Tach/Transmission Problems

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Mar 3, 2011 | 01:50 PM

nse1355

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Tach/Transmission Problems

Hi guys and gals...

Another forum suggested I post this problem here.

90 F350 diesel automatic transmission

Problem: Dead tach and erratic shifting.

Things I know...

The RPM/tach sensor measures 2,475 ohms when removed and held away from metal.
With sensor in but disconnected the volts increase when I rev up the engine.
I've replaced the fuses under the dash.

Any ideas on what to check next and how to check it?

Thanks!

Mar 3, 2011 | 02:53 PM

Lazy K

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https://www.ford-trucks.com/forums/1...and-trans.html Have a read through this link. May be worth checking the wire harness mentioned.

Mar 3, 2011 | 04:06 PM

nse1355

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Thanks Lazy K...

Sounds a bit over my head. Maybe I should just go ahead and have a shop run a diagnostics test and tell me exactly what it is.

There was one interesting point. Check the RPM sensor away from metal then against metal which I didn't do nor know what the measurement should be against metal. Also read that the sensor passed these tests but was still bad.

I didn't' do anything to disturb the wiring harness as in the link but it is old so the problem may be there.

Mar 3, 2011 | 08:11 PM

gumper 123

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Is your od light working?

Mar 4, 2011 | 09:34 AM

nse1355

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Shoot! I forgot to include that in my first post.

No, the OD light doesn't work no matter how many time I push the button. I pulled the plug on it yesterday but didn't see anything wrong so I plugged it back in. No change.

Thanks

Mar 4, 2011 | 02:23 PM

gumper 123

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cup your hand around the light and see if it is on but dim.

Mar 7, 2011 | 05:43 PM

nse1355

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Sorry for the delay.

I waited until it was dark and there's no OD light. However, I also noticed I don't have dash lights and I don't know if this makes a difference. I'll investigate the dash light problem.

Obviously I don't drive this truck much.

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Mar 7, 2011 | 09:00 PM

gumper 123

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check fuse 17 under dash

Mar 8, 2011 | 11:54 AM

#10

nse1355

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All fuses are good. I have no dash lights, fender marker lights, or tail lights. I have dome, turn, brake lights. Radio, power windows and horn works. I don't mind the lights not working as I never drive this truck at night which is why I never noticed the dash lights were out.

Do you think the light problem could be related to the RPM sensor problem?

Can the RPM sensor pass the two tests above (ohms and voltage) and still be bad?

Thanks for you help

Mar 8, 2011 | 01:00 PM

#11

gumper 123

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did you do a visual on fueses every once ina while they can look great and not be good i always test both sides with test light to check. I dont remember what is all on that fuse i will look.

Mar 8, 2011 | 01:37 PM

#12

nse1355

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They looked good but I'll do a light test on each. thanks

Apr 27, 2011 | 02:22 PM

#13

nse1355

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Sorry for the long delay. The truck is in a comma but I'm pretty sure how to get it out. Weird things started happening such as some lights worked whereas others didn't. Couldn't get it to shut off until I unplugged a wire on top of the engine. Finally it just gave up hope and wouldn't start. The batteries were dead so something was probably draining them. Replaced the batteries and it cranks but no start.

I finally took a shop vac to it and cleaned out all the leaves under the hood and found squirrels had eaten the wiring harness into the wiring block down to nubs. The also got into some of the in-line wires.

Neither the wiring block or harness are available from Ford or aftermarket.

I'm now looking for a wiring harness from salvage yards.

Thanks for all your help!

Apr 27, 2011 | 02:37 PM

#14

Aune163rd

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From: Victor, MT

Check with international for the wiring harness, if not. Alot of local salvage yards have them or can get them. a good one I go to alot being from montana is Spauldings Auto Salvage in Spokane, WA. I was gonna say if it wast the harness, the exterior lights might not work because of a bad ground, or your turn signal switch is bad.