7.3 IDI with Allison 1000 overheating

[AzironaZack]

Hey there, folks! I've got a 1991 International 3700 based short school bus with a 7.3 IDI, original injection pump, non-wastegated Banks Turbo, and intercooler. This bus is an RV for me and my wife.

I am having trouble with overheating.

The radiator, thermostat, fan, and viscous clutch are all new in the last couple years. The radiator shroud is in great shape and fits the fan well.

I recently swapped the AT545 transmission for an Allison 1000 six speed. I have 4.44 gears in the rear (originally had 5.38 ratio).

On a drive up Mt Lemmon (local mountain for us in Tucson) on a 105 degree day the coolant temperature light and alarm came on around halfway up. The coolant temperature was 210 degrees (according to the tiny, old gauge on the dash) and the transmission sump temperature was 210. I pulled off and allowed the bus to idle cool in the shade, but it still is disconcerting.

The radiator is a Northern Radiator 232124, which is brass and truly enormous (31 x 26 1/8 x 2 1/16 (4 Row)). The radiator cools both the engine and the transmission fluid.

I'm hoping folks can provide some advice on the overheating.

I suspect the new gearing with the six speed has dropped my engine speed to the point where it's affecting the cooling capacity of the fan. This is just a guess.

When I installed the intercooler I cranked up the injection pump fuel screw. I'm making great power now, but I wonder if this is part of the problem too.

I'm curious what the experts in here think.

Would it be worthwhile to switch to a big electric fan set up? What about switching back to the 5.38 gears in the rear end? Swapping to the higher gears would make my engine speed higher for a given road speed. Right now I seldom see sixth gear, but with the higher ratio I'd probably see sixth more regularly.

What do you think? Thanks!

 

[PorchDog]

If you are lugging-- that contributes to heating. On my bus with a DT466 and 5 speed, when it begins to heat on a grade, I will slow down if necessary, and shift to a lower gear, which usually fixes the problem in less that a minute.

 

[AzironaZack]

On the lead-up to the coolant temperature warning coming on I was doing 30 mph in 2nd gear at around 2000 RPM for a good long time. It didn't feel like it was lugging.

With the old AT545 I'd be doing 30 at closer to 2800 RPM in 2nd.

 

[nikitis]

Who did you get to swap out your transmission? I want to do this exact swap.

 

[AzironaZack]

I did it my own self, with the help of Jason at TransmissionTuner.com. Jason provided the DIY wiring harness kit and the computer and the tuning. I did all the work. It's a huge job.

 

[magnakansas]

30 mph, 2000 rpm. Was the clutch in the torque converter locked? If it was locked, was it slipping? Was the fan roaring?

My experience says 30 mph, not enough speed to push air through the radiator. You must have the fan locked and full boogie. The fan will roar locked at 2000 rpm.

I think I would like to see 2200rpm-2300 rpm at a sustained full load run. Do you have a way to see your boost level AND egt-exhaust gas temperature?

Were you foot to the floor at 30 mph?

William

 

[AzironaZack]

30 mph, 2000 rpm. Was the clutch in the torque converter locked? If it was locked, was it slipping? Was the fan roaring?

The torque converter was locked, yes. No slipping indicated in the log.

The engine is always loud, so I can't really say if the fan was roaring or not. The fan clutch was replaced in 2023 so it's unlikely to have failed, but I suppose it's possible.

My experience says 30 mph, not enough speed to push air through the radiator. You must have the fan locked and full boogie. The fan will roar locked at 2000 rpm.

The engine and fan are always roaring loud when climbing mountains. I'll try turning off the motor when it's all the way up to temp someday and see if the fan clutch freewheels. Do you have any other tests for the fan clutch?

I think I would like to see 2200rpm-2300 rpm at a sustained full load run. Do you have a way to see your boost level AND egt-exhaust gas temperature?

My boost and EGT gauges are analog so I don't have that data logged, but EGT never exceeded 900 degrees. I don't remember what the boost was, but it wasn't unusually low or high. It's typically around 7-8 PSI during slow-speed, low throttle uphills. If I wind the motor up to 2800-2900 it'll be up at 11-12 PSI.

Were you foot to the floor at 30 mph?

No. Throttle in the logs fluctuates between 35% and 50% for this period of the drive.

Thanks!

 

[nikitis]

I did it my own self, with the help of Jason at TransmissionTuner.com. Jason provided the DIY wiring harness kit and the computer and the tuning. I did all the work. It's a huge job.

Cool, I actually have some questions.

I was talking to Jason for the options.

1.) Did you have to get a new torque converter or adjust the drive shaft at all? Or a new flywheel?

2.) What engine do you have? Edit: nm, I see post title 7.3 IDI.

