New Tach Sensor - Tach still doesn't work - Diesel Forum

iowa-rob · 10-30-2012, 06:23 PM

Hey Guys
Looked at all threads and saw that more than likely it was a bad Tach sensor (transmission was also in failure mode). Purchased a new sensor from Ford and installed tonight. Tach still does not work.
Tach moves slightly on start but that is it. Can't see any bare wires.

Any ideas??

Thanks

chuckster57 · 10-30-2012, 08:11 PM

I am not an auto electric guru by any means, but it sounds like there may be a problem with chaffing. It would probably be best to inspect the wiring harness from the sensor as far as you can get. If that doesn't reveal any chaffed or bare wires, you might run a continuity test between the wires, I would expect some resistance due to the dash gauge, but I don't know what it should be. Maybe someone can point you in the right direction.

iowa-rob · 10-30-2012, 08:19 PM

Thanks Chuckster.
I tried to look them over and didn't see much but have not yet removed all the covering from the wires.
I saw in another post someone mentioned a fuseable link that was blown, said it was near passenger side battery. Have you ever seen this fuseable link?
I looked and can't find one.
Thanks again!!

chuckster57 · 10-30-2012, 08:25 PM

I doubt that's the issue. If it was, the tach would be dead no matter what. Fusible links are just a fuse built into the wire and once they blow, that circuit is dead. You have to replace the wire.

Using my iPhone4S right now

iowa-rob · 10-31-2012, 09:26 AM

I'm getting no reading at the tach (needle only moves slightly when first started.)
So may not be getting any power.
Also OD light does not flash it just stays illuminated.
Does that give you any ideas?

drinkypoo · 10-31-2012, 09:37 AM

on the transmission electronic control ass'y (TECA) pin 4 ckt 11 color T/Y (tan/yel?) is RPMS+, and pin 44 ckt 644 color DG (dark green I hope) is RPMS-

RPMS failure should set code 14

If the O/D light is steady and won't go off when you push ISTR that indicates some kind of failure, possibly of the TECA. But maybe the wire is just shorting out in the column, which has supposedly been known to happen.

iowa-rob · 10-31-2012, 12:15 PM

Ok great. I'll check that tonight.

Anyone have a wiring diagram for that system?

iowa-rob · 11-04-2012, 01:45 AM

So I have not gotten it in to get codes pulled yet, but where is the port? I thought it was on the passenger side under the hood but I don't see it.
I'm not very good at electrical troubleshooting but bought a multimeter today and trying to learn.
So cleaned all connections at harness to the transmission but no changes and all the pindway looked good. Checked voltage across all the but not sure I know anything from that. Is there a schematic that would tell me what I should have at each one?

I also checked voltage at the connection to tach sensor but got no reading but not sure I did it correctly what is the proper procedure and what should I see? (I put black on one pin and red on the othee but I think I should have put red on one and grounded black?)

Thanks guys. I really appreciate the help.
Rob

drinkypoo · 11-04-2012, 05:16 AM

Codes are on the driver side, there is supposed to be a plastic cover for them to tuck up into but mine is missing so they just hang near the brake booster.

If you check voltage it doesn't matter how you have the leads hooked up. But I think that is a voltage generating sensor. At this point I'd be checking continuity of the leads. The TECA is behind the dash...

iowa-rob · 11-04-2012, 08:15 AM

Thanks DP!
Ok so pretty dumb about multimeters. Mine does have the continuity function.
But to check continuity do I have a lead at each end of the wire or how do I do that?
And are you saying to go to the teca and continuity there?
Thanks again.

drinkypoo · 11-04-2012, 08:17 AM

Yeah, I gave you those wire colors so you could trace to the other ends. You will probably need a test lead (clips on the ends) and a long piece of wire to reach both ends with your meter.

Just check resistance, if it's much higher than 0 there's a wiring fault someplace. That's a continuity test. Some meters have an audible continuity test that beeps when resistance is 0 on one of the higher scales.

Also I believe only one of those wires, if any, should short to ground.

iowa-rob · 11-07-2012, 09:27 PM

Ok so I have checked continuity from the RPM sensor connector back to the control module and continuity is good.
so dismantled the dash and checked continuity from the control module into the connection at the tachometer and it was good. So checked the continuity of the ground for the tachometer and it was good.
Started it up and to see if tach was working (just hoping) and it still wasn't and od light is still lit up.
Anyone have other ideas?
I have not checked codes as I don't have a code reader and couldn't find a manual way to check codes becasue I only have a digital mulitmeter (not analog).

Is there a way to manually check codes with digital multimeter?
Electrical problems are tough. My son really wants his truck back as he is stuck driving a ford freestyle - not quite the same hot rod as an F350!!
Thanks.

Rob

chuckster57 · 11-07-2012, 09:34 PM

You can still buy a Ford OBD I code reader. That may be your only option at this point, and IIRC they are less than $25.00 at the A/P store. Just make sure you ask for the FORD code reader.

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drinkypoo · 11-08-2012, 09:43 AM

OK, here's some more data. a white/pink wire feeds tach signal from TECA to cluster.

The TECA has a diode and a relay in the under-hood power distribution box. The relay feeds a red line that goes to a splice numbered 157 in the engine harness, which feeds both the transmission itself via a connector very near the box, and the TECA via two red wires, to pins 37 and 57 on the TECA

as for reading codes you can try this http://www.hotrodders.com/forum/read...ter-63787.html

I have a Ford OBD-I flasher and recently got a Ford OBD-I code reader with a screen on it and everything so I'm set... out here
You probably should get a flasher for twenty or thirty bucks like Chuck says, for the future. Or just watch yard sales and flea markets, I got my actron scanner for $10. he wanted $25 but I talked him down

iowa-rob · 11-08-2012, 09:07 PM

I stopped and bought a code reader tonight. Told them I needed the Ford - and of course they started with the $200 model.
Finaly gave me the Innova 3145 which is a OBD 1 EEC IV MCU.
BUT when I got home I see on the back it says under trucks 85-94 7.3L excludes 1994 models!!! (but does not say diesel.)
So does it mean PS and so will work on mine??
I assume it will but it cost $30+ so don't want to open it to have it not work.