My 92 f250 7.3/zf5 has a tach issue. It's kind of jumpy, and when I run over 2300 rpms, it pegs all the way up until I let off the throttle.

The problem was intermittent until I removed the gauge cluster for some dash light replacement. I cleaned the circuit connections on the back of the cluster at that time. I also removed the sender, and cleaned it (not really dirty at all and no metal flakes stuck on there). I don't remember what the exact ohm reading was when I tested it, but it tested ok, according to what I've found in my post search on this forum. The wiring on the sender showed bare wire right at the back of the sender, but I saw no broken copper strands. So I added some liquid electrical tape goop to make sure no unwanted contacts were happening there, and put it all back together, including a new oring on the sender.

Now it does all the same things, but more regularly and with more enthusiasm. On searching the previous posts here, it seems that mostly the sender is blamed for low reading/ no reading issues. I'm assuming that if the wires are grounding/shorting I'd get no reading rather than a spike

Do I have a sender issue, a gauge issue, an overall voltage issue of another type I don't know about, or a bad connection somewhere inline that I haven't found? All other gauges and electrics work fine. Sorry so long-winded, I just wanted to get everything I think I know out there early.

 

Sounds like the gauge to me. As you said, you'd see a low reading from a faulty sender, not a high one.

If you can solder, I'd pull it, look for any capacitors and replace them. Also, look for any 'dry solder joints' and re-solder them. Check Youtube for tips.

 

I'd replace the sender. I had a FoMoCo unit about a year old so the same sort of crazy intermittent and inaccurate reading thing, replaced it with an aftermarket part and rock steady accurate.

 

Okay, it's been an ugly week at work but I've got the truck in the shop again. I got some advice about checking the vss in the rear axle, and did that. It tested in the "normal" range (1292 ohms) and i figured that if I had a problem there, it would show up on the speedometer.

However, I noticed as soon as I drove it again that all my spikes at higher rpms have gone away, and now the rpms keep more steady but the readings are way low for what I know the engine is turning. This looks more like what I'd expect from a failing sender, so I'm thinking to replace it and see where that leaves me.

Also, when I fire it up, the voltage meter goes crazy for a few seconds, then settles down, and runs up to show charge. I don't think it's just the gauge, because the lights get bright/dim several times as well. This truck has done this since before I owned it, and it doesn't seem to affect anything seriously, so I haven't worried about it. But now the tach mirrors those surges, and settles down when the voltage levels out.

Macrobb, I have seen some videos on board repair on Youtube, which is where I found out about the little doohickey that is supposed to rectify the power input, but I'm not confident in my ability to repair the thing if it has a problem. I am suspicious that the power fluctuations may have finally damaged the power input on the unit, though.

Cadunkle, I didn't know there were after market senders for these motors. Every thread I've searched says "Ford only part". Where did you get yours?

 

 

The tach sender is a "Variable Reluctance Sensor", and there are a few compatible ones. I know someone who used an ABS sensor off a Chevy something.

I personally have been using these:
Honeywell Variable Reluctance Speed Sensor 2874A003 Perkins Engine 728062-02 | eBay

I have to turn the OD down a bit to get it to press into the factory steel hex nut piece, then solder on the two wires. Works just fine; I've done this on 3 different trucks so far.

 

Thanks for the info. I'll get a sensor ordered and hopefully that will solve that problem. Today I'm trying to replace my glow plug harness. Just a precaution, but all the plastic has decayed off the connectors, so I may as well fix it before it fails.

 

Wow that sensor is cheap! I'll have to order that next time I need one

My old sensor was showing wire at the end and I did the same thing with the electrical tape goo, didn't help but a new one did.

 

^^^ What's needed is a mini-write-up on how to mount that sensor in the big nut that goes on the IP cover. I presume you have to gut an old broken sensor and use that nut as the host for the new one, right? I would also reckon that you have to be careful about how "deep" you mount the sensor into the nut, in order to for it to pick up the IP gear teeth properly and obviously not to make contact with them. My sensor's good so far, but I had to repair the wire insulation right where they come out of the sensor so I gotta believe it's prone to failure.

 

you said your alt gauge freaks out, this could be the after glow of controller for glow plugs, the dimming is normal too. if you have a bad conection and voltage spikes high it could cause tach to freak out??

 

Yes, Speedwrench, the alt does freak out on startup, but would afterglow happen once the motor is running? That voltage fluctuation has been happening every time it is cold-started since I started driving the truck. The new thing is that the tach follows the voltage spike while it's idling, and it never used to do that.

The replacement (Honeywell) sensor came today. Thanks for the link, Macrobb. I removed the old one, drilled out the "adapter" and filed down the plastic body so it would slip inside. Then I soldered the wires on to the connector, and now I have to figure out what I want to use to glue it in there. My technique left me a little shy of "press fit". I'm thinking black RTV but maybe JB Weld. Any suggestions?

 

The key word there is AFTERglow. As is after the engine has started, so yes, it will cause voltage fluctuations right after cold start.

I'm trying to remember what I did when I replaced the tach sensor with the ebay sensor (I think the honeywell one, but don't recall right now). JB weld should be fine, black rtv would probably be fine as well. Its not like a mission critical fit up, you just want to make sure the sensor is close to the gear without actually touching it.

 

Thanks for the info. I had assumed that the glowplug circuit shut down at ignition. It's good to know my truck is normal. Even if the owner's only aspiring in that direction. I intend to install the new sensor flush to the bottom of the thread on the metal housing, which is where the original one was.