 Chassis, 2001 GMC Savanna Van
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Discussion Starter · #9 ·
Not exactly, when you turn on the key, engine off, the ECM turns on the fuel pump relay. If it doesn't see any DRPs within 2 seconds, it'll shut off the relay. Then, when you start cranking AND it sees DRPs, it turns the relay back on until it doesn't see any more (once the engine is shut off). If the engine relied strictly on the OPSU you'd experience longer cranking times.
Sorry if I wasn't clear. I was strictly speaking about the Hot Fuel module's involvement. The HFM sees nothing but ignition and ground. As you already know, it has no view of engine status. If the engine stalls within the initial delay time it will still keep the pump running for the remaining time as long as ignition is still on. Once ignition is turned off, it will no longer provide power to the fuel pump under any circumstances as long as it's working properly.

As for the OPSU, looking at the schematics, it looks like the OPSU is in parallel with the fuel pump relay contacts.

Even the ECU can't stop the fuel pump as long as the OPSU sees pressure. Not likely to be an issue unless the OPSU gets stuck in the shorted state.

I did have another person that had the HFM drive transistor short out. Actually a common failure mode. It basically kept the fuel pump running as long as the key was turned on. Basically defeating the safety feature of the OPSU. The original design of the HFM inherently provided low voltage to the fuel pump due to two devices that were in series that both had significant insertion loss in the range of a volt +- a tenth or two. When his shorted out, it allowed a low voltage to keep the pump running for an extended time and burnt out his fuel pump. Admittedly his problem was mostly due to him keeping the key on for an extended time while he was working on the vehicle and the engine was not running. He was trying to trace down another no start problem and ended up creating a second problem because of his damaged HFM.
 

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Sorry if I wasn't clear. I was strictly speaking about the Hot Fuel module's involvement
Ahhh, OK. My mistake. I misread your post. I'm curious, how does the hot fuel module know it's hot?
 

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1992 Airstream Landyacht on P32 Chassis, 2001 GMC Savanna Van
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Discussion Starter · #11 ·
Just for additional information, here's a schematic diagram of the original HFM that I derived from one that was sent to me for repair.

And here's a close approximation of the board layout. Not exact, but close enough to reproduce the design:


And here's a 3D model:

 

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LMAO! But seriously, does the HFM only come on when the engine is warm or does it come on every time you turn on the key?

Edit: I think I answered my own question - it looks like the HFM comes on every time, if I read your schematic correctly. It feed back through the fuel tank selector switch, huh?
 

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1992 Airstream Landyacht on P32 Chassis, 2001 GMC Savanna Van
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Discussion Starter · #14 ·
LMAO! But seriously, does the HFM only come on when the engine is warm or does it come on every time you turn on the key?
Every time the key is turned to ON it will power the pump for as long as the key is ON and the duration has not expired. Actually I found that it turns on twice. First when the key is turned to ON and a second time when the key cranks the engine. Seems that ignition is lost for a short time between those two events. It's one reason I decided to make the initial default time delay 12 seconds instead of the factory 20 but still gave provisions for a full 20 second delay.
 

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1992 Airstream Landyacht on P32 Chassis, 2001 GMC Savanna Van
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Discussion Starter · #15 ·
Any additional input on timing requirements? Just how long should the fuel pump run with no engine start?
 

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1992 Airstream Landyacht on P32 Chassis, 2001 GMC Savanna Van
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Discussion Starter · #16 ·
LMAO! But seriously, does the HFM only come on when the engine is warm or does it come on every time you turn on the key?

Edit: I think I answered my own question - it looks like the HFM comes on every time, if I read your schematic correctly. It feed back through the fuel tank selector switch, huh?
Yep, feeds back through the fuel tank selector on models so equipped. On my motorhome, it only has one 80 gal. fuel tank, so it goes straight to the fuel pump.
 

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1992 Airstream Landyacht on P32 Chassis, 2001 GMC Savanna Van
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Discussion Starter · #17 ·
Currently have 3 made up for anyone looking for one.
 
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