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Hot Ford 7.3L

I look forward to reading your magazine each month, primarily because you haven’t forgotten the older diesels. I have a 1988 Ford F-350 equipped with a 7.3L non-turbo diesel engine, manual four-speed OD transmission, and 4.10 gears. I’ve been unsuccessful in locating someone locally who can help with a problem I’m having with my truck’s cooling system.

The problem occurs while towing my bass boat, in that the engine will begin running a little warmer (220-225° F) than I remember. I’m told that the fan-clutch disengages at a certain rpm because Ford seems to think it will get enough air through the radiator at 55-60 mph. This is fine while not towing, but not so good when towing. I installed two electric fans in front of the radiator, but they helped very little. I was wondering if anyone offers a clutch with an electric lockup so the fan would operate at any temp over 195-200. I think this would help me a lot. Or maybe someone makes a thermal clutch that would lock up at high temps. Any help on this would be appreciated. By the way, I have a new radiator, water pump, belts and hoses.

Thank you,

Robert Gooding
Birmingham, AL


Hi Robert,

I spoke with Dick at Horton Engine Cooling Solutions a few years ago about their line of electromagnetic fan-clutches. For a time, they offered a version of their fan-clutch for the Dodge Cummins-equipped trucks, but discontinued the kit. At the present time, Horton doesn’t offer an electromagnetic fan-clutch for any Ford diesel pickup engine. They do offer a fan-clutch kit for the International DT444E (kit #996124), but this won’t work in a Ford truck because the water pump used by Ford is different than that used by International.

I visited the Hayden Auto website, the manufacturer for many viscous fan-clutches used by engine manufacturers over the past few decades, including your 7.3L diesel engine. According to Hayden, the viscous fan-clutch factory replacement you need is part number 2830, which is a standard-rotation severe-duty thermal fan-clutch. You can order this fan-clutch online from Amazon, Summit Racing, Rock Auto, and from many other vendors. Prices begin in the $125 range.

This may be all you need, and would undoubtedly be less expensive than any electric fan-clutch. You’ll also need a special tool set to remove and re-install the fan-clutch. These are available from a variety of sources, including Auto Tool Express.

I suspect your original fan-clutch isn’t engaging when it should. A poorly performing fan-clutch will produce a chronic overheating condition. Depending on engine load, you may see above normal engine temperatures or you may see excessive temperatures while towing up grades that cause you to pull over. What follows is basic troubleshooting tech for discovering whether your fan-clutch is operating correctly.

The factory fan-clutch should engage when the air temperature between the rear surface of the radiator and the fan-clutch reaches its pre-set engage temperature of somewhere between 165-205° F, depending on application. Factory fan-clutch calibration can sometimes be a little loose, and having a fan-clutch with a calibration near the upper limit (or not working at all) can result in chronic overheating and poor air conditioning performance.

A bi-metallic coil on the face of the viscous clutch controls engagement. As the air temperature at the coil increases, the coil expands, which in turn rotates a shaft and plate, allowing the silicone fluid to pass into the working chamber of the fan-clutch. Opening this valve allows the silicone fluid to circulate between a close-fitting grooved rotor and stator, which increases fluid coupling. This causes the fan to turn at a speed approaching that of the fan pulley. Engagement never reaches a full 100 percent, but fan speed is typically in the range of 80-95 percent of pulley speed, depending on engine rpm and fan loads.

The silicone fluid is continuously cycling between the reservoir and the coupling plates while the valve is open. Once the temperature drops below the engagement point, the bi-metallic coil closes the valve, and most of the silicone fluid becomes trapped in the reservoir. This uncouples the rotor and stator, and the fan-clutch disengages.

You should hear more fan noise when the fan-clutch is engaged. Some people describe this as a “roar,” but there’s no mistaking the sound when the fan-clutch is fully engaged. You should hear the fan while pulling away from a stoplight on a hot day in city traffic or when the engine temperature reaches or exceeds 210 while towing up grades. A correctly operating fan-clutch is as important as any other single component in the cooling system.

Many people don’t realize that a viscous fan-clutch doesn’t last forever. Over time, heat slowly breaks down the silicone fluid or the fluid can sometimes slowly leak out of the clutch. Replace the fan-clutch whenever it doesn’t engage in situations like those described above. Good luck.


Half-Ton Towing?

Hi, I’m considering buying a late 1990s GMC 1500 4×4 pickup with a 2007 Duramax diesel. The engine has approximately 6,000 miles on it, and the truck looks to be well taken care of. The reason I haven’t seen the truck personally is because it’s two [Canadian] Provinces away from where I live.

If this all works out I will use the truck to pull a horse trailer, which weighs about 3,200 lbs. empty or about 5,000 lbs. with two horses. I don’t know what the future might hold, but a three-horse trailer is something I’m considering.