3.) Additionally, If your bus is a '91, it wouldn't have a computer driven engine. So what all did you hook the Allison 1000's harness to? Can Jasons TCM just operate interdependently without electronic input from the engine, and just detect the RPM's coming from the engine spline and work with a Throttle position Sensor mod?

I know this is a lot of questions, but I'm looking to do this as I'm in a similar boat, and I would love clarification from someone who's actually done this. I have a J1708 network which can give RPM's etc to the AT1000, but still requires a TPS, or an AU translator device. The AU Device translates Throttle from the first computer driven engines starting in '94 to a J1939 driven AT1000, I think you had to have done it without an AU device, and I really would like to know how this works. It would help a lot of people here.

Also to help you here, I know Jason can send you a better tune so your bus wouldn't lug, but he charges for time.

 

[BamaBus]

WARNING! Hijack in progress

 

[AzironaZack]

Cool, I actually have some questions.

I was talking to Jason for the options.

I'd rather keep this thread focused on the issue at hand: my overheating.

I'll write up a conversion post at some point about my experience with the AT545 to 1000 process, and then we can chat about that there.

 

[AzironaZack]

Here's a giant post about my AT545 to Allison 1000 conversion:

AT545 to Allison 1000 Conversion Report

Below is the story of my AT545 to Allison 1000 conversion, performed in late 2024 and early 2025. This is a long narrative, not a how-to guide. I hope it will give you an idea for what you are in store for. If you have detailed questions, please post them and I'll do my best to answer. Here we...

www.skoolie.net

I'm hoping folks will still chime in here on the overheating situation, please and thank you.

 

[La Camioneta]

Cool, I actually have some questions.

I was talking to Jason for the options.

1.) Did you have to get a new torque converter or adjust the drive shaft at all? Or a new flywheel?

2.) What engine do you have? Edit: nm, I see post title 7.3 IDI.

3.) Additionally, If your bus is a '91, it wouldn't have a computer driven engine. So what all did you hook the Allison 1000's harness to? Can Jasons TCM just operate interdependently without electronic input from the engine, and just detect the RPM's coming from the engine spline and work with a Throttle position Sensor mod?

I know this is a lot of questions, but I'm looking to do this as I'm in a similar boat, and I would love clarification from someone who's actually done this. I have a J1708 network which can give RPM's etc to the AT1000, but still requires a TPS, or an AU translator device. The AU Device translates Throttle from the first computer driven engines starting in '94 to a J1939 driven AT1000, I think you had to have done it without an AU device, and I really would like to know how this works. It would help a lot of people here.

Also to help you here, I know Jason can send you a better tune so your bus wouldn't lug, but he charges for time.

All the A-2000 swaps need externally is a TPS signal. Jason works the 0-100% values into his programming magic.

 

[nikitis]

Is your radiator stock or a replacement one? There have been issues with some stock radiators being somewhat inadequate from factory. Cadillackid often mentioned this being an issue. Wondering if the trans upgrade was just enough to push it over the edge. There's not a ton of room for a larger radiator in these as well. Also are you using radiator fluid through your transcooler with your new setup?

 

[Bjorkinman]

If the fan clutch is relatively new I’d wanna make certain that it’s working first. If you have a hydraulic fan (just guessing, my pusher is hydraulic) I wouldn’t swap that out for an electronic. Even with a loud engine those fans are really loud and you should hear it.

 

[La Camioneta]

Here's a giant post about my AT545 to Allison 1000 conversion:

AT545 to Allison 1000 Conversion Report

Below is the story of my AT545 to Allison 1000 conversion, performed in late 2024 and early 2025. This is a long narrative, not a how-to guide. I hope it will give you an idea for what you are in store for. If you have detailed questions, please post them and I'll do my best to answer. Here we...

www.skoolie.net

I'm hoping folks will still chime in here on the overheating situation, please and thank you.

I have a 7.3 psd in a F-250 and rarely hear the fan except towing on hot summer days.

If you grab a blade and the fan freely spins with a cold engine, the clutch is no good. Its overlooked because it spins with the engine whether its freewheeling or being driven to spin.

On a 100• day uphill you should hear it cycle on and off, similar to a semi-truck. You know that fan sound you hear as they drive by and it kicks on?

Most of the times you get enough airflow on flat ground that the fan freewheels. If you're not getting a noticeable fan roar when the coolant gets hot the clutch has failed.

I did the A-2000 5spd swap with a 5.9/4.11s/11R22.5s. 5th is .74? and your sixth is .64? so you might have seen 6th with the 5.38's but will not have the power to push that tall of an OD without massive power upgrades. The high-dollar motor coaches you see flying by are pushing 500-600hp. https://www.rvtechlibrary.com/engine/Cat_RV_Performance.pdf

My 5.9 has timing, better turbo, governor spring and needs an injector upgrade to get the egt's down. It keeps up with semi's except long uphill pulls - egt's, but have seen 88mph on level/downhill, so your swap as it is will go faster than you'd probably care to.

