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David, it seems like the chart NumberDummy posted shows that the 3500 rpm tachs were on Diesel engine trucks and 4500 rpm tachs were on gas engine models. Some are not specified in the chart, but none contradict that trend either. This would be consistent with how the terminals are labeled as well (the 3500 rpm tach has the same S- and S+ terminal markings as the light duty diesel pick up tachs with the yellow and red zones). I am guessing that the 4500 rpm tach that you sent to me is for a gas engine.
There are many different part numbers for 3500 rpm tachs, with very specific applications regarding the transmission etc., and my theory is that the diesel medium duty trucks may have used a sensor that counted flywheel teeth. The 6.9/7.3 diesel tach sensor counts the 52 teeth of the injection pump timing gear. I remember reading that flywheel teeth was another commonly used gear for diesel tach signals, and if so that would explain why there were so many different 3500 rpm tach models to correspond to different flywheel teeth counts. Just a theory ...
The windshield trim might be really useful. David gave me 4 of the 5 pieces in very good condition, for which I'm eternally grateful. But, since they do have some minor scratches I thought about getting NOS ones - until I discovered that a full set is about $500. So, I ordered the 5th piece to complete the set, and even then it was pushing $100 with the part and shipping.
Gary...Are the scratches beyond polishing out? Mine are not scratched but I have them off and am using a three step rouge polish kit with layered polishing wheels on the Dremel. I stripped my wiper arms and used the same process to get the old school stainless wiper arm look to match the windshield trim. If the scratches are not too deep, they can be wet sanded out, feathering well beyond, then polished.
The windshield trim is anodized aluminum, and if you polish very far you take the coating off. I've done some polishing and have gotten some of the scratches out, but don't dare go further.
My trim was "washed out" (wadding polish only went so far) so I took a chance figuring that I had nothing to loose. It turned out pretty decent. I'll take some pics and start a new thread when I get the chance.
After looking at the chart posted by number dummy and looking at the terminal markings, David and I think the 4500 tach he sent me will work for a gas engine, and the 3500 tach was for some kind of diesel application. I returned the 4500 tach to him and he sent me the 3500 model to try with my 6.9 IDI diesel truck once I get it running. I suspect the signal will be different. There are many 3500 tachs on number dummy's chart and they seem to be transmission specific. This makes me think the signal may indeed come from flywheel teeth. Nevertheless I will play around with it just to see what it does. The IDI counts injection pump gear teeth, 106 teeth = 53 pulses per crankshaft revolution. The 3500 tach is really neat looking. It has more marked increments than the other low rpm tachs, it has a WHITE needle, and a horizontal rectangle under the RPM x 100 text. Very cool to see in person, for everyone else here are pictures
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