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Old 07-12-2021, 02:28 PM   #1
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P30 "Hot start module"

After reading PeterH-360LE's thread on fuel pump wiring I decided to fill a need that may help others along the way. Since the module is no longer available from GM, or any other source for that matter, I decided to design a replacement board for the module. With the generous contribution of Peter's old damaged module I was able to get a good start.

My first step was to learn about what the module does and how the original module was designed. I have been able to produce a schematic diagram for the original module. I've identified most of the parts and those components (2) that I haven't been able to definitively identify I have been able to find replacements for. Once all the parts arrive, I'll fix Peter's module and then I'll breadboard the original design from the schematics I generated and make sure that it functions like the original.

The next step will be to produce a newer, more updated, design using modern components. Current plan is to use a micro controller to create the timing pulse and then use an N-Channel MosFET as the output driver. The original design used a simple transistor but the issue related to using that method is a much higher internal resistance that creates a voltage loss and also generates a lot of heat which eventually destroys the output transistor as noted by Peter's unit that failed. The MosFET will have very little voltage drop across it so most of the voltage will be delivered to the pump and less heat will be generated inside the module. One advantage of using the micro controller will be that I can program it for any time delay I want or even allow the user to select various time delays by simply jumpering out a couple of different pins. Since I can program it for virtually any time delay, I can allow the user to select times like 15 sec, 20 sec, 25 sec or 30. Any suggestions on what run times would be desired or any other features needed would be greatly appreciated.

Since this module had a limited production time I think that only a few here would be in need, but I'm pretty sure that the truck community as a whole might have a higher need over all. Seems like this module was used on many big block equipped medium and light duty trucks in the early 90's. I might have to get in contact with some GM truck forums to see what their needs are as well.

I'll keep you guys up to date as I make progress. That assuming there's interest here.
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Old 07-12-2021, 04:34 PM   #2
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Most certainly interest from me. Most of the technical stuff is unfortunately above my paygrade, but I do get the general idea.

As i understand it, the main reason for the module was a "fix" for vapor lock on heavy and long vehicles. What puzzles me a little bit, is absence of general knowledge about this. Yes, after looking very hard I found some references, but for the normal operator of a vehicle like that, one has to get into pretty deep.

Do folks just turn the ignition on to allow for the mystery module to do its job. Probably not. I have in the past turned the ignition on/off/on hoping it would pressurize the system with what I thought to be a 2-3 second event.
My 2019 Chevy van never gets to enjoy much time in the just ignition on mode.
I suppose the info came with the operators manual when the coaches were new?
If it would not have been for my DW to say in passing (whilst I was really busy trying to get that darn radiator passed the power steering pump reservoir), "hey Peter your 350 is making a funny noise in the back" I ignored her, but hours later I heard it too. Then I realized I had left the ignition on and probably fried the fuel pump
The long odyssey of trying to figure out what caused this is well documented here

The danger of a defective module as in my case, can not be underestimated.
1. Burning out the fuel pump.
2. Unsafe power to the fuel pump with the engine not running (oil pressure switch over ride)
As I mentioned before, it would be safer to remove the module than having it supply power to that Tan/white fuel pump circuit by its own accord. no fail safe in place. The ECU/PCM is completely ignorant to the fact. It just passes on the flame to the module, but never checks what the modules does with it.
no wonder its no longer available. Makes me wonder how many trucks/vans/Rvs were and are on the road with a defective module??
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Old 07-12-2021, 05:11 PM   #3
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I understand your concern but...

In all cases, even yours, the module would do nothing if the ignition was turned off. The only time it would be an issue would be if the module was shorted AND the ignition was left on. Don't know too many that have left there ignition on for an extended time. Even in this case it would not really present many problems. Leaving the fuel pump running would not cause the pump to go bad either. The pump and pressure regulator system is designed to be on for extended periods of time. Think about it... All the time that the engine is running, the pump is on and producing pressure. Any extra fuel that is not consumed by the engine is just routed back to the tank. If the engine is not running, the injectors would not allow gas to pass into the engine so no fuel leak danger there. The only potential issue is during an accident, if the engine is cut off and a fuel line is ruptured. In that situation it would present a real danger.

I think the real reason that there are none on the market, is because there was relatively few of these made and were from so long ago that there is no profit in producing them. This along with the fact that disconnecting it would fix most issues. The original problem of prolonged sitting and hot start would still exist with it disconnected, but the engine would eventually start if the module was disconnected.

I still plan to produce a working unit. If anything, just for the fun of it.
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Old 07-12-2021, 06:07 PM   #4
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Quote:
Originally Posted by 92landyacht View Post
I understand your concern but...

