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New Member

8 Posts

Posted - 11/25/2021 :  09:43:08 AM  Show Profile  Visit daley's Homepage  Reply with Quote
Hey folks,

I have a Thomas 2001 Saf T liner HDX and I have been busy working on the inside of it but I may have to move it soon.

I have been having trouble with starting it from the ignition key.

The starter solenoid is usually mounted in the rear electrical box, when I was doing some work in the past to get it going it got removed from there and is hanging loose.

My question is, on this bus, is the solenoid grounded when it is bolted to the rear electrical box, or is it grounded from the 4th pole which has a black wire going to the rear electrical box?

If I run a test wire to the 4th pole of the solenoid to the ground terminal in the electrical box then it will start on the key, however, I am a bit confused as this 4th pole shows 12v when the ignition pole is powered and it is not grounded?

The 4th pole is connected to a wire that goes to the rear electrical box and the wiring diagram lists this fused wire as 'ether start signal from solenoid'

I am a bit confused. Do I have a bad ground? Is this wire 'ether start signal' supposed to arm the solenoid in some way and something is preventing this?

I don't think I want to add a ground wire just to make it start from the key. I would rather figure out what is not happening correctly Any help or advice much appreciated.

Senior Member

65 Posts

Posted - 11/25/2021 :  8:20:11 PM  Show Profile  Visit TRex's Homepage  Reply with Quote
So what may be going on is this. By turning the key you supply power to what is a coil. Your meter is providing the ground path that the 4th pole provides. To check try this. Measure the voltage where the key switch wire attaches to the solenoid. Now measure it at the 4th post. You will find it will be slightly lower by a 1/4 to 3/4 of a volt due to the resistance of the coil.

The 4th post is to assure a good ground rather than relying on the case grounding to whatever it is bolted to. At least that is what I think is going on. Bosch relays used to confuse me until I figured this out.
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New Member

8 Posts

Posted - 11/27/2021 :  06:46:41 AM  Show Profile  Visit daley's Homepage  Reply with Quote

Thanks for the info,I got things working again, I bought a new solenoid and grounded it out and now bus works on the ignition.

New problem though, now none of the temperature gauges on the dash work and the display says ' no data '

I will go look for wires not connected and see if anything is obvious.

Thanks again
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Senior Member

145 Posts

Posted - 11/27/2021 :  1:40:04 PM  Show Profile  Reply with Quote
Stop and retrace your first repair and make sure the first repair did not create the second problem. One major rule I always had my guys follow was to ask this question. Had someone just been in this area? So many times things get bumped, moved, diagnosed wrong, repaired wrong and causes other problems.

One other thing I would suggest. Solenoids all look alike, but do not work the same. Take a few minutes and get a catalog like Cole-Hersee. They not only list all of the solenoids, they show how they are wired internally.

You need to understand how they work internally, so they can be connected correctly externally. I had always drawn these diagrams on my inventory boxes so my guys would not grab the wrong solenoid. When working on wheel chair lifts, lift gates, starter solenoids, constant duty, intermittent duty, and so on. They all look alike, but do not work the same. Just my opinion, thanks.

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New Member

8 Posts

Posted - 11/30/2021 :  1:53:31 PM  Show Profile  Visit daley's Homepage  Reply with Quote
Hi again,

Thanks very much for the info, I made sure to get the correct solenoid.

I managed to get the cluster to work again. I cleaned all the grounds and major terminals all over the bus, on the engine, in the electrical boxes, and up front where it comes into the bus.
I did that and also took the cluster out and unplugged the connectors and put them back in and all is well again on that front, mileage shows and gauges work.

I haven't used this bus for a year or more and the last time it was driven it wouldn't rev very high or go more than 10-15mph.

Along with the electrical issues I also fixed a section of exhaust pipe that was missing between the turbo and the silencer/filter.

When I first started the bus up after fixing this it would rev all the way up quickly and I believe I could here the whooshing of the turbo. I thought I had fixed it!

Anyhow, after a minute or so it was back to revving very slowly up to 1500rpm max. I switched it off and called it a day.

Where should I start to look? It obviously was happy for a minute but something has derated it. I don't have any check engine lights on the dash and it starts without any hesitation.

Should I be looking at simple things like fuel and air filter? I feel like not though as it did rev to max fine at first.

I do have CAT ET(it is a 3126 engine), but I have no idea how to use it or what to be looking for.

I am really low on funds, I might have to move this thing very soon, and I love figuring out problems, have tools and not afraid to get dirty or stuck in. If anyone has any information about where to look next, or can guide me on what to look for on CAT ET I would be more than extremely grateful.

Many thanks again

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New Member

8 Posts

Posted - 11/30/2021 :  1:56:17 PM  Show Profile  Visit daley's Homepage  Reply with Quote
Oh also, when I press the accelerator pedal I can hear a relay click in the front electrical section to the right of the dash.
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