kcinga Posted December 26, 2022 Share Posted December 26, 2022 Just stumbled across this forum as I’ve been conducting many internet searches to gather more information on my truck. I recently purchased a 1986 F350 diesel with dual wheels and a crew cab. Previous owner couldn’t get it on the road himself, so now it’s my turn! I’ve never owned a diesel in my life. It starts and runs beautifully, but the issue with the tachometer has prevented the transmission from engaging properly. I love that it has A/C! I live in Georgia, so it helps to have some way to deal with the heat. I’m moving on from my existing truck, which is a 1971 Ford F100 that I have owned since 2009. It’s been great for piddling: making trips to the dump or picking up loads of mulch or dirt for landscaping around the house. It has no A/C. I’m anxious to get my ‘86 F350 on the road again! Link to comment Share on other sites More sharing options...
Gary Lewis Posted December 26, 2022 Share Posted December 26, 2022 Welcome! Glad you joined. And that you posted an intro here. What part of GA? I ask because we have a map (Bullnose Forum/Member's Map in the menu) and we can add you with a city or zip. Link to comment Share on other sites More sharing options...
grumpin Posted December 26, 2022 Share Posted December 26, 2022 Welcome! Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted December 26, 2022 Share Posted December 26, 2022 Welcome! I wasn't aware the tach has anything to do with shifting a diesel C6 You might need to address the VRV (vacuum regulator valve) on the injection pump, but I'm not an expert IDI mechanic. Maybe our member Jonathan could better address your issue, but you might post a new topic in the main forum so more eyes get to see your questions. 💡 Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted December 26, 2022 Share Posted December 26, 2022 The engine speed sensor is a closed loop on diesel models, as can be seen here in the 1986 EVTM. The fact that tach/engine speed sensor is optional points hard to the idea that it is not required for shifting a C6 (as many trucks sold drive just fine without a tach cluster ever being installed. Link to comment Share on other sites More sharing options...
kcinga Posted December 26, 2022 Author Share Posted December 26, 2022 The engine speed sensor is a closed loop on diesel models, as can be seen here in the 1986 EVTM. The fact that tach/engine speed sensor is optional points hard to the idea that it is not required for shifting a C6 (as many trucks sold drive just fine without a tach cluster ever being installed. I’m just going by what I see on the internet. Plenty of other people have posted transmission issues related to tachometer (only on automatic transmission, not on manual) on diesel engines. If tachometer not working, then transmission won’t function properly, either. I think they call it “stuck in limp mode”. Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted December 26, 2022 Share Posted December 26, 2022 I’m just going by what I see on the internet. Plenty of other people have posted transmission issues related to tachometer (only on automatic transmission, not on manual) on diesel engines. If tachometer not working, then transmission won’t function properly, either. I think they call it “stuck in limp mode”. While that's definitely true of later diesels that use a VSS to control shift points through the PSOM, in 1986 everything is analog. There is no 'limp mode' for a diesel Bullnose truck. Can you be a little more specific about what your transmission is doing (or not doing)? Link to comment Share on other sites More sharing options...
kcinga Posted December 26, 2022 Author Share Posted December 26, 2022 While that's definitely true of later diesels that use a VSS to control shift points through the PSOM, in 1986 everything is analog. There is no 'limp mode' for a diesel Bullnose truck. Can you be a little more specific about what your transmission is doing (or not doing)? Previous owner told me about limp mode. Transmission was replaced, but since tachometer wasn’t working he thought that was the reason for it barely moving. Tends to work better in reverse. I had it shipped to me and it was driven to my house from semi trailer (he couldn’t get all the way to my subdivision) in reverse. I did drive it in subdivision in drive and got as high as 20-30 mph, but I didn’t want to push it too hard-based on what previous owner was telling me. It does move, but it’s like it really doesn’t want to go anywhere. Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted December 26, 2022 Share Posted December 26, 2022 Previous owner told me about limp mode. Transmission was replaced, but since tachometer wasn’t working he thought that was the reason for it barely moving. Tends to work better in reverse. I had it shipped to me and it was driven to my house from semi trailer (he couldn’t get all the way to my subdivision) in reverse. I did drive it in subdivision in drive and got as high as 20-30 mph, but I didn’t want to push it too hard-based on what previous owner was telling me. It does move, but it’s like it really doesn’t want to go anywhere. Did the new C6 come with a warranty??? Have you confirmed the fluid level and line pressure? The fact that is works in reverse but is problematic in forward gear makes me think the pump and torque converter are okay and either the clutches are smoked or the solenoids are not applying enough band pressure. They are very robust and simple transmissions best known for their ability to convert horsepower into heat. As you can see from the above factory illustration the speed sensor only sends a signal to the tach. Beyond that one instrument it is completely isolated from the truck wiring. I'm sorry to say that I think the PO was either very ignorant or very underhanded and took advantage of you. Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted December 26, 2022 Share Posted December 26, 2022 ......barely moving...... Tends to work better in reverse...... It does move, but it’s like it really doesn’t want to go anywhere. Define "barely moving". Does it just rev like there's no load attached to the engine? How free does it roll in neutral? You probably know where I'm going but it pays to ask stupid questions first. Link to comment Share on other sites More sharing options...
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