In all cases, even yours, the module would do nothing if the ignition was turned off. The only time it would be an issue would be if the module was shorted AND the ignition was left on. Don't know too many that have left there ignition on for an extended time. Even in this case it would not really present many problems. Leaving the fuel pump running would not cause the pump to go bad either. The pump and pressure regulator system is designed to be on for extended periods of time. Think about it... All the time that the engine is running, the pump is on and producing pressure. Any extra fuel that is not consumed by the engine is just routed back to the tank. If the engine is not running, the injectors would not allow gas to pass into the engine so no fuel leak danger there. The only potential issue is during an accident, if the engine is cut off and a fuel line is ruptured. In that situation it would present a real danger.

I think the real reason that there are none on the market, is because there was relatively few of these made and were from so long ago that there is no profit in producing them. This along with the fact that disconnecting it would fix most issues. The original problem of prolonged sitting and hot start would still exist with it disconnected, but the engine would eventually start if the module was disconnected.

I still plan to produce a working unit. If anything, just for the fun of it.

May be I had a defective fuel pump, but it did killed it leaving it on for 5 hours. or maybe the low voltage did it?

If I would not have been able to find the cause of the problem, I would have wired a manual over ride switch for my rear airbags. The reason is the Tag Axle. If Airstream MH owners ever get into the need of being towed, it will must likely be a nose up tow. The higher the nose the higher the airbags will inflate, since they are controlled by the tag axle. Leaving the ignition on for the tow, will be your chance to safe the Tag axle mounts from self destruction
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Old 07-12-2021, 07:56 PM   #5
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Wouldn't a simple 555 timer circuit controlling a relay or mosfet pretty much work the same? In fact you can buy those type of modules for cheap. https://www.amazon.com/NE555-Switch-...57200951&psc=1
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Old 07-13-2021, 04:59 AM   #6
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Originally Posted by PeterH-350LE View Post
May be I had a defective fuel pump, but it did killed it leaving it on for 5 hours. or maybe the low voltage did it?
Low voltage was most likely the culprit in your failed fuel pump. They are designed to run in the 12 to 14 volt range and you were providing something like 9 volts.

When voltage goes down current goes up so yes, I believe low voltage killed your pump.
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Old 07-13-2021, 05:12 AM   #7
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Low voltage was most likely the culprit in your failed fuel pump. They are designed to run in the 12 to 14 volt range and you were providing something like 9 volts.

When voltage goes down current goes up so yes, I believe low voltage killed your pump.

Then that would beg the question, how would one check what voltage is going to the fuel pump with ignition on and how long does it take to do harm to the fuel pump. (the replacement module I installed has the same low voltage as discussed above)
Other than poking the wire (since most have an in tank pump) the place to check is the upper left pin (490) on the ODB1 or ALDL.
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Old 07-13-2021, 06:06 AM   #8
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Then that would beg the question, how would one check what voltage is going to the fuel pump with ignition on and how long does it take to do harm to the fuel pump. (the replacement module I installed has the same low voltage as discussed above)
Other than poking the wire (since most have an in tank pump) the place to check is the upper left pin (490) on the ODB1 or ALDL.
Sounds to me like you have another bad module. I would think no matter where you check for pump voltage it should read above 12 volts, i.e. ignition voltage.

I wouldn't think 20 seconds is going to damage the pump.

Are you seeing less than 12 volts on the pump wire with the engine running? If you are then I would be concerned.
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Old 07-13-2021, 06:21 AM   #9
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Wouldn't a simple 555 timer circuit controlling a relay or mosfet pretty much work the same? In fact you can buy those type of modules for cheap. https://www.amazon.com/NE555-Switch-...57200951&psc=1
I originally looked at using the 555 but the problem is getting the one shot to fire once on powerup without any false triggers. The 555 works well when triggered properly, but the potential for misfires is high on a "trigger up on power on" without adding some other components and even then it's prone to false triggers.

One of the design constraints I have is the physical size of the original housing. I wanted the design to be plug and play using the original box since the board inside is easily removed and the housing can be reused.
The board inside just plugs into an edge connector at the bottom. No new connectors or housing would be needed.

The micro controller I'm planning on using is an ATTINY85 which actually costs less than a 555 and gives me the reliability of function on startup. It also allows me to adjust the delay as needed without changing any external components. The chip is completely self contained and comes as a standard 8 pin dip package or a SOP surface mount package. The only additional component needed to drive it is a simple 5v regulator.

I looked at using a relay, but I'm still trying to find one that has the current capability and can fit into the original housing. If I gave up on using the original housing I could already have it finished but... what would be the fun of that.
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Old 07-13-2021, 06:31 AM   #10
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May be I had a defective fuel pump, but it did killed it leaving it on for 5 hours. or maybe the low voltage did it?