I’m familiar with the Dodge diesel trucks, and have towed with one. I like the looks of the GMC pickups, but don’t know enough about them. My question is, do you think the half-ton chassis will work for this type of towing? If you would give me your opinion and thoughts I would really appreciate it.

Sarah Heland
Via Email


Hello Sarah,

Towing capacity is based on several factors, such as engine performance and cooling system capacity, transmission durability and rating, braking performance, frame construction, suspension, and axle and tire/wheel weight ratings.

The 1998-1999 model year auto transmission-equipped 1500 series GM 4×4 trucks were factory-rated to tow up to a 6,500-lb ball hitch trailer if the truck is equipped with a 3.42 differential gear ratio, or up to a 7,500-lb trailer if the truck is equipped with 3.73 differential gears. This assumes a weight-distributing hitch. So, it appears the truck you’re considering will have the factory-rated towing capacity you need.

If you don’t know what differential gear ratio the truck was factory equipped with, you can scan the rows and columns of three-digit codes on a sticker in the glove box. Somewhere in the mix you should find either a code GU6 (which indicates 3.42 ratio) or GT4 (which indicates a 3.73 ratio). This assumes a previous owner hasn’t changed the gearing.

However, the late 1990s GM trucks did not come from the factory with the Duramax 6600 diesel engine. If the GMC you mentioned actually has a Duramax installed, it was a custom installation. Like most things custom, the quality, safety and functionality of the truck all depend on the skills of those doing the conversion. I would suggest you have the truck examined by a knowledgeable third party for a second opinion. The 1990s GM diesel pickups were available with GM’s 6.5L turbo diesel, which can be perfectly adequate for its rated towing capacities, but it’s not as powerful as the Duramax.

Let me know how it goes or if we can help further.


Delayed Shifting

Hello, I have a 2005 Chevy Duramax. I use this vehicle mostly to pull a fifth-wheel travel-trailer that weighs nearly 15,000-lbs. when loaded. We have towed this trailer all over the West during the past two years, so we’ve seen what mountain passes are all about.

I have a Banks programmer and 4-inch exhaust system on our truck, which produced a dramatic improvement in performance. While towing, I never exceed level 3 on the programmer, and always use the Allison’s Tow/Haul mode. I usually set the cruise control at about 65 mph while towing.

The truck is currently equipped with 3.73 differential gears, which seem to cause a delay in shifting from fourth back into fifth following a downshift. Up-shifting back into fifth sometimes takes a long time even after returning to the original cruise speed. Can I install 4.10 differential gears in this truck to help with the shifting problem?


Carl Penny
Via the Internet 


Thanks Carl,

I spoke with Kevin at Randy’s Ring & Pinion concerning what gear ratios are available for the GM/Dodge AAM1150 rear differential. Kevin provided this list:





4.63 (2WD only)



5.38 (upcoming) 

The 3.73 differential ratio is the only ratio offered in the Duramax diesel-equipped 2500HD/3500 series trucks. I’ve always thought that GM should have offered the 4.10 ratio option, especially for those owners who use their truck to tow at or near the maximum gross combined weight rating. 

Regarding the delay in shifting back into fifth (overdrive), what you experienced is somewhat normal. GM Powertrain uses somewhat sophisticated programming with their Allison automatic, which can result in delayed up-shifts while towing in hilly terrain. One design feature programmed into the TCM (Transmission Control Module) of special interest is called “Shift Stabilization.” Owners of 2000 or earlier (pre-Duramax/Allison) GM trucks having either the 4L60-E or the 4L80-E automatic transmission usually have to be more careful while towing in hilly country. Frequent shifts in and out of overdrive, or any other gear, can reduce transmission life. 

The standard recommendation for owners of trucks equipped with either the 4L60-E or 4L80-E is to pull the gear selector down a gear to help prevent this “shift busyness.” The Allison programming does this for you, as well as eliminating frequent up/down shifts while driving at slow speeds, such as you might encounter in a parking lot or in heavy city traffic. The Allison programming will generally select the most appropriate gear, and keep it there for a longer period of time. Usually de-selecting Tow/Haul mode will allow the Allison to up-shift, if you want the transmission to shift into the next higher gear. 

Another aspect of “Shift Stabilization” can appear while towing a heavy trailer up a long, steep grade. If the rate of acceleration slows, and to ease the burden on the automatic transmission, the transmission programming could force a downshift and hold it there for the duration of the run, even if the grade flattens and you could run faster. This helps to protect the transmission, and is undoubtedly one of the factors in producing the 26,000-lb Gross Vehicle Weight rating for the 2005 Allison 1000 (in the medium-duty applications). 

Incidentally, the various aftermarket programmers allow owners to correct for speedometer error when changing tire size or differential gearing. Changing gearing could produce problems with the truck’s anti-lock braking system unless an aftermarket programmer is used to correct for speedometer error and recalibrate the ABS system. It’s do-able. Thanks for writing. DW

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