 

[Bjorkinman]

If the fan clutch is relatively new I’d wanna make certain that it’s working first. If you have a hydraulic fan (just guessing, my pusher is hydraulic) I wouldn’t swap that out for an electronic. Even with a loud engine those fans are really loud and you should hear it.

Also, looking at your picture you have a ball valve on a coolant line that’s closed. Should it be open?

 

[La Camioneta]

Also, looking at your picture you have a ball valve on a coolant line that’s closed. Should it be open?

Most likely for the heater.
That's another cheat, when you get over-temp, turn the heat on max and roast yourself, but it will bring your engine temps down...

 

[La Camioneta]

On the lead-up to the coolant temperature warning coming on I was doing 30 mph in 2nd gear at around 2000 RPM for a good long time. It didn't feel like it was lugging.

With the old AT545 I'd be doing 30 at closer to 2800 RPM in 2nd.

Def check the fan clutch.... not a lot of airflow at 30mph, especially with the intercooler pre-heating the radiator air.

 

[bluebird90]

Hey there, folks! I've got a 1991 International 3700 based short school bus with a 7.3 IDI, original injection pump, non-wastegated Banks Turbo, and intercooler. This bus is an RV for me and my wife.

I am having trouble with overheating.

The radiator, thermostat, fan, and viscous clutch are all new in the last couple years. The radiator shroud is in great shape and fits the fan well.

I recently swapped the AT545 transmission for an Allison 1000 six speed. I have 4.44 gears in the rear (originally had 5.38 ratio).

On a drive up Mt Lemmon (local mountain for us in Tucson) on a 105 degree day the coolant temperature light and alarm came on around halfway up. The coolant temperature was 210 degrees (according to the tiny, old gauge on the dash) and the transmission sump temperature was 210. I pulled off and allowed the bus to idle cool in the shade, but it still is disconcerting.

The radiator is a Northern Radiator 232124, which is brass and truly enormous (31 x 26 1/8 x 2 1/16 (4 Row)). The radiator cools both the engine and the transmission fluid.

I'm hoping folks can provide some advice on the overheating.

I suspect the new gearing with the six speed has dropped my engine speed to the point where it's affecting the cooling capacity of the fan. This is just a guess.

When I installed the intercooler I cranked up the injection pump fuel screw. I'm making great power now, but I wonder if this is part of the problem too.

I'm curious what the experts in here think.

Would it be worthwhile to switch to a big electric fan set up? What about switching back to the 5.38 gears in the rear end? Swapping to the higher gears would make my engine speed higher for a given road speed. Right now I seldom see sixth gear, but with the higher ratio I'd probably see sixth more regularly.

What do you think? Thanks!

2 things. It would be worth it to remove the water pump and check the impeller. It could be rusted away and not pushing water. Also, 210 isnt bad for pulling a hill. I think the alarm should be going off at 230. but don't mark my words on that. It's possible the overheat alarm system is bad.

 

[AzironaZack]

Also, looking at your picture you have a ball valve on a coolant line that’s closed. Should it be open?

That picture is dated; the ball valve is open currently. It's there to cut off coolant to the interior heaters, if needed.

Is your radiator stock or a replacement one? There have been issues with some stock radiators being somewhat inadequate from factory. Cadillackid often mentioned this being an issue. Wondering if the trans upgrade was just enough to push it over the edge. There's not a ton of room for a larger radiator in these as well. Also are you using radiator fluid through your transcooler with your new setup?

This is a replacement radiator, put on last year. Northern Radiator 232124, which is brass and truly enormous (31 x 26 1/8 x 2 1/16 (4 Row)).

I do not have a separate trans cooler; it is integrated into the radiator. I could add an additional supplementary trans cooler, but I suspect the heat is coming from the engine and not the trans.

If the fan clutch is relatively new I’d wanna make certain that it’s working first. If you have a hydraulic fan (just guessing, my pusher is hydraulic) I wouldn’t swap that out for an electronic. Even with a loud engine those fans are really loud and you should hear it.

Thanks. It's a viscous clutch, driven by the water pump pulley.

Def check the fan clutch.... not a lot of airflow at 30mph, especially with the intercooler pre-heating the radiator air.

That's what I'm thinking, too. If the fan clutch is okay, I guess I may just have to take it a little easier on grades when it's 105 outside.

2 things. It would be worth it to remove the water pump and check the impeller. It could be rusted away and not pushing water. Also, 210 isnt bad for pulling a hill. I think the alarm should be going off at 230. but don't mark my words on that. It's possible the overheat alarm system is bad.

I'll check the manuals. I'm not sure I've ever seen a spec for when that alarm is supposed to come on. 210 certainly did seem little conservative to me, too.

The water pump was replaced along with the radiator and thermostat last year.