If I would not have been able to find the cause of the problem, I would have wired a manual over ride switch for my rear airbags. The reason is the Tag Axle. If Airstream MH owners ever get into the need of being towed, it will must likely be a nose up tow. The higher the nose the higher the airbags will inflate, since they are controlled by the tag axle. Leaving the ignition on for the tow, will be your chance to safe the Tag axle mounts from self destruction
I've had my 36' Land Yacht, with tag axle, towed a couple of times. In both cases I never had to turn the ignition on. Yes, it was towed from the front but it never needs to have the airbags inflated to do so. The equipment needed to tow an RV is the same equipment used to tow a tractor trailer and they don't need to lift the front very high. 6 inches if I remember correctly.
In both cases the airbags were dumped by the leveling system.

The first was when I bought the RV and had it dragged home. A little over an hour ride. In the second case it was because the ECU was causing a no start issue. This time it was only a 20 minute ride. No tag axle damage in either case.
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Old 07-13-2021, 06:58 AM   #11
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Here's a sneak preview of what I'm planning. Still have a lot of work to do but you get the idea:

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Old 07-13-2021, 07:22 AM   #12
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Sounds to me like you have another bad module. I would think no matter where you check for pump voltage it should read above 12 volts, i.e. ignition voltage.

I wouldn't think 20 seconds is going to damage the pump.

Are you seeing less than 12 volts on the pump wire with the engine running? If you are then I would be concerned.

engine start and run the ECU takes over and supplies the proper voltage
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Old 07-13-2021, 08:59 AM   #13
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engine start and run the ECU takes over and supplies the proper voltage
If you have the proper voltage once the engine starts, then you're ok.

The original module did have a voltage drop when under load. Once the engine starts, power from the fuel pump relay is in parallel with the module for the 20 second delay and then is the only supply source after that.

One of the reasons I decided to use a MOSFET is because of the voltage drop through the PNP transistor in the original design. A relay would be simpler, but to drive the pump you need 10 amps. Any relay should be rated above 10 amps for margin. Finding one small enough to fit in the original case is difficult and would be expensive at that rating. FETs are more reliable over many cycles as well. The only downside to using the FET is making sure you use the right driver in order to reliably turn it on and off.
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Old 07-15-2021, 06:47 PM   #14
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Just dawned on me that I completely overlooked a "little" issue. 20 amp required for the Walpro Gsl 395. 10 amp what appears to be supplied by the GM System for that circuit, right?
So, I will add a relay at the pump to take care of multiple issues, one of them being potential low voltage from the module. As I understand it relays will function at ~80% supply voltage. 80% of 12v=9.60V

Is my logic correct? any other issues I should consider? What gauge wire for the 12-15' run?
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Old 07-16-2021, 06:13 AM   #15
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Just dawned on me that I completely overlooked a "little" issue. 20 amp required for the Walpro Gsl 395. 10 amp what appears to be supplied by the GM System for that circuit, right?
So, I will add a relay at the pump to take care of multiple issues, one of them being potential low voltage from the module. As I understand it relays will function at ~80% supply voltage. 80% of 12v=9.60V

Is my logic correct? any other issues I should consider? What gauge wire for the 12-15' run?
I think you have your numbers wrong on how much current that pump draws. 20 amps seem very high. Here's the graph from Walbro:


Looks to me like it never gets above 4 amps.
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Old 07-16-2021, 06:21 AM   #16
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I think you have your numbers wrong on how much current that pump draws. 20 amps seem very high. Here's the graph from Walbro:


Looks to me like it never gets above 4 amps.
Maybe they suggest using a 20 amp fuse? Just guessing here....
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Old 07-16-2021, 06:31 AM   #17
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Maybe they suggest using a 20 amp fuse? Just guessing here....

Yes, I didn't make that clear, 20amp fuse required per Walpro
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Old 07-16-2021, 06:42 AM   #18
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A 10 amp fuse would work just fine.

PS: I was able to pick up the needed transistor and verified that your damaged module is now functional again. Now I have a working unit that I can do more testing on.
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Old 07-18-2021, 10:48 AM   #19
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Ordered a PCB for the new design today. Board layout was pretty straight forward. All the components needed to build the first prototype once the board arrives are in hand. Should have the new PCB here in a few weeks.

I'll continue to verify my tracing down of the tracks on the OEM board. Once I've verified that my schematics are in order, I'll post a pick of the schematic for those interested. The only component that seems to be vulnerable to failure is the output transistor. I've seen posts all over the NET that show it listed as a "voltage regulator". It is not a voltage regulator. It is a simple PNP power transistor. I'll post a viable replacement part once I've determined what's available.
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Old 07-18-2021, 11:17 AM   #20
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Following... interesting thread